Lecture Notes Foundation Engineering 6532r

l/6 the computed minimum pressure is negative soil pressure. This is an indication of a tensile stress between the soil and footing. This in not feasible and the soil pressure has to be evaluated neglecting any soil tension. The eccentricity is said to be outside mid-third. For eccentricity outside middle third with respect one axis the maximum soil pressure redistributes itself since the base cannot take negative pressure. The distribution of pressure is triangular and is shown on Figure 1.20. The equations applicable in this case can be derived as follows:P M

B

L L’

L’/3

e=M/P

P Figure 1. 4 Eccentrically loaded rectangular out of middle third L' L q   e and P  ( BL ' ) 3 2 2 Solving the two equations to obtain the maximum soil pressure q, Equation 1. is obtained 2* P q 1.3 3B(l / 2  e) Rectangular combined footings It may not be possible to place columns at the centre of spread footings if they are near the property line, near mechanical equipment or irregularly spaced columns. Columns located off center will result in a non uniform soil pressure. In order to avoid the non uniform soil pressure, an alternative is to enlarge the footing and place one or more of the columns in the same footing to enable the center of gravity of the columns loads to coincide with the center

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of the footing (Figure 1. . The assumption here is that the footing is rigid. The column loads are taken as point loads and distributed into the footing. The footings are statically determinate for any number of columns. The column loads are known and the resulting pressure is shown in equation 1.37 q  P / A

1. 4

P1

P2 Variable

S

Figure 1.5 Combined rectangular footing Trapezoidal shaped footings A trapezoidal shaped footing is required when a combined rectangular footing will not result in uniform pressure. This is usually so when the space between the combined footings is constricted as shown on Figure 1.22.

b

a

X’ X1 L

Figure 1. 6 Trapezoidal footing From Figure 1.22 the position of the centre of area of the footing is x’. The centre of the area is to coincide with the center of gravity of the loads from the two or more columns being ed by the trapezoidal footing. The position of the base cannot be extended beyond the length dimension L. L is therefore a known dimension. The value of the area of the foundation is obtained from the allowable bearing pressure and the total column loads ( A  P / q a ). . The area of the base is shown in Equation 1.38 and the position of the centre of the area is shown in Equation 1.39. The solution to the two equations leads to unique values of a and b representing the dimensions of the trapezoidal footing.

A

ab L 2


1. 5

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ab 1 L L L * x 1  (b  a) * L *  a * L * 2 2 3 2 Therefore L 2a  b x1  * 1. 6 3 ab A

From Equation 1.39 and Figure 1.22 it can be seen that the solution for a=0 is a triangular footing and for a=b it is a rectangle. The solution for a trapezoid footing exists only for

L 1 L x  3 2 Strap or cantilever footings A strap footing is designed to connect an eccentrically loaded column to an interior column as shown on Figure 1.23. The strap is used to transmit the moment caused by eccentricity to the interior column footing so that a uniform soil pressure applied to both footings. The strap serves the same purpose as the interior portion of combined footing and is used in lieu of combined rectangular or trapezoidal footing. Equations 1.40 through 1.43 are used to proportion the footing dimensions. The value of eccentricity e is chosen arbitrary by the designer. Unique solution of the strap footing is not always possible R1 * S 1  P1S

S S1 R2  P1  P2  R1 R1  P1

1. 7 1. 8

L1 / 2  e  x

1. 9

R1  B1 * L1 * q a

and S

P1

x

R2  B2 * L2 * q a

e

1. 10 P2

L2

L1/2 S1

R1

R2

Figure 1. 7 Typical strap footing Three basic considerations for strap footing design are:-


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The strap must be rigid (Istrap/Ifooting>2. This rigidity is necessary to avoid rotation of the exterior footing. The footing should be proportioned to approximately the same soil pressures and avoidance of large differential settlements The strap should be out of with the soil so that there are no soil reactions and is weightless

A strap footing is to be considered only as a last option when other options would not work. The extra labor involved in the forming of the deep beam and accompanying costs make it only an attractive alternative when other options have been exhausted. Raft foundations A raft foundation is a large concrete slab used as a foundation of a several columns in several lines. It may encom the entire foundation area or only a portion. Raft foundations are generally used to storage tanks, several pieces of industrial equipment or high rise buildings. Figure 1.24 shows some typical raft foundations A raft foundation is used where the ing soil has a low bearing capacity. Traditionally the raft is adopted when pad and structural wall foundations cover over half the area enclosed by the columns and the structural walls. However this should be evaluated on a case by case basis since the raft foundations end up with negative moments and top and bottom reinforcement. This arrangement could end up being more expensive than closely spaced pads which require only bottom reinforcement.

(a)

(b)

(c)

(a) Flat slab; (b) Thickened under columns or beam slab (c) Basement walls as part of the raft or cellular construction

Figure 1. 8 Common types of raft foundations


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The advantages of the raft foundations over the other foundations include:a)

b) c)

The effect of combining the column bases is increase in the bearing capacity of the foundation. This is because the bearing capacity increases with the breadth of the base. The raft foundations bridge over the weak spots They reduce settlement and are particularly suitable for structures sensitive to settlement.

Raft foundations are usually designed as infinitely rigid in comparison to the ing soil. This assumption simplifies the pressure under the raft to a linearly distributed pressure. The centroid of the pressure coincides with the line of action of the resultant force of all the loads acting on the raft. Figure 1.25 shows the pressure distribution and the resultant of the vertical loads. Resultant of column and wall loads

σmin

σmax Resultant of soil pressure

Figure 1. 9 Linear pressure distribution below a rigid raft

A raft foundation is considered as rigid if the column spacing is less than 1.75/λ. λ is given by Equation 1.44

 K *b   s   4 * Ec * I 

1/ 4

1. 11

Where Ks = coefficient of sub-grade reaction B = width of strip of the raft between centers of adjacent bays Ec = modulus of elasticity of concrete I = the moment of inertia of the strip of concrete λ. = characteristic coefficient Bowles (1982) suggests that the coefficient of subgrade reaction be estimated from Equation 1.45. K s  40 * F * q a 1. 12 Technical University of Mombasa – ECE 2414

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Where F = the factor of safety applied to the ultimate bearing capacity qa = the allowable bearing capacity Equation 1.44 is applicable when the column loads do not vary in magnitude by more than 20%. The column loads should also be uniformly spaced. The design of the raft follows the following basic steps a) Compute the maximum column and wall loads b) Determine the line of action of the resultant of all the loads c) Determine the pressure distribution using Equation 1.46. Figure 1.26 shows the arrangement of the columns and the eccentricities with respect to x and y axis.

 ( x, y ) 

P P * e y * y P * e x * x   A Ix Iy

1. 13

Where ∑P=total loads on the raft A = Total area of the raft x, y =Coordinates of any point on the x and y axis ing through the centroid of the raft Ix and I y = moment of inertia of the area of the raft with respect to the x and y axis respectively ex and ey = the eccentricities of the resultant force in the x and y direction It is conventional to obtain the pressures at the four corners and then interpolate in between to enable the determination of moments and shears for the structural design of the raft y P1

My

PP2 2 ex

P3 ex

∑P

B

Mx x

ey ey

P4 P5

P6 P9

P7 P8

L

Figure 1. 10 Raft foundation plan showing column loads 1.3.2 General consideration in the selection of the foundation depth Once the geometry of the foundation of the foundation has been found, it is necessary to determine an appropriate depth of the foundation. The following are general considerations which the designers should take into consideration.


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b) c)

d)

e)

f)

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Usually the foundation should be placed below the depth with minimum moisture variation over the years. This eliminates the shrinkage and collapse effects of the foundation soil. In this country a depth of between 1.0 and 1.5 metres is usually sufficient. The foundation should be placed below top soil and below depths with roots of tress. The roots are potential water paths which weaken the foundations. The foundations should be sited with due consideration to existing nearby structures. The exaction of the foundation in the vicinity of the existing structures could lead to loss of lateral of the neighboring structures. Special attention should be taken to foundations ed on expansive soils and those on loose sandy silts which are likely to be saturated during the lifetime of the structure. For water structures viz: - river bridges it is necessary to take extra care to ensure that scouring of the foundation vicinity does not impair the safety of the foundation. It is usual to use gabions in areas where scouring is likely to erode the foundations such as downstream of box culverts and around abutments and pier foundations It is preferable to place foundations at one level throughout. None the less if it is not practical to have the foundations at one level, the change of level should be at one plane. Sloping foundation levels should be completely avoided even if they are on rock. There is a risk of the foundation sliding.

1.3.3 Foundations for common buildings This section deals with foundations for ordinary common buildings. These are single and double storied buildings with structural walls as the main form of . The spans should generally not be bigger than six metres. The buildings are generally on good bearing soils. The bearing soils include red coffee soils, gravelly soils and firm sandy, gravelly clays. The footing for these common buildings is shown on Figure 1.27. The 600 mm width is a practical width which allows masons to maneuver into the trench.

200-150 mm masonry wall

thick 100mm slab with BRC no 65 at the top face

200-150 DPC Damp proof membrane 150 mm minimum drop dropasountonsd

100-200 mm thick hardcore

A minimum of 1000 mm depth of foundations 600mm wide x 200mm deep mass concrete foundation

Figure 1. 11 Typical strip footing for an ordinary building


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The following are the general considerations in the usage of the standard footing. a) No reinforcement is needed for strips where the load can be distributed through 45o. b) The foundations should be excavated and the last 150mm excavation be finalized when the concreting can be done without further delay. This minimizes the softening of the foundation c) The mass concrete is in mass concrete usually by volume batching to achieve grade 15 concrete. A ratio of 1:3:6 for cement sand and ballast respectively is generally sufficient. d) Reinforced concrete foundations are done for areas with concentrated loads. These are usually column s. Grade 25 concrete is the lowest class of concrete allowed in the new BS 8110, but grade 20 of concrete can be considered.

1.4

Foundations on difficult soils

1.4.1 Foundations on expansive clays Introduction The problems associated with expansive soils arise as a result of alternate heaving and shrinkage of the clays. These soils are typically black or grey and are referred to as black cotton soils in this country. The cycle of expansion and shrinkage is a result of ability of the clays to take in water and retain it in its clay structure. The water absorption leads to expansion of the clay and causes strains in the foundation and the structures ed thereupon. The strains eventually cause the cracks to appear on the walls. The result is structural safety and aesthetics of the buildings are compromised The clay minerals include montmorillonite, illite and kaolinite as discussed in FCE 311. The montmorillonite clay mineral is particularly prone to heaving and shrinkage. Soil having more than 20% of montmorillonite are particularly prone to swelling problems In addition to visual identification the expansive soils can be identified by assessing the swell potential of the soils. This is done by conducting an odometer test which measures the free swell and the swell pressure attained in an odometer when a sample held in an odometer ring is kept at the same volume as swelling is induced by allowing the sample to take in water. Some of the Nairobi black cotton soils have been found to have a swell pressure of up to 350 kN/m2. Chen ( ) has related swell potential to plasticity index as shown on Table 1.2. The following methods can be applied to mitigate damage control a) b) c)

Moisture control Soil stabilization Structural measures


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Table 1.2 Relationship of swelling potential and plasticity Swelling Potential Plasticity index (PI) Low 0-15 Medium 10-35 High 20-55 Very High Over 55 Source (Chen, ) Moisture control The main course of heave and shrinkage is the fluctuations of moisture under and around the structures in question. Depending on the topographical, geological and weather conditions the natural ground water fluctuates during the year. This seasonal fluctuation decreases with depth. In some areas the depth to the fluation zone is as low as 1.5 meters. In other areas it will be deeper going down to over three meters. In addition to the ground water fluctuation the surface water from rains or bust pipes seeps into the foundations and course moisture migration. A satisfactory solution to the problem would to devise an economical way of stabilizing the soil moisture under and around the structure. It does not matter whether the moisture is maintained high or low in so far as it can be maintained throughout the year. An effective procedure of achieving this is to provide a water tight apron of approximately one metre round the building. A subsurface drain one metre round the building is provided with augur holes provided at every 2 meters. The holes are filled with sand and interconnected at the top. In effect the augur drain is and the impervious apron ensures that the moisture at the foundation area remains the same. Figure 1. 12 shows such an arrangement of the drains for ensures that the moisture content of the foundations remain the same The subsurface drain is used to intercept the gravity flow, or; perched water of free water to lower ground. It also arrests capillary moisture water movement. The subsurface drain should be lend to a positive outlet. In general the ground surface around the building should be graded so that surface water will flow away from the building foundations all h the time.


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Positive drain to outfall away from the building

Building

a)

Location of sand drain around a building

Ground floor with double mesh A142 Masonry walling

Original level

ground

2 meter wide water tight apron Compacted granular material at high water content

Coarse sand drains at 2 metre intervals

Expansive soil

b) Sand drain and apron detail Figure 1. 12 Typical sand drain treatment of a building

Soil stabilization Soil stabilization consists of one of the following operations (a) Pre-wetting or flooding the in-situ soil to achieve swelling prior to construction. (b) Compaction control (c) Soil replacement (d) Chemical stabilization Pre-wetting or flooding the in-situ soil to achieve swelling prior to construction involves the flooding of the site under consideration prior to construction. The soil would heave and the Technical University of Mombasa – ECE 2414

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potential danger of cracking is eliminated. Pre-wetting has been used with success when the active zones are not large. It is very difficult to saturate high plasticity clays. There is danger that expansion of the clays could continue after the construction has taken place. This procedure should be considered for stabilizing pavement or canal linings. In only rare cases should the method be considered for use below ground floor slabs. Its application below building foundations is risky and questionable. Compaction control has been used in pavement construction. Expansive clays expand very little when compacted at low densities and high moisture contents. But will expand considerably when compacted to high densities at low moisture contents. The approach is to compact swelling clays at moisture contents slightly above their natural moisture content for good result. In this method it is not necessary to introduce large amounts of water into the soil. Dry compaction of expansive soils was done along the Lodwar-Kakuma road. Soil replacement is the simplest an easiest solution for slabs and footings founded on expansive soils. The expansive foundation soils are replaced with non-heaving materials. The method requires the selection of the replacement material and the depth to replacement. In Nairobi the depth of the expansive black cotton soils is in the region of 1.0 to 1.5 metres. In this case it has been found desirable to remove the entire expansive soil below buildings and replace with suitable granular material. When the expansive soil is deeper building slabs can be constructed above the compacted soil covering the expansive soil but the foundation of main structure needs further consideration. This method is particularly useful for the construction of highway pavement in a site completely overlaid with expansive soils where the alternative to reroute the road is not viable. In this case it the lower expansive soils are overlaid with the compacted replaced material to a depth of 1.5 metres. Chemical stabilization is the process of mixing additives like cement and lime to expansive soil to alter its chemical structure and in the process retard its potential expansiveness. Lime reduces the plasticity of the soil and hence its swelling potential. The amounts used range from two to eight percent by weight. Cement on the other hand reduces the liquid limit, plasticity and potential volume change. Stabilization has been used mainly in highway and airport construction. Structural measures include several methods have been reported in literature such methods include (a) (b) (c)

Floating foundation Reinforcement of brick walls Foundation on piles

Floating foundation concept is a providing a stiffened foundation. This is essentially a slab on ground foundation with the main ing beams resting on non-cohesive non heaving material. The slabs are designed fixed on the beams that assuming a heave pressure of 20 Technical University of Mombasa – ECE 2414

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kN/m2. This magnitude is small considering that the swell pressure of the expansive soils commonly found in Kenya has been estimated at between 300 and 500 kN/m2. Results of such an approach have been mixed where they have been tried. This method needs further research. Reinforcement of brick walls have been tried in South Africa. In this method reinforcement is placed in brick walls. The reinforcement is placed where cracking usually takes place. This is typically above and below openings. The structure is made also semi flexible by providing ts in the brickwork so that when heave takes place the building will conform to the new ground shape and consequently reduce the bending moment induced in the walls. The ts are typically 1.5cm. Foundation on piles is a very successful procedure which ignores the heave by placing the footing to a sufficient depth (Figure ). The depth of the pile should leave an expansion zone between the ground and the building to allow the soil to swell without causing detrimental effect to the building. One way of installing the piles is to provide a pile with bell at the bottom. The bell or under reamed section should be well below the active zone. The bell is installed with special equipment and anchors the pile into the ground. The pile can be installed in an oversize shaft which is subsequently filled with straw saw dust as filler to eliminate uplifting of the pile by heaving soil. Alternatively the pile could be a straight and the effect of the uplift calculated using Equation 1.47 The friction below the active zone is utilized in the calculation of the bearing capacity of the pile. 1. 14 Where

= the total uplift D = the diameter of the pile h = the depth of the pile in the active zone u = the swelling pressure f = the coefficient of friction between the pile and the soil f may be taken as 0.15 while the swelling ;pressure varies between 250 and 500 kN/m2

Straight pile Figure 1. 13 Pile systems for expansive soils Technical University of Mombasa – ECE 2414

Under ream pile

Uplift Skin friction

Stable zone Active Stable zone Zone

Skin friction

Uplift

Beam

Active Stable zone zone

Beam

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1.4.2 Foundations on loose sands Foundations on loose sands are particularly difficult due to the likelihood of collapse in the event of large storms. The storms result in the realignment of the sand particles and consequent settlement due to repacking of the sand . This has resulted in large cracks in buildings which have been placed on this type of foundation soils. The foundation soils subsequently loose there bearing capacity and the result is settlement of the foundations. The superstructure has to absorb the settlement usually with resultant cracks of walls and structural elements. A real case story is one of the Garissa teachers college whose buildings were placed on sand strata. The area is generally dry but when the rain comes, it usually very heavy and comes in large storms. The performance of the three building types of structures adopted at Garissa teachers college forms a case study whose findings are used to suggest a construction procedure for foundations and masonry superstructures on loose sands. The main teaching bungalow consisted of buildings constructed with a ground beam which was framed with columns and a concrete roof slab. The masonry was thus reinforced at the corners with columns and subsequently bound at he top by a ring beam and at the bottom with a ground beam. These types of buildings were found to have performed well several years after construction. This type of construction produced a satisfactory type of constructed and when the buildings were inspected ten years after construction the structural frames and the infill masonry walls were performing well. The second type of buildings consisted of three and four and three storied flats. As in the case of the previous buildings these types of buildings were found to have performed well ten years after construction The third type of the buildings was the staff residential bungalows. These were constructed with a ground beam and masonry walls. The roof of the buildings was a concrete slab. However as the rains came and went in there stormy characteristics the residential houses developed cracks in the walls. The cracks were particularly severe in the external walls and after about 10 years of service and needed attention (Plate 1.1 Based on the satisfactory behavior of the framed structures it was found prudent to introduce columns at the masonry wall corners in a repair scheme. Plate … It is therefore recommended for foundations on loose sands the masonry should be reinforced with columns at the corners. In addition the foundations should be kept as far as is possible free from percolating water. In this way the in the event of settlement the frame will be able to absolve the stressed attributable to additional settlement and reduce the severity of the cracks.


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Plate 1.1 Cracks in the walls occasioned by settlement of the foundation

Plate 1.2 Introduction of columns to stiffen the walls

1.5

1)

Tutorial examples on chapter one

You are responsible for the design of a combined footing to two columns as shown in the figure below. The vertical dead loads on column A and B are 500 and 1400KN respectively. The design requires that the resultant of the column loads acts through the centroid of the footing. In addition the dead loads, columns A and B also can carry vertical live loads of up to 800 and 1200 KN respectively. The live loads vary with time, and thus may be present some days and absent other days. In addition the live load on each column is independent of that on the other column. Check that the design meets all eccentricity requirements if the worst possible combination of live loads is imposed


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2) A column is carrying a load of 1200kN. The column is located 300mm form the boundary of wall. Calculate the pressure distribution if the column is founded on a square base of 1500mm x1500mm. is the foundation safe if the allowable bearing pressure is estimated at 300kN/m2 3) An internal column is carrying a load of 2400kN. It is located 3000mm from the column described in Question 1 Design:a. a suitable combined base for the two columns b. A suitable strap footing for the two columns 4) Your client acquires the next plot and you are not limited by the boundary wall. Calculate the safe bearing pressure below the columns described in questions 1 and 2. Assume a detailed site investigation has established the following strength parameters. C’ = 10kN/m2, φ’ =20o, γsat = 18 kN/m2, γb= 16 kN/m2, 4

Four columns are carrying a tower. If the columns are on a square grid of 2.5mssquare, calculate the pressure at each of the four column positions if a raft foundation of 3 mmx3m is designed to carry the foundation loads estimated at 4000kN, 5000kN, 6000kN and 7000kN


Chapter two: Deep Foundations

Deep foundation can be categorized into three major types. These include i. ii. iii.

Pile foundations Drilled piers Caisson foundations.

The ground and structural conditions which require the use of the two types are discussed under each of the sections dealing with the two types of the foundations.

2.1

Pile foundations

2.1.1 Introduction Pile foundations are structural used to transmit surface loads to lower levels in the soil mass. They are used when soil beneath the level at an appropriate raft or conventional footing is too weak or too compressible to provide adequate to the structure load. The piles have small cross-section area compared to their lengths. The pile materials generally include timber, steel or concrete. The transfer is by vertical distribution of load along the pile surface and at the pile end point. Piles may be used in the following circumstances a) To transfer loads to a suitable bearing layer when weak strata is ignored and the load is transferred to an overlying strong bedrock or compact layer. b) To transfer load through the shaft friction when compact layer is very deep and would be impractical to reach it c) To structures over water where conventional exaction and construction of the foundation is not possible or very expensive to achieve. d) To reduce settlement and in particular differential settlement e) Based on cost. It might prove economical to drive piles down the strata and then build on top of the piles instead of having to excavate deep layers and then construct ordinary foundations f) In structures which have considerable uplift, horizontal and/or inclined forces. This is especially true for marine and harbor works. g) To increase the bearing capacity by vibration and compaction of granular layers of soil. h) In soils where deep excavations would result in damage of existing buildings. Technical University of Mombasa - ECE 2414

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Piles can be distinguished by the function they are intended to perform or by the material and construction procedures used in their construction. The various types of piles by function are shown on Figure 2.1. The main function of the piles is to take the loads by end bearing or by friction or by combination of the two. Other functions exist and two which can be sited here include tension piles and fender piles. The tension piles take lateral forces in place of traditional retaining walls while fender piles also referred to as dolphin piles are marine structures principally for taking horizontal loads from vessels in the docking areas. Section 2.2 is presentation of piles by their material and construction procedures.

Soft soil Soft soil

Soft soil Friction resistance

Firm strata

Hard strata

End bearing pile

Friction pile

Combination

Impact from floating object

Tension resistance

Tension pile

Dolphin or fender pile

Figure 2. 1 Types of piles by function 2.1.2 Classification of Piles by materials and construction Piles are constructed in a variety of properties of materials, construction methods and functions. This makes as simple classification difficult. Notwithstanding theses difficulties they are classified in accordance with the pile materials and method of construction (Figure 2.2). This classification also identifies the pile materials. The principal timber materials are timber, concrete and steel.


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Types of piles Driven piles Large displacement Preformed. Solid or hollow tubes closed at the end and left in position

Solid Pre-cast concrete or Timber. Formed to required lengths as units with mechanical

a) H and pipe piles

Cast in place formed by driving closed tubular sections and then filling the void as the tube is withdrawn

Bored piles Small displacement

Replacement

Steel sections H Piles Open ended tubes unless a plug forms during driving

A void is formed by excavation. the void is filled with concrete sides may be ed or uned

Hollow Steel or concrete tubes closed at the bottom. Filled or unfilled after driving

b) RC Precast pile

The ing may effected permanently by casing or Temporarily by casing or drilling mud (Betonite) or By soil on a continuous auger

c) Shell Pile

d) Cast in-situ tube withdrawn

e) Bored pile

Figure 2.2 Principal Types of piles

2.1.3 Driven piles To install prefabricated and some form of cast in place piles it is necessary to displace soil by driving the piles. The piling is commonly done by means of a hammer. The hammer operates between guides or leads by use of lifting cranes. The leads are carried by the cranes such that they can drive vertical or raking piles. The piling assembly may be mounted on base suitable for operation on land or on a floating pontoon in the case of piling in the sea. Technical University of Mombasa – ECE 2414

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The hammers may be free falling operated by a clutch release mechanism. Alternatively they are powered by diesel or steam. There are several forms of mechanical devices and equipment in the market used by piling contractors. In order to reduce the impact stresses on the hammer and the piles it is normal to strike the pile through a hammer cushion. The elements of cushion vary but are mainly wood packing in a steel cap or dolly. The various elements in the cushion not only protect the top of the pile but have a significant influence on the stress waves developed in the pile during the driving. The rating of a hammer is based on the gross energy per blow. For a drop hammer the rated energy is the product of the hammer and the height of fall. The efficiency of the hammer is the defined as the energy delivered at impact divided by the gross rated energy. Energy having been lost in the dropping of the hammer to pile. For driving piles to great length the hammers have energies of between of between 50kNm to over 180kNm. Piles are installed by impact hammers and driven to a resistance measured by number of blows required in the final stages of piling. For wood piles the energy would be limited to about 3 to 4 blows per inch when energy of 15kNm is applied by the hammer. If the pile is to be driven through heaving strata then, it might be necessary to predrill the borehole where the pile is to be driven. This eliminates undesirable heaving. Additionally if the pile is to be driven through dense layers of sand and gravel it is possible to loosen the hard strata by sending a stream of water jet with specially adapted equipment. The various types of driven piles are now described. Timber Piles Timber piles are made of trunks of timber. The timber should be preserved to prevent decay. Untreated timber embedded below the ground water table has a long life. If the timber is exposed to alternating wetting and drying it is subject to decay. These types of piles are not very common. Steel Piles Steel piles (Figure 2.2a) are usually in form of H-Piles and pipe piles. H piles are preferred where high depth is required while the pipe piles are usually filled with concrete after driving. In the case of H-Piles the flanges and the web are equal thickness in order to withstand large impact forces. Steel H piles penetrate the ground more readily than other pile types because of the relatively small cross-section area. They are subsequently used to reach stronger bearing stratum at great depth. Steel H piles have also relatively large bearing capacity of between 500 and 2,000 kN per pile depending on the size of the H section. The pile H sections are usually 250x250 to 350x350 with varying section thickness. Pipe piles are of the range of 250mm to 750 mm diameter. The wall thickness is usually over 2.54mm. In the event that the wall thickness is less than 4.54mm the pile has to driven with a mandrel. When the thickness of wall is over 2.5mm the pipe acts with any concrete in carrying the load. Pipe piles are usually driven with the lower end closed with a plate. In some instances conical driving shoes have been attached. The advantage is not significant. Steel piles are subjected to corrosion. The corrosion is minimal when the entire pile is embedded in natural soil. However, the corrosion can significantly increase in the event of Technical University of Mombasa – ECE 2414

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Shallow foundations

entrapped oxygen. Zones of water table variation are particularly vulnerable. Severe attacks are encountered on sea structural sections exposed to high and low water tides where the salt sprays can significantly cause corrosion. The standard practice is to use piles which have a factory applied epoxy coating. The most vulnerable sections of the piles should be encased in concrete. Hard driving and driving through obstructions causes the piles to twist and bend. They can easily go out of plumb without the piling team recognizing since the depth is at depth. Deviations from the vertical of below 10% are usually accepted. A penetration of 2 to 2.5mm per blow should be considered as refusal and further driving would generally cause deterioration. Pre-cast Concrete Piles Pre-cast Concrete Piles (Figure 2.2b) are usually cast in a casting yard and transported to the construction site. Where hard driving is expected the tip of the pile is fitted with a driving shoe. They are usually of square or octagonal section. The reinforcement is necessary within the pile to withstand both handling and driving stresses. It is necessary that the exact length to be installed be determined accurately. If the required length is underestimated, the extension can be done only with a lot of difficulties. If the length provided proves to be longer than needed at the site, the piles have to be cut again with a lot of difficulties. Pre-stressed concrete piles are used and generally have less reinforcement. The prestressing reduces the incidence of tension cracking during handling and driving. The difficulties related to the pre-cast concrete piles also apply to the pre-stressed concrete piles Pre-cast concrete piles have relatively large bearing capacity of between 800 and 2,000 kN per pile. The presence of high concentrations of magnesium or sodium sulphate in the piled environments causes the piles to deteriorate. The deterioration is in the form of rust in the reinforcement, cracking and spalling. The best practice is dense concrete of high quality or the use of pre-stressed piles which are not so much susceptible because tension cracks are minimized. Driven cast in place piles Driven cast in place fall in two categories namely case or uncased type. In the cased type also known as shell the shell type a corrugated steel or pipe which is driven into the ground. The driving is terminated when the desired length of the pile has been achieved. The concrete is poured in the shell and left place. In the shell is then left in place. Figure 2.3 shows the schematic installation of a shell type pile.


- 27-

Shallow foundations (1) RC shells threaded on mandrel and set in position (2) Pile driven to the required set (3) Mandrel is withdrawn and top shells above the top of the pile are removed. A cage of reinforcement is introduced (4) Core concrete is inserted

(1)

(2)

`

(3)

(4)

Figure 2.3 Shell type of pile In the uncased type a steel tube is driven into the ground and tube is withdrawn upon concreting. Figure 2.4 shows the schematic installation of a typical driven cast in situ pile where the casing is withdrawn. The pile illustrated is also known as a Franki pile. (1)

(2) (3) (4) (5)

(1)

(2)

(3)

(4)

A gravel pug is compacted at the lower end of the pile tube Pile driven to the required set Plug broken and a concrete plug is formed Core concrete is inserted Tube is withdrawn as concrete is placed

(5)

Figure 2.4 Installation of a Franki pile Difficulties encountered in the installation of driven piles The installation of driven piles has difficulties due to various factors incidental to the installation procedures and to the ground encountered at the sites. These difficulties are varied but the main ones include:a)

Handling of the preformed sections which could lead to damage of the piles before installation.


- 28b) c)

d)

Shallow foundations

Noise arising from the hammer dropping on to the pile. This can be particularly undesirable in sites in the busy neighborhoods. Spoiling of the pile in the driving operations include the spoiling of pile heads and or pile toes. This usually takes place due to overdriving piles when refusal has been reached. It is usually sufficient to achieve a penetration of 2-2.5 mm per blow in the last stages of piling. Piles of small cross-section especially H piles driven in boulderly strata could easily alignment. Vertical piles could end up having bent up shapes and hence lose their carrying capacity.

2.1.4 Bored piles Bored piles are also known as cast in place concrete piles (Figures 2.2c-e. The borehole is effected by various methods using piling equipment. The bore is ed by casing or by drilling mud (bentonite suspension). At the required depth boring is stopped and the hole is filled with concrete. If required a cage of reinforcement is placed before concreting is done. With the use of bored piles larger diameter piles have been installed with corresponding high bearing capacities. They are constructed in diameters ranging from 300mm to as high as 2400mm. They have been performed to depths of 70 metres and below and can be constructed vertically or in rakes of up to 1:4. They are thus ideal for many site conditions. The construction sequence of bored piles depends on the method of construction adopted. The main construction methods include bored piles with casing and bored piles with bentonite . Bored piles with casing In this type of pile the casing is advanced by a crane and a casing oscillator. The material below the casing area is excavated and brought up for examination and testing where necessary. After the depth needed has been achieved the reinforcement cage is inserted followed by concreting as shown on Figure 2.5

Bored piles with bentonite In this type of pile a lead casing is advanced into the soil. The material below the casing area is excavated and brought up by use of drilling equipment with a bucket which can bail out the drilled soil. The excavated soil is examined and tested where possible. The drilled hole is ed by drilling mud After the depth needed has been achieved the reinforcement cage is inserted followed by concreting as shown on Figure 2.5


- 29-

Shallow foundations

Install casing using an oscillator

Advance the casing and excavate with grab

Insert reinforcement cage

Place concrete with a tremie pipe as casing is withdrawn

Complete pile

This installation is particularly desirable in gravelly and boulderly conditions

a)

With casing

Install starter casing

Advance into the soil by drilling and ing with bentonite

Insert reinforcement cage

Place concrete with a tremie pipe and recycle bentonite

Complete pile

This installation is suitable in all soils

b)

With betonite

Figure 2.5 Installation of a bored pile with drilling mud Difficulties encountered in the installation of bored piles The difficulties associated with the installation of bored piles are also varied but the main ones include:i.

Poor base preparation after the bearing strata has been reached. Loose particles will have reached the bottom of the bore and will be difficult to detect or remove. The base the pile will consequently have a lower bearing capacity than would have been expected


- 30ii.

iii.

Shallow foundations

Poor concreting control where the pile is being cast under artesian conditions. This usually results from poor shaft control as the concreting continues. The result is necking of the concrete and/or washout of various sections of the pile. Under ideal conditions the concreter under tremie conditions should always be placed inside the wet concrete. Vibration and movement of the ground in the vicinity of the pile under construction.

It is to be noted that these difficulties are also present in the driven cast in place piles where the casing is withdrawn as concreting proceeds

2.1.5 Determination of pile load carrying capacity Determination of load carrying capacity by soil mechanics Pile design is preceded by extensive site investigation to establish the geotechnical properties of the soil where the piles will be installed. The parameters obtained in the investigations are then used in the estimation of the load carrying capacity of the piles. Piles derive their capacity from base resistance and from side friction. The ultimate load that can be carried by a pile is then given by Equation 2.1. The are explained in Figure 2.6. The accuracy of the equation depends on the determination of the parameters used in the determination of Qb and Qs.

Where = Ultimate Load carrying capacity of the pile Ultimate Load carrying capacity of the base of the pile = Ultimate Load carrying capacity of the pile side friction 2. 1 Where Ab= Area of the pile at the toe of the pile qf = Ultimate bearing capacity at the toe of the pile = Surface area of the pile shaft = Ultimate shearing resistance of the shaft of the pile generally referred to as the shaft friction An appropriate factor of safety is applied to the ultimate load. It is prudent to apply different values for the base and the side friction. This is primarily because the movement needed to mobilize the friction resistance is much less than the movement needed to mobilize the base resistance. Initially as the pile is loaded the load is taken by the side friction and as load is increased the base takes more load. At failure the proportion of load ed by friction may actually decrease slightly due to plastic flow of the soil near the base of the pile. Equation 2.2 shows the allowable load when allowing for a factor of safety of 2 and 3 for side friction and base resistance respectively. Technical University of Mombasa – ECE 2414

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Shallow foundations

2.2

Qs

Qb

Figure 2.6 Load distribution of load on a pile Cohesive soils Base resistance: The base resistance Qb of piles in cohesive soils is based on the bearing capacity factor Nc .

2. 3 Where = bearing capacity factor which is usually taken as 9.0 = undisturbed un-drained shear strength of the soil at the base of the pile = the cross section area of the pile at the base In the case of driven piles the clay adjacent to the pile is displaced both laterally and vertically. Upward movement of the clay results in heave of the ground around the pile and can cause reduction of the bearing capacity of the pile. The clay in the vicinity of the pile is completely remolded during driving. Excess pore water pressures are set up during driving. This pore pressure dissipates in a few months and in any case before significant load is applied to the pile In the case of bored pile, the clay area around the pile will be remolded. Additionally as the water seeps towards the created borehole their softening of the soil in the vicinity of the pile. Water can also be absolved from the wet concrete when it comes in with the clay. The upshot of this is and subsequent reduction of the pile bearing capacity. Side resistance is based on the friction mobilized on the surface of the pile. Equation 2.4 and 2.5 shows the estimation of the side friction


- 32-

Shallow foundations

̅̅̅

2. 4

̅̅̅

2. 5

Where = adhesion factor between the pile and the soil ̅̅̅ = the average undisturbed shear strength of soil ading the pile = the shaft area which contributes to the friction resistance Most of the load of a pile installed in a clay soil is derived from the shaft friction and the problem usually revolves accurate determination of the value of α. For soft clays driving of piles tend to increase strength around the pile. A value of α equal to 1 can be used. It is however unlikely that the soil will not in the long run return to its original soft status after some time. In over-consolidated clays the value varies from 0.3 to 0.6 (Smith and Smith, 1998). A value of 0.45 is usually used for design purposes. An alternative is approach is to express skin friction in of effective stress. The rationale of this approach is that the area of disturbance during pile installation is relatively small. The excess pore water pressure induced in the installation process dissipates ahead of the application of load. ̅̅̅

́

2. 6

Where Ks = the average coefficient of earth pressure and ̅̅̅ = the average effective overburden pressure adjacent to the pile shaft ́ = the angle of internal friction of the remolded clay. The cohesion intercept of remolded clay in an drained triaxial test being zero. Cohesionless soils Base resistance: The ultimate bearing load carried by a pile depends mainly on the relative density of the sand in which it is driven. The ultimate bearing capacity at the base of the pile is given by ̀ Where = The bearing capacity coefficient. ̀ = The effective overburden pressure at the base of the pile It is to be noted that the bearing capacity attributable to Nγ usually ignored in pile design as the value of B is usually small. The values suggested by Berezantzv et al (1961) are often used and are shown on Figure


Value of Nq

- 33-

Shallow foundations

100 Nq 10 25

35

45

φ in Degrees Figure 2.7 Bearing capacity factors for use in pile design Source Berezantzv et al 1961

Side friction: Meyerhof (1959) suggested the average value of friction to be estimated from Equation 2.6. As can be seen from the Equation the value of fs continues to increase as the effective overburden increase. However field tests have shown that the maximum value of fs occurs when the embedded length of the pile is between ten and twenty diameters. In practice a maximum value of 100 kN/m2 of fs is taken. ̅̅̅

2.7

Where Ks = the average coefficient of earth pressure and ̅̅̅ = the average effective overburden pressure adjacent to the pile shaft = the angle of internal friction between the soil and the pile. Typical values of and Ks are given on Table 2.1 after Smith and Smith (1998) are shown on Table 2.1. The ultimate load that can be carried by the pile is therefore given by Equation 2.7. Table 2.1 Typical values of

and Ks

Pile material Steel Concrete Wood

o

20 0.75φ 0.67 φ

Ks Loose 0.5 1.0 1.5

Dense 1 2.0 4.0

Source Smith and Smith (1998) ̀ ̅̅̅

2.8

Equation 2.8 shows the allowable load when allowing for a factor of safety of 2 and 3 for side friction and base resistance respectively. ̀

̅̅̅̅


2.9

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Shallow foundations

Determination of piling parameters from in-situ tests The above equations pose difficulties with respect to determination of parameters for a cohesionless soil which is difficult to sample in the field in undisturbed condition for accurate determination of Nq which depends on the internal angle of friction. The value of the angle of internal friction between the soil and the pile remains at best an estimate. Consequently it has been found preferable to use empirical correlations based on the results of standard penetration and those of the Dutch cone penetration equipment. Meyerhof (1976) proposed the values given on Table below.

Table 2.2 Pilling parameters from standard penetration tests Driven piles Type of soil Sands and gravels

qb (kN/m2)

Non plastic silts Bored piles Any types of soils

fs (kN/m2) Large diameter - ̅ Average diameter - ̅ Large diameter - ̅ Average diameter - ̅ 0.67 ̅

Source Smith and Smith (1998) Where N = the uncorrected blow count at the base of the pile ̅ = the average uncorrected value of the blows over the embedded length of the pile D = is the embedded length of the pile in the bearing stratum B = the width or the diameter of the pile. An alternative to the use of the Standard Penetration tests is to use the Dutch cone test results. The cone penetration results can be seen in Figure 2.8. The ultimate base resistance is taken as average value of Cr over a depth of 4d as shown on Figure 2.8. The ultimate skin friction can be obtained from Table 2.3.


- 35-

Shallow foundations

Estimated of the pile

Depth (m)

depth

Cr (kN/m2)

3d

d

Figure 2.8 Typical results from a Dutch Cone Test

Table 2.3 Skin friction (fs) values from Dutch cone test results Type of pile fs kN/m2 ̅̅̅ Driven piles in dense sand Driven piles in loose sand

̅̅̅

Driven piles in non plastic silts

̅̅̅

Where ̅̅̅ is the cone resistance along the embedded length of the pile The allowable bearing load of the pile as before based on the Dutch Cone Test results is given by Equation 2.9 2.10

2.1.6 Determination of load carrying capacity dynamic methods Determination of load carrying capacity dynamic methods is applicable to driven piles. The basis of derivation of dynamic formula is that a relationship exists between the pile capacity and the driving behavior during the last stages of driving. The energy from the hammer to the pile is transformed into useful energy and can be represented by Equation 2.10 in the last stages of the pile driving 2. 11 Where


- 36-

Shallow foundations

M = the mass of the hammer g = the acceleration of the hammer h = the drop the hammer R = the pile capacity S = the settlement of the hammer as result of the drop h In practice the above Equation has been modified to take of several losses which take place during the driving process. The main losses of energy occur as a result of sound, heat, friction, quake, losses associated with elastic behavior of the pile and those associated with the pile head compression. The net energy is equated to the work done in penetrating the ground by the pile. Figure 2.9 shows the sequence of the pile driving and the

Wh efWh

efeivWh

h

a) Variation of energy upon falling of hammer on to a driven pile

Permanent +Elastic penetration (sso + spp) +(sep +ses) (sso + spp =set =s) (sep +ses )=c)

(sso+ses) (sso) (ses) b) Penetration of pile upon falling of hammer on to a driven pile Figure 2.9 Energy and penetration of a pile during driving The potential energy of the hammer is Wh. Upon with the pile the available energy to drive the pile into the ground is ef.eiv.Wh, where ef is the efficiency upon falling and eiv is the efficiency upon impact. The penetration of the pile as shown on Figure 2.9b can be shown to result in permanent ;penetration attributable to the pile and soil spp aand sso. In addition there will be elastic penetration sep and ses attributable to the pile and soil respectively. The work done and the pile resistance equation can now be rewritten as shown on Equation 2.11.


- 37-

Shallow foundations

2. 12 Where R = The ultimate load capacity of the pile = the overall efficiency factor Equation 2.10 is known as Hiley formula. In the field the final stages of the pile are monitored and recorded as can be seen on Figure 2.10. It is usual to drive the piles to a minimum set of 2.5mm. Harder driving only goes to damage the toe of the pile and could reduce the pile capacity in the process. Pile driving formulas should be used in the piles driven in sand and gravel and in any case should be calibrated with a load test.

Elastic comp = c3 Elastic comp = c2

Elastic comp = c1 set = s3 set = s1

set = s2

Figure 2.10 Pile driving trace of the final stages

2.1.6 Determination of load carrying capacity pile testing The load test is the most reliable of all the methods used in the determination of load carrying capacity of a pile. In this method a full scale test is carried out on a working pile. Essentially the pile is loaded and a plot of load versus settlement is recorded. From the plot the allowable load is computed by one of the many formulas available from literature. Full scale piles are then constructed to the same specification as the test pile The test is conducted by loading the pile with kentledge load or by use of tension piles (Figure 2.11). In some piling contracts the working piles cannot be used as tension piles for testing purposes. This is primarily because in the cause of piling test the tension piles are lifted slightly. This could lead to weakening of the working piles.


- 38-

Shallow foundations

Kenteledge Kentledge

a)

Jack Test pile

Existing ground level

Load resisted by kentledge

Jack

Existing ground level

Test pile Tension pile

b)

Tension pile

Load resisted by tension piles

Figure 2.11 Methods of testing piles in the field

If the test pile is a purely test pile ahead of the main installation of the pile the maximum load to be applied is equal to two and half times the estimated safe carrying capacity of the pile. It is usual to load the pile to 1.5 times the design allowable pile load when a working pile is tested for ascertaining the integrity of the piles installed. Maintained load test The load is applied by maintaining the load in a series of increments. The increments are usually equal to 20 to 25percent of the design working load of the pile. The subsequent increments are carried out when the settlement has reduced to less than 0.25mm per hour. The load is subsequently withdrawn in the same stages as the loading to trace the unloading curve. Constant rate of penetration In this method the load is applied by a constant rate of penetration by a jack in order to maintain a constant penetration rate (Figure 2.11b). it is usual to maintain penetration rates of 1.5mm per minute and 0.75mm per minute in the case of sands and clays respectively. Interpretation of test results The results are plotted on a load settlement curve as shown on Figure 2.12. In the two procedures ultimate pile load is taken as the load which achieves a settlement equal to 10 percent the diameter of the pile as is seen in test pile a Figure 2.11b. (BS 8004). The ultimate pile load could also be reached when the shear failure of the pile soil interface or the pile toe occurs (Figure 2.12b). The allowable pile load is obtained by dividing the ultimate load by an appropriate factor of safety. The factor of safety usually ranges from 1.3 to 2.0


Shallow foundations

Load

- 39-

Settlement

Load

Settlement

Time

a) Maintained load test results

a

Ultimate load (a) b

Penetration = 0.1 pile diameter

Load

Ultimate load (b)

Penetration b) Constant rate penetration test results Figure 2.12 Pile test load results The above failure criterion is applicable to normal size piles. In the case of large diameter piles on rock the ultimate load depends on the capacity of the concrete. This depends on the stress in the concrete.

2.1.7 Negative skin friction Negative skin friction is a phenomenon or which occurs in piles when a force develops between the pile and the ading soil in a direction which increases the load on the pile and or the pile groups. This phenomenon develops when a compressible layer of clay, silt, or mud etc settles on of consolidation which may be initiated by ground water lowering or increase in overburden pressure. As clay layer settles, piles are dragged into the soil by the consolidating soil and the overburden soil. The direction of the friction is reversed increases the load on the pile. The friction generated on the perimeter of the pile due to this dragging is carried by the column instead of assisting in carrying he pile load. The effect is to reduce the carrying capacity of the pile. This is the phenomenon known as negative skin friction


- 40-

Shallow foundations

l-fill

Fill Compressible clay

l-clay

Length of settling soil=l

Figure 2.13. The negative skin friction may be estimated from Equation 212 for single piles and Equation2. For group piles

Figure 2.13 Negative skin friction 2.13 For cohesive soils fs is can be approximated to ̅ . while for cohesionless soils fs is equal to ̅ . Where the value of fs is estimated from triaxial testing for cohesive soils the fs can be taken as 0.5Cu Where = the ultimate force generated by the negative friction = the shearing resistance of the soil = length embedded above the bottom of the compressible layer = the pile diameter = the coefficient of earth pressure at rest = angle of shearing resistance in of effective stress ̅ = average effective overburden pressure

2.1.8 Pile groups In practice piles are designed and constructed to work in groups. In construction of a group a pile cap is cat on top of the piles. The cap is usually in with the soil on top of the piles. The bearing capacity of the group is an arithmetic sum of the piles and that of the cap. Banerjee (1975) showed that the pile cap could up to 60% of the applied load. If the cap is clear of the ground surface piles in the group are referred to as free standing piles. Bearing capacity of groups Except for the large diameter piles of over 700mm diameter the piles are usually designed in groups of three or more piles under a column. The minimum under a foundation wall would be two per typical cross-section. Typical arrangement of the piles is given on Figure 2.14. In general the ultimate load capacity of the pile group is not the sum of the loads of the piles in Technical University of Mombasa – ECE 2414

- 41-

Shallow foundations

the group. The ration of the ultimate load for the group to the sum of the loads carried by individual piles is the efficiency factor of the group.

3 – Pile

4 – Pile

5 – Pile

12 – Pile Figure 2.14 Typical arrangement of pile groups For piles in sand, the group action is complicated by dilatancy and densification characteristics of the sand. When the spacing of the piles is less than eight times the pile diameter, group action takes place (Department of Navy, Naval Facilities Engineering Command, 1982). In dense sand the effect of driving piles is to loosen the sand and hence the angle of internal friction of the sand in the vicinity of the piles. This results in overall reduction of the pile bearing capacity. The group efficiency factor is less than one. In loose sand the effect of driving piles is to increase the density of the sand. The bearing capacity of the loose sand will therefore be increased. In this case the efficiency factor is more than one. An efficiency factor of 1.2 is often used. In the case of bored piles in sand the resulting loosening of sand in the boring operation results in efficiency factors less than 2/3. The difficulties in the quantification of the design parameters of either loosened or densified sand strata in piling operations remains a real problem for engineers (Mwea, 1984). Nonetheless experimental evidence has it that the piles at the centre of a group in sand carry more load than the piles on the periphery. For piles in clay the effect of the pile group is to reduce the bearing capacity of the pile group. This is because the effect of placing piles in a group is to have one large block taking friction on the sides and base resistance over the block base. The spacing of piles in clay is of the order of two times the pile diameter to four times the diameter. The efficiency of the groups range from 0.6 to unity as the pile spacing increases from two diameters to four diameters. The ultimate load in the case of a pile group is given by Equation 2.13. In the case where the pile cap rests on the ground the ultimate load should be taken as the less of the block capacity or the sum of the individual piles on the group. 2. 14 Where

= The width of the group


- 42-

Shallow foundations = Length of the base of the group = Depth of the group = Bearing capacity factor of the clay = The average undrained strength of the undisturbed clay

Whitker (1957) in a series of model tests showed that block failure as a group in clays occurs when the spacing of the piles is not more than 1.5d apart. General practice is however to space the piles at between 2 and 3d. In such cases the efficiency of the group is approximately 0.7. Settlement of groups The settlement o a group of piles can be estimated by assuming that the entire load acts at a depth as an equivalent raft. In clays the raft is assumed to be located at a depth of 2/3 D where D is the depth of the pile group. The load is at spread of 1:4 from the underside of the pile cap to allow for friction transfer. After the assumed depth of the raft the load is distributed at a spread of 1:2 (Error! Reference source not found.a). Immediate settlement nd consolidation settlement can then be estimated for the layers of soil below 2/3D by application of normal methods. For groups in sand the equivalent raft is at a depth of 2/3Db from depth 2/3D. The spread from the perimeter of the piles is 1:4 followed by a spread of 1:2 Error! Reference ource not found.b). The settlement of the underlying sand stratum is then gotten from application of standard penetration data and or the cone penetration resistance

D

1:4

2/3D

2/3D

Db

1:2 Position of equivalent raft Clay stratum

1:4 1:2

2/3Db Position of equivalent raft Sand stratum

Figure 2.15 Equivalent raft concept for piles

2.2

Drilled piers and Caisson Foundations

2.2.1 Drilled piers The term drilled pier foundations is used in a number of situations which to refer to deep foundations which method of construction is fundamentally different from that of piles. A large shaft performed in soil and then filled with concrete may be termed as a drilled pier. ACI (1972) refers to all shafts where a person may enter and work as a drilled piers. In this definition all shafts larger than 750mm diameter can be referred to as drilled piers. Figure Technical University of Mombasa – ECE 2414

- 43-

Shallow foundations

*** shows typical piers used in practice. In general drilled piers are used where the soil has a low bearing capacity and it is necessary large loads to firmer stratum and the following conditions preclude the use of smaller piles. i. ii. iii.

Pile vibrations are not acceptable. Pile are too small for the loads. A large bearing end is needed for higher load capacity

Straight pier

Underreamed pier

Pier socketed Rock

Into

2.2.2 Caisson Foundations The term caisson is also used to refer to box type structures consisting of many cells built in, concrete or steel or combination of both. They are built wholly or partly at higher ground and sunk to final position. They are used to transmit large loads through water and soil to firm strata. They are used in large bridges, shore protection structures. They are generally used under the following conditions. i. ii. iii.

The soil contains large boulders which would otherwise obstruct the penetration of piles and or construction of cast in place piles. A massive substructure is needed to extend below the river bend to provide resistance against floating objects and scour. Foundation is subjected to very large lateral forces.

Caissons may be divided into three categories i. Open caissons ii. Pneumatic caissons iii. Box caissons or floating caissons Open caissons An open caisson essentially consists of a box open at the top and bottom ( Figure 2.16). the soil is removed from the caisson by grabbing, dredging from inside the caisson. The sinking of the caisson proceeds by the caissons self weight assisted by cutting edges of the walls. When the desired level has been reached concrete is poured under onto the base of the caisson by tremie pipe. In some cases the caisson has been pumped out. But in most of the cases the caisson has been left in place. The bearing capacity of the soil below is usually determined by normal bearing equations.


- 44-

Shallow foundations

The concrete seal at the bottom is placed as a plug at the bottom of the caisson but later serves as a permanent base of the caisson. Its thickness can be obtained from the equations below For circular caissons √

For rectangular caissons √

Where = thickness of the seal σo = pressure or hydrostatic pressure R = radius of the caisson in the case of circular caisson fc = the allowable concrete stress in tension (0.1 to 0.2cube strength) b= width or the short side of the caisson in the case of a rectangular caisson l= length or the long side of the caisson in the case of a rectangular caisson β = coefficient which depends on the l/b ratio

Water level Ground surface Cutting edge

Circular open caisson

Box caisson

Figure 2.16 Open Caissons Pneumatic caissons Pneumatic caissons provide an airtight enclosure (Figure 2.17). In effect water is prevented from getting into the enclosure and the workers can excavate and pour concrete under dry conditions. The reliability of the quality in this case is better in so the mechanical ventilation is carried out to the strictest of the specifications. Pneumatic caissons are costly and should be considered only with the following conditions in mind: i. ii.

pay because of associated health hazards Overall safety requirements are high


- 45iii.

Shallow foundations

Much of the effort is towards making the work environment suitable for the workers

When the excavation has reached the desired stratum the concrete is sent down to the working chamber carefully to fill any weak points on the exposed strata. After this initial filling the area is filled except a small portion of the chamber below the roof of the chamber. This final portion is filled with grout which also fills any spaces which might have been left behind during the concreting. The seal design and estimation of the bearing capacity is the same as that of the open caissons

Compressed air working chamber

in

Figure 2.17 Pneumatic caissons

Box caissons Open caissons are usually cast on the ground and then towed to the site. They area then lowered to a prepared ground. They are carefully aligned on place and then made stable by placement of ballast. The design and construction of box caissons do not bring any new design requirements. The ground upon which the caisson is being laid needs to have been exhaustively investigated to ascertain the foundation depth and any likely difficulties likely to be encountered. After the caisson is in place it may be filled with either sand concrete or sand. The caisson should be checked against stability as it is floated to the final place of the intended foundation. Design of caissons The caissons will be designed to resist vertical loads including superstructures, own weight minus buoyancy forces. The lateral forces will typically include forces due to wind, earthquake, earth and water pressures, and traction from traffic and pressure from current flow. The forces acting on a caisson must be estimated as accurately as can be to enable a safe design. There are many methods adopted by various geotechnical engineers but the for stability of the caisson the following combination of forces will suffice


- 46i. ii. iii.

Shallow foundations All forces are resolved into A single vertical force Two horizontal forces in the direction across and along the caisson.

It has been found out that analysis of the caisson in a direction transverse to the direction of the axis is more critical. From Figure ***-* the three equations of static equilibrium are solved. This are W = Base reaction + skin friction Q = ive pressure created on BF – ive pressure on DE – Base friction Q (H+D) = Moment of all the forces From structural analyses

Q

Q h

D

From geotechnical analyses

O

Qmax =Area ABC-Area FEC Qmax =1/2 γD2 (Kp - Ka)- ½*2* D (Kp - Ka)*D1 Moments about O: Qmax (H+D)=1/2 γD2 (Kp-Ka)D*1/3- ½*2* D (Kp-Ka)*D*D1*1/3


- 47-

Shallow foundations

Therefore D1 and Qmax can be calculated and necessary adjustments of the caisson are made depending on values of Kp and Ka 2.4

Examples of Piling Schemes

Sutong bridge in China Sutong bridge in China (Plate 1), which has a centre span of 1088m, designed in an area of high winds and likely to be hit with massive earthquakes (Bitener et al, 2007). The foundation strata presented the designers with particularly difficult task. The soils at the site consisted of firm to stiff clay extending to 45 metres below the sea bend. This clay strata was underlain with a medium to very dense coarse sands, silty sands and occasional loam layers matrix to a to of 250 metres below the sea bed where the basement rock was encountered. The designed pile groups covered a plan area of 113.8x48.1m. The design consisted of 2.8 and 2.5 diameter piles. Permanent casings were installed to a depth of 40 metres. The overall depth of the piles was of the region of 110 metres. The shafts were designed to mainly be carried by friction since the displacement needed to mobilize the end bearing is two to three times that needed to mobilize the skin friction The tips of the pile shafts were however grouted to increase the bearing capacity of the piles. This procedure densifies the soil below the shaft and any debris left during the drilling operations. The increased the pile capacity end bearing capacity is of the order of 20%.

Plate 1 of the Sutong Bridge in China (1088 m center span)

The Nyali bridge in Mombasa This is a pre-stressed concrete bridge founded on seabed which had coral deposits, sand and clay soils matrix proved to a depth of 100metres below the sea bend. The designers depended on the skin friction for the centre piers. The design consisted of 2.0metre diameter shafts drilled down to depth of 50 metres. On plan the piles have a rectangular layout of 3x8 piles per pier.

2.5 1)

Tutorial examples on chapter two A single pile 0.6 m diameter is bored into sand strata six meters thick overlying a clay stratum of infinite depth. Detailed investigations have established that N value in the


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2)

Shallow foundations

sand zone increases with depth (n=3Z). The undrained cohesion increases with depth (Cu = 5+4Z). Assuming the adhesion factor α = 0.35, determine a) An equation for the estimation of pile working load if the pile is to terminate in the sand zone. b) An equation for the estimation of the pile working load if the pile is to terminate in the clay zone. A precast reinforced concrete pile measured 450mm x450mm. The pile was driven to a depth of 15 metres to a set of 3mm by a drop hammer of 2.5 tones freely through 1.5 metres. The piling arrangement was changed to have a 4.2 tone hammer falling through 2 metres. Assuming the same resistance with the new hammer, determine the set achieved if the following information is also available. 2.5 tone hammer 0.5 4mm 4.5mm

Overall efficiency factor Elastic compression of pile Elastic compression of soil

4.2 tone hammer 0.35 4mm 5.0mm

3)

A pile under test has started showing considerable settlement under load of seventy tones. The pile diameter is 500mm and a length of 8.5metres in stiff clay. Assuming below the 8.5metres the clay was soft clay and did not contribute to any resistance evaluate the magnitude of the unit shear along its skin. (Answer 10.5tones per m2).

4)

A 500mm diameter bored pile is to be made in stiff clay to a depth of 20metres. The un-drained strength of the clay varies with depth as shown in the following table Depth 4 2 Cu (kN/m ) 78

6 86

8 102

142 132

16 157

20 184

24 212

Determine the maximum load that may be applied to the pile. The following factors may be taken. Adhesion factor α = 0.45 Overall factor of safety = 2 Nc for piles is usually taken as = 9 (Answer 1025kN).


Chapter Three: Retaining Walls

3.1

Introduction

Retaining walls are used to retain soils between two different elevations in areas of terrain possessing undesirable slopes or in areas where the landscape needs to be shaped severely and engineered for more specific purposes like hillside farming or roadway overes. The most important consideration in proper design and installation of retaining walls is to recognize the tendency of the retained material to move. This creates lateral earth pressure behind the wall which depends on the angle of internal friction (φ) and the cohesive strength (c) of the retained material, as well as the direction and magnitude of movement the retaining structure undergoes. Earth pressures will push the wall forward or overturn it if not properly taken into . Any groundwater behind the wall that is not dissipated by a drainage system causes hydrostatic pressure on the wall. If the wall is not designed to retain water, a proper drainage system behind the wall in order to limit the pressure to the wall's design value is needed. Drainage materials will reduce or eliminate the hydrostatic pressure and improve the stability of the material behind the wall. 3.2

Types of retaining walls

3.2.1 Gravity walls Gravity walls (Figure depend on their mass (stone, concrete or other heavy material) to resist pressure from behind and may have a 'batter' setback to improve stability by leaning back toward the retained soil. For short landscaping walls, they are often made from mortarless stone or segmental concrete units (masonry units). Dry-stacked gravity walls are somewhat flexible and do not require a rigid footing in frost areas. Tall gravity retaining walls are increasingly built as composite walls such as reinforced earth with precast facing; gabions; crib walls; or soil-nailed walls

Technical University of Mombasa - ECE 2414

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Retaining walls Road way

Road way Mass Stone Wall Reinforced earth wall

Crib Wall

Gabion Mattress Wall

Figure 1.14 Different types of retaining Walls

3.2.2 Cantilevered retaining walls Cantilevered retaining walls are made from an internal stem of steel-reinforced, cast-in-place concrete or mortared masonry (often in the shape of an inverted T). These walls consist of a cantilever stem, cantilever heel and toe For high walls in excess of eight meters deg counterfort on the back of the wall, or buttress in the front, improves their strength resisting high loads. This type of wall uses less material than a traditional high cantilever wall when designed carefully. The horizontal load is taken by spanning horizontally


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Retaining Walls

Ground Floor Ground

Floor

Original ground

Upper Upper basement

Basement Basement wall

Lower Lower Basement Basement

Bridge Abutment waalls

Basement Wall

3.2.3 Sheet pile wall Sheet pile retaining walls are usually used in soft soils. Sheet pile walls are made out of steel, vinyl or wood planks which are driven into the ground. They are usually driven 1/3 height above ground, 2/3 below ground. This however may be altered depending on the environment. Taller sheet pile walls will need a tie-back anchor, placed in the soil a distance behind the face of the wall that is tied to the wall, usually by a cable or a rod. Anchors are then placed behind the potential failure plane in the soil.

3.2.4 Bored pile Bored pile retaining walls are built by assembling a sequence of bored piles, preceded by excavating away the excess soil. Depending on the project, the bored pile retaining wall may include a series of earth anchors, reinforcing beams, soil improvement operations and


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Retaining Walls

shotcrete reinforcement layer. This construction technique tends to be employed in scenarios where sheet piling is a valid construction solution, but where the vibration or noise levels generated by a pile driver are not acceptable. 3.2.4 Anchored An anchored retaining wall can be constructed in any of the aforementioned alternatives but also includes additional strength using cables or other stays anchored in the rock or soil behind it. The anchors are driven into the material with boring; anchors are then expanded at the end of the cable, either by mechanical means or often by injecting pressurized concrete, which expands to form a bulb in the soil. Technically complex. This method is very useful where high loads are expected, or where the wall itself has to be slender and would be too weak to retain the soil

3.3

Design of retaining walls

The Design of any Retaining Wall is concerned with The stability of the retaining wall is due to its self weight and the dead weight on top of the heel. The wall is designed to obtain an acceptable factor of safety with respect to a. b. c. d.

Overall slope stability failure of the soil around the wall Overturning. Sliding. Ensuring that allowable soil bearing pressures is not exceeded at the base of the wall. This is critical at the toe of the wall

These design stability failure modes are shown on Figure

a)

Overall slope stability failure

b)

d) c)

Sliding

Figure 3.1 Retaining wall failure modes


Overturning

Overturning

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Retaining Walls

The design steps of a retaining wall

i.

a. b. c. ii. iii. iv. v.

vi.

vii.

viii.

Start with an assumed geometry of the wall. The first trial experience but the following dimensions are generally good for the start in the case of a cantilever retaining wall The footing width, to be about 0.4 to 0.7 of the height of the wall The toe projection is with 1/3 to 1/4 of the height of the wall. The footing thickness and the stem width at the footing is1/10 to 1/14H of the height of the wall. Compute overturning moments, calculated about the front (toe) bottom edge of the footing. Compute resisting moments based upon the assumed footing width, calculated about the front edge of the footing. An overturning factor of safety (resisting moments/ overturning moments) of at least 1.5 is considered safe. Check sliding. A factor of safety with respect to sliding of 1.5 is considered safe. Calculate the eccentricity of the total vertical load. Is it within or outside the middlethird of the footing width? Calculate the soil pressure at the toe and heel. If the eccentricity, e, is > B/6 (B = width of footing) it will be outside the middle third of the footing width (not recommended!), and because there cannot be tension between the footing and soil, a triangular pressure distribution will be the result. if this condition cannot be avoided, then adjust the wall dimensions Design the stem. Start at the bottom of the stem where moments and shears are highest. Then, for economy, check up the stem to determine if the bar size can be reduced or alternate bars dropped. The thickness of the stem may vary, top to bottom. The minimum top thickness for reinforced concrete walls is usually 150mm to properly place the concrete200mm at the bottom. Design footing for moments and shears.


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Retaining Walls

Example Design a cantilever retaining wall to retain earth for a height of 4 meters. The backfill is horizontal. The density of the retained soil is 18kN/m3. The safe bearing capacity is 200kN/m2. The angle of friction for the backfill is 350while that of the base is 400 i) Assumed geometry Assume a depth of foundation of 1.2m. Therefore total height is 5.2m. Total height for stability = 5.2+.32 Try 5.52m Width of the base, 4*5.52 to, 7*5.52: 2.208 to 3.864 Try 3.0m Thickness of the base 1/10 to 1/14H 0.552 to 0.392 Try 450mm Width of the toe of the base 1/3 to 1/4B 1,0m to 0,75m Try 750mm Width of the heel of the base =3-.75-.45 Try 1800mm hs = height of slope 1.8*tanβ Try 320mm Thickness of stem at base 1/10 to 1/14H 0.552 to 0.394 Try 450mm Thickness of stem at top 200 to 400mm Try 200mm Thickness of heel =3-.75-.45 Try 1800mm β=100

0.2m

0.32m W1 Pa W2 W4

4.0m β=100

H=5.52m

W3 H/3=1.84m 1.2m

W5 T .75

1.8

H

.45 B=3m

Stability analysis Note that all the loads and actions are per metre length of the retaining wall Assume that Pa is the Rankine lateral force and has two components of the vertical force and horizontal force From ECE 2406


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Retaining Walls

√ √ o

Cos 10 = 0.98 Cos 35o = 0.82 √

=

√

=

= 0.195

Stability Computations Take moments about the Toe (T) Area W1 = W2 = W3 = W4 = W5 =

Force .5*1.8*0.32 *1*18 = 5.184 1.8*4 *1*18= 129.6 0.2*4 *1*24= 19.2 ½*0.25*4 *1*24= 12 0.25*4 *1*24= 12

Lever arm 2/3*1.8+.45+.75=2.4 1/2*1.8+.45+.75=2.1 1/2*0.2+.25+.75=1.1 2/3*0.25+0.75=0.917 2/3*0.25+0.75=0.917

Moment 12.4416 272.16 21.12 11 11

Area

lever moment dimentions

W1 = W2 W3 W4 W5

0.50 1.00 1.00 0.50 1.00

1.80 1.80 0.20 0.25 1.20

0.32 4.00 4.00 4.00 1.80

density Force

Dimensions

LA

18.00= 5.18 18.00=129.60 24.00=19.20 24.00=12.00 24.00=51.84

1.20 0.90 0.10 0.17 1.50

0.75 0.75 0.75


0.45 0.45 0.45 0.75

arm Moment

2.40= 2.10= 1.30= 0.92= 1.50=

12.44 272.16 24.96 11.00 77.76

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Retaining Walls

Design steps of a restrained retaining wall i. ii. iii.

iv. v.

vi.

vii.

Start with assumed geometry of the wall Compute all applied loads Select restraint – level and base of stem design assumptions: pinned - pinned; pinned fixed; or fixed - fixed. Then based on statics determine the reactions at the top and at the base of the wall. If a floor slab is present at the top of the footing, check its adequacy to sustain this lateral sliding force. Design the stem. If the stem is assumed pinned at the base and at the top, the maximum moment will be a positive moment near mid-height—usually the same material (concrete or masonry) and thickness will be used for the full height. Some degree of fixity at the top of the wall even with a pinned Design the footing. If the stem is assumed fixed at the base check the soil pressure and design for the moments and shears. If the stem is assumed pinned at the footing interface, try to centre the footing under the wall to prevent eccentricity. Check sliding. If a restraining floor slab is not present, a key or adjusting the footing width or depth may be required

3.2.5 Examples on retaining walls A retaining wall is needed to retain a highway as shown in the figure below. Design a suitable wall if it is to a 10kN/m2 surcharge as shown. The backfill is made up of compacted granular material for density 20kN/m3 and Ø = 350 Assume an allowable bearing pressure of 300 kN/m2. The strata at the base has a Ø= 400


Chapter Four : Site Investigation

4.1

Introduction

Site investigations are also referred to as soil exploration. It consists of investigating the condition on which construction is planned. From site investigation it should be possible to obtain information for the following geotechnical engineering activities i. ii. iii. iv. v.

Design of new foundations Modification of existing foundations Location of materials of construction of roads, runways, etc Identification of materials needed for the construction of pavement structures for roads, runways etc Identification of ground to be excavated in the construction of various facilities including water pipe lines, building foundations, earthworks in cut areas etc

The site investigation should form a part of a coordinated chain of design from inception of the project through preliminary to the final detailed design of a civil engineering project. It should indeed continue post construction monitoring of the completed schemes. Because of the diversity of civil engineering schemes a set of standard procedures is not possible for all site investigations. The varying civil engineering schemes require a variety of options in breadth and detail needed for the various schemes. The objectives for which a site investigation is carried out also differ with various schemes. The main objectives of carrying out a site investigation are now presented i) Suitability of site for particular works In the case of option of site for particular works a detailed site investigation should be able to enable determination of the most suitable site. Thus it is possible to shift a bridge from one location which would call for expensive deep foundations to one where ordinary shallow foundations would be sufficient. ii) Adequate and economic design A site investigation leads to safe structures during and after construction. Additionally sufficient information is obtained for quantifying the excavations needed in the preparation of the bills of quantities. This should minimizes the possibility of cost overruns due to unexpected ground conditions being met at construction time.

University of Nairobi –FCE 511 Geotechnical Engineering IV

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Site Investigation

iii) Planning construction By identifying different materials along the construction paths and their locations a systematic procedure of carrying out the works is evolved. In the case of road works materials from the cut areas are analyzed for use in the fill areas. It is then possible to proceed with construction of the fills and cuts methodically with minimum haulages and waste of materials. iv) Prediction in changes in structure Carefully and well executed site investigations should enable the prediction of the likely settlement of structures under construction. Equally important is the ability to predict the effect of excavations on the neighboring structures. v) Safe structural design of large structures Heavy modern structures require more detailed site investigations. Today we are seeing higher buildings, larger bridges and installations sensitive to settlement. Structures and civil engineering schemes are being put up very quickly. Immediate and consolidated settlement is taking place when the works are commissioned. Further settlement takes place during the useful life of the civil engineering installation. Accurate estimation of the settlement regime is particularly important considering that clients are becoming more and more sensitive to the performance of structures and the argument that cracks are minor and do not pose any danger to the structure is no longer good.

4.1.2 Planning a site investigation Table 4.1 shows a schematic way in which various activities with respect to site investigation can be performed at various stages of a project. It is clear from the table that site investigation should not be treated as an afterthought but rather should grow with the project from conceptual initial design to eventual post construction period.


Table 4.1 Stages of a site investigation Phase Pre-construction Conceptual Stage Initial design Preliminary design Conceptual Main activity design Design Alternatives

Site investigation activity

SI Reports

Detailed design

Detailed Site Detailed investigations Desk study of SI – -Boreholes Review of existing -Trial pits etc Define Scope of data Preliminary trial Laboratory and field SI pits tests of reference and i) Preliminary SI bid documents investigation report Detailed design report ii) Cost estimate of SI -SI report

Construction Supervision construction

Post Construction of Operation Maintenance

Construction control

&

Field observations – field densities - field moisture contents -

-Performance Monitoring and checking performance – - pore water pressures Settlement Inclinations

As built SI report -

-Maintenance reports -Performance reports -Research reports


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4.2

Preliminary and detailed stage site investigations

4.2.1 Preliminary stage site investigations This should lead to information needed for the design of the various alternatives at the preliminary stage of the study. The activities in this stage can be summarized as follows: i) ii)

iii)

iv)

A study of any existing site investigation reports for the area or in the neighborhood should form the basis of this stage of investigations. A study of geographical a geological maps of the site in the case of large sites. Topographical characteristics should lead to useful information such faulty areas. Heavily forested areas are an indication of deep rooted top soils. A site inspection of the existing buildings and any existing structures. Any signs of distress which can be related to the settlement of the foundations. Any information from archives, previous records held by the local authorities. Inspection of the soil profiles, in cut areas, old used quarries. Structured questions to local people with regard to the geotechnical information being sought yields considerable information. Such questions are: a) b) c)

What is the depth of the pit latrines in the area? At what depth murram encountered? At what depth was water struck?

v)

Aerial survey of the site could give useful information with regard to land formations and soil profiles. vi) Seismic refractions could be carried out at this stage of investigations. Usually a specialist is needed to interpret the results. vii) Preliminary trial pits Geophysical methods Geophysical methods involve sending of seismic or electrical waves through the ground. The determination of the soil strata is based on the fact that the velocity or the resistance seismic wave transmission or resistance to electrical flow differs with different rock types and soils. The method allows the boundaries of the soils to be determined seismic refraction is described below Seismic refraction is conducted by having a source of seismic waves (Figure 4.1). The seismic waves are induced by detonating a small explosive or by striking a metal plate hard. Waves are subsequently emitted in all directions, through the air, and through the soil in all directions. Seismic wave transducers called geophones are placed radially from the epicenter. A circuit connects the geophones and the detonator for accurate determination of time. A direct wave will reach the geophone first since it is the shortest distance covered. When there is a dense stratum at depth a refracted wave will travel along the top of the bed rock. As it travels it leaks energy to the surface which can be picked by the geophone.

Foundation Engineering I: - ECE 2406

60

Seismic source Geophones

Figure 4.1 Seismic refraction – arrangement of equipment

Time

For short distances the direct waves reach the geophones first. For longer distances the refracted wave reaches first though the distances is longer than t he surface direct distance. This is so because the speed of the wave in the dense material is higher than that in the overburden material of less density. The geophone has a mechanism which records the first wave and ignores the others. This enables a plot of arrival time versus the distance. The first section of the graph represents the direct wave measurements while the second section represents the refracted wave measurements ( Figure 4.2). The inverse of these curves are the velocities of the seismic waves. The general types of the rocks are determined by geophysics from the knowledge of velocity versus rock type. It is also used in the determination of depth to water table and thicknesses of multiple strata. The depth D to the bedrock can be estimated from the formula.

d

Distance

Figure 4.2 Time versus distance for seismic waves

√

4.2.2 Detailed stage site investigations At this stage the aim is to obtain data for use in the final design of the works. The investigation is carried out by use of trial pits, sounding and boring. The extent of the use of these methods depends on the type of the project at hand and the geotechnical parameters being sought. The trial pits


61

The pit and shaft technique supplies the most detailed and reliable data on he existing soil conditions. Once the trial pit has been dug stratification of the soil should be done usually in the field. In addition as much information should be recorded. This information includes i. ii. iii. iv.

Depth to ground water table. Field assessment of the bearing capacity. Depth of the various strata encountered in the trial pit. The encountered soils should classified by visual inspection a. b. c.

v.

Coarse grained soils should be described with adjectives such as angular, rounded with traces of fines etc Fine grained soils should be studied to indicate whether they are loamy, of low plasticity, whether they are sandy clays etc All soils should be described indicating their color and odour if any. Decaying organic matter if encountered should be mentioned.

Obtain undisturbed samples when you can for the different layers of strata encountered. These samples can then be taken to the laboratory for tests

For large sites the pits should then be surveyed and located in a grid system for incorporation into the site investigation report. Sounding tests These are basically are penetration tests carried out to supplementing trial pits and borings. The penetration resistance is measured and related to the bearing capacity. They are widely used in site investigations. They consist of the cone penetrometer already presented in chapter 1. The other commonly used penetration equipment is the dynamic cone penetrometer used in the estimation of the California bearing ratio (CBR) of road pavement layers. This enables the design of the pavement layers to be carried out Boring methods When a deep stratum has to be investigated it will usually be necessary to perform boring operations to ascertain the strata below the ground to be used in the of the proposed structures. Several boring methods are available and are summarized as follows Percussion drilling consists of a derrick, a power unit and a winch carrying a light steel cable which es thorough a pulley. The unit can be towed by a vehicle after the assembly is folded. The assembly drops a chisel on the ground and strata being drilled


62

Rod Chisel

Figure 4.3 Schematic presentation of a drilling chisel

The excavation is effected by the drilling chisel. The drilling rods provide the necessary weight for the penetration the strata. Further weight may be added when need arises. The winch raises and lowers the chisel and its attachments Below the water table the loosened soil forms slurry. Above the water table water is introduced to form the slurry. Periodically the slurry is bailed out by a shell or a bailer to make progress into the soil. In boreholes which are liable to collapse the borehole must be cased. In some cases the casings slide on their own weight. On completion of the job, the casing is jacked out. Percussion drilling is usually done in diameters of 150mm to 300mm. the borehole depth investigated by this drilling method can be up to 50 to 60 metres. This method of drilling can be done on virtually all types of soils including those with boulders and cobbles. The rig is versatile enough to place mechanical augers and penetrating testing equipments at appropriate depths. Power operated augers are usually on vehicles. Downward pressure is applied by pressure or dead weight. The augurs are 75-300mm diameters. Augers are usually used in self ing soils. Casing is usually not needed since the augers have to be removed before driving. In full flight augers the rod and the helix cover the entire length being investigated. The augur is then brought up. The soil is ejected by reverse rotation. The likely hood of soil from different strata being mixed up is very high. In the short flight augur the auger is advanced into the soil and then raised. The soil is also ejected by reverse rotation.


63

Full flight augur

Short flight augur

Figure 4.4 full flight and short flight augurs The continuous flight augurs are sometimes fitted with a hollow stem which is plugged during the drilling operations. When samples are needed the plug and the rods are removed and a sampler is introduced for the recovery of a sample. The sample may be undisturbed depending on the sampler utilized. The flight augurs are not suitable for use in loose soils which are likely to collapse as the augur is inserted and removed from the hole. Hand and portable augers are usually operated by persons by turning the handle of the augur. The hand augers are typically of 75 – 300mm diameters. The soil is locked in the auger and frequent removal is needed to ensure that the augur does not get stack in the soil. Undisturbed samples may be obtained by introduction of small diameter tubes which are hammered into the strata under investigation. This method is suitable for self ing soils. It is not possible to penetrate coarse granular soils.

Figure 4.5 schematic representation of a hand augur

Wash boring is a method of boring where water is pumped through boring rods and released through narrow holes in the chisel attached at eth lower ends of the boring arrangement (Figure ****).


64

Water from pump Tiller

To sump

Drilling bit

Figure 4.6 schematic representation of a hand augur

In this method the soil is loosened and broken by water jet. This is aided by the up an down movements of the chisel. An attachment to the rods called a tiller enable the rotation on the drilling bit. The drilling winch is able to raise and lower the chisel and hence get the chopping action of the chisel. This method is suitable for most soils but progress is slow if the particles of coarse gravel larger particles are present. The accurate identification of the soil types is difficult. The method cannot be used to recover soil samples for testing. However tube samplers can be advanced into the borehole for obtaining relatively undisturbed samples. Rotary drilling is done by use of drilling bits that cuts and grinds the subsoil or rock at the bottom of the borehole. Water is usually pumped down hollow rods ing under pressure through to the drilling tools. This cools and lubricates the bits. The fluid also provides for the borehole where there is no casing. Two methods of rotary drilling are available. The first is open drilling where the soils and rocks are broken within the diameter of the hole. Subsequently the tubes are removed and tube samplers and testing continues below the borehole. This advances the drilling. The second method is known as core drilling and involves creation of an annular hole in the material and intact rock enters the drilling core. This advances the drilling and enables samples to be retrieved from the borehole. The sample is then subjected to immediate field description and taken to the laboratory for various tests. Typical core diameters range from 41mm to 165mm. The method is fast, but in large gravelly soils the speed is slowed by rotation of the bit without advancement into the ground.


65

4.2.3 Sampling Disturbed samples Disturbed samples are recovered from trial pits and along drilling tools where there is no attempt to retain the soil constituents. Disturbed samples should however be collected carefully and placed in airtight tins or jars or in plastic sampling bags. The samples should be labeled to give the borehole or trial pit identification number, depth of recovery and field description should be done. The disturbed samples are used for identification tests namely Field moisture content, PI, grading, compaction and CBR. Undisturbed sample – cohesive soils Undisturbed samples are recovered from trial pits and along drilling tools where there is an attempt to retain the soil constituents. Such a sample is taken in an airtight container with wax at both ends to prevent moisture from escaping during transportation to the laboratory. In trial pits the samples can be obtained by pressing a sampler into the ground at the appropriate depth. The sampler is typically 100mm diameter by 150mm long. In the hand augur a 38mm sampling tube with a length of 200mm is fitted to the rod after the removal of the augur. The tube is pressed into the soil and given half a turn to break the soil. The sampler is then removed and the ends are waxed. In boring rigs a 105mm diameter sampler is introduced to the borehole to recover a 100mm diameter sample. The sample is usually 381mm long and is fitted with a cutting shoe of about 110mm diameter. The sample is driven by a falling weight. Any entrapped air or water is expelled from the top through a non return valve. For soft clays thin walled samplers are preferred to minimize disturbance. Inevitably there will be some disturbance in the process of retrieving soil samples from the ground. The least disturbance is for shoes samples cut from the floor of trial pits. Sample tubes, inserted by pressing, jacking or steady hammering produce some form of disturbance depending on the thickness of the sampler walls. The degree of disturbance is related to the area ratio of the sampler tube as given by Equation ****** In general good samplers have and area ratio not exceeding 25%. Area ratios less than 10% are very good and are used for very sensitive soils. x100%

De Di

De Di

Sampler tubes

Sampler tubes fitted with a cutting shoe

Figure 4.7 Typical sample tubes


66

Undisturbed sample – cohesionless soils Various methods have been employed to obtain undisturbed sand samples. These include freezing, chemical application, and use of compressed air (Smith and Smith, 1998). Whatever method is employed eventual disturbance occurs as the soil is transported to the laboratory for testing. In light of these difficulties it is prudent to assess the engineering properties of cohesionless soils through field testing such as penetration. Quality class for soil sampling Table ** below based on Rowe (1972) shows the quality classes for soil samples obtained from various site investigation operations. Table 4.2 Quality class for soil sampling Quality class 5 4 3

Method of sampling

Use of sample

Material brought up by drilling tools an no attempt is made to retain all the soil constituents As for 5 but all soil constituents are retained as far as possible. Bulk an jar samples. Plastic bag samples Pressed or driven thin or thick walled samplers with water balance in very permeable soils

Rough sequence of strata Sequence of strata and remolded properties As above and examination of soil fabric As 3 and γ, n, mv, cu, c’ θ’ As 2 and cv and k

2

As for class 3 above but with water balance all the time

1

Thin walled piston samplers with water balance

Borehole logs Borehole logs summarizes all the laboratory an field tests carried out on samples representing the various strata encountered in the boring operations. All ground conditions encountered at the site are also included. The log enables a rapid accurate assessment of the soil profile on a vertical scale. The details of the various strata encountered including all their geological formation details which can be inferred are given. The details captured should include the depth to which ground water was encountered. The description is based on particle distribution and plasticity based visual inspection and feel. Soil color should also be recorded.


67

Courtesy of Norken Engineering Consultants

Figure 4.8 Borehole logs


68

4.2.4 Scope of Site Investigation Spacing of the trial pits and or boreholes The scope of site investigation is dependent on the effect of the construction on the ground. The scope should be commensurate with the needed geotechnical parameters. Table 4.3 shows the suggested minimum number of borings for the various structures. Table 4.3 Recommended spacing of investigation trial pits and boreholes Project Type of soil/Distance between borings Minimum no Uniform Average Erratic Multistory 45 30 15 4 1 to 2 storeys 60 30 15 3 Bridge piers and 30 30 15 1 – 2 per unit abutments For highways and runways during preliminary design the subgrade soils along the proposed alignment should be sampled at 1000metres and the samples should be tested to establish the in-situ CBR, grading and plasticity of the materials. At this stage the material site should be investigated at 60 meter intervals. In the detailed stage the subgrade is sampled at 500meters while the material sites are sampled at 30metres. Depth of investigation The depth should be such as to capture the geotechnical information needed for the design of the facility. Equally important is to capture the information needed in the quantification of the bill of quantities to ensure an accurate specification of the works is carried out. The recommended depths below the formation of investigation for the various civil engineering schemes is shown on Table 4.4 based on Figure 4.9 below. Table 4.4 Depth of investigation Project Column foundations Raft foundations Bridge piers and abutments Earthworks in fill for highways Earthworks in cut highways Pipe works

Depth 1.5B-3B 1.5B 1.5B-3B

In rock 1.5-3m 1.5-3m 1.5-3m

Parameters to be established C, θ, N, RQD,TCR C, θ, N. RQD,TCR C, θ, N, RQD,TCR

0.5L

0.50m

0.5H

0.50m

D

0

PI, CBR for fill material Strength of Establish the type of excavated material and strength of Investigate type of excavated material and strength of


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Raft foundations

Column

foundations B

Piled foundations

B L B H

Retaining walls B a)

Structural foundations L

L In cut H

In fill

b)

Highway earthworks

D c)

Pipe works

Figure 4.9 Scope of foundation investigations

4.2.5 Site Investigation Reports List of suitable headings Title page Gives the title of the project at a glance Abstract The abstract should be approximately 200 words. It is a very important element of the project and should be prepared with care. It must convey the essence of the site investigation and all the important findings without ambiguity. List of contents Guides the reader to the various chapters Field work A brief and complete description of what was done in the field. Boreholes, and trial pits performed, field testing etc. Actual procedures of standard tests need not be repeated. A


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mention of the tests performed is sufficient. New procedures and peculiar fieldwork should be explained. Laboratory work A brief and complete description of what was done in the laboratory work carried out . as in the case of field testing actual procedures of standard tests need not be repeated. A mention of the tests performed is sufficient. New procedures and peculiar laboratory equipment and procedures should however be explained Site description and geology An engineering summary of the nature of the site an its geology, including aspects such excavated areas and what was found, stability of natural slopes, drainage etc Engineering properties of soils an rocks A summary of the results of field and laboratory tests and other observations made at the site Discussion A reasoned discussion of what design and construction problems are likely to be encountered in relation to the site and its geological situations. Recommendations and conclusions A brief but clear statement of the recommended geotechnical parameters investigated. The treatment of the various aspects of design should come out clearly and without doubt. Values of use in design and construction should be summarized viz, allowable bearing capacity, estimated settlement, suitable types of foundations, construction requirements namely grouting, compaction etc References A list of the books, papers, referred to in the work Appendices Appendix A – should contain site plan, borehole logs, photographs, etc Appendix B – should contain tables of results of field and laboratory test those not included in Appendix A Appendix C – Any special or unusual test procedures adopted in the investigation References: Craig FR, 1987, Soil mechanics, Van Nostrand Reinhold (International) London Bowles JE , 1982, Foundation Engineering, McGraw-Hill international book company, Tokyo. Tomlinson MJ and Boorman R (1986), Foundation and construction, Longman scientific and technical, England Franklin JA and Dussealt MB (1989) Rock Engineering, McGraw-Hill international editions, London Chen FH (1975) Foundations on expansive soils, Elsevier scientific Publishing Company


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Chapter Six :

Shoring and underpinning

Syllabus Shoring and underpinning

6.1

Shoring

Shoring is the process of ing a building, a structure, or trench with props when in danger of collapse or during repairs or alterations. Shoring comes from shore a timber or metal prop. Buildings Raking Shores consist of one or more timbers sloping between the face of the structure to be ed and the ground. The most effective is given if the raker meets the wall at an angle of 60 to 70 degrees. A wall-plate is typically used to increase the area of . Foundations Shoring is commonly used when installing the foundation of a building. A shoring system such as piles and lagging or shotcrete will the surrounding loads until the underground levels of the building are constructed. Trenches During excavation, shoring systems provide safety for workers in a trench and speeds up excavation. It is designed to prevent collapse Concrete structures shoring, in this case also referred to as falsework, provides temporary until the concrete becomes hard and achieves the desired strength to loads.

b) Sketch of a timber single flying shore between adjacent buildings. a) Sketch of a timber double raking shore. Projected centre lines of floors and shores meet.


Figure 6.1 Examples of Shoring

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shoring and underpinning

c) Schematic sketch of a modern steel trench shore being lowered into a trench.

d) Traditional trench shoring or Timbering.


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6.2

Underpinning

Underpinning is the process of strengthening the foundation of an existing building or other structure. Underpinning may be necessary for a variety of reasons: • The original foundation is not strong or stable enough. • The usage of the structure has changed in which case additional load is being transmitted to the foundation. • The properties of the soil ing the foundation may have changed or were mischaracterized during design. • The construction of nearby structures necessitates the excavation of soil ing existing foundations. • To increase the depth or load capacity of existing foundations to the addition of another storey to the building (above or below grade). • Natural causes have caused the structure to move, thereby requiring stabilisation of foundation soils and/or footings.  Underpinning may be accomplished by extending the foundation in depth or in breadth so it either rests on a more ive soil stratum or distributes its load across a greater area. Use of micro piles and jet grouting are common methods in underpinning. Mass Concrete Underpinning Mass concrete underpinning method is an old tradition established over the years. This underpinning method strengthens an existing structure's foundation by digging boxes by hand underneath and sequentially pouring concrete in a strategic order. The final result is basically a foundation built underneath the existing foundation. This underpinning method is generally applied when the existing foundation is at a shallow depth but reports of fifteen meter depths have been made. Heavy machinery is not called for in this method due to the tight nature of the boxes being dug.

1

2

3

3

1 2

Ground floor

2

1

3 3

a) Mass concrete below the wall

b)

2

1

Sequence of operations

Figure 6.2 Underpinning a wall Beam and base underpinning The beam and base method of underpinning is a more technically advanced adaptation of traditional mass concrete underpinning. A reinforced concrete beam is constructed below, University of Nairobi –FCE 511 Geotechnical Engineering IV

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above or in replacement of the existing footing. The beam then transfers the load of the building to mass concrete bases, which are constructed at designed strategic locations. Base sizes and depths are dependent upon the prevailing ground conditions. Beam design is dependent upon the configuration of the building and the applied loads. Ground floor Beam

New wall or series of columns New base Figure 6.3 Beam and base new foundations

Mini-piled underpinning Mini-piles have the greatest use where ground conditions are very variable, where access is restrictive, where environmental pollution aspects are significant, and where structural movements in service must be minimal. Mini-piled underpinning is generally used when the loads from the foundations need to be transferred to stable soils at considerable depths in excess of 5 m. Mini-piles may either be augured or driven steel cased, and are normally between 150 mm and 300 mm in diameter. Piling rigs for this type of underpinning are designed to operate in with limited headroom and limited space. The equipment is capable of constructing piles to depths of up to 15m. Ground floor

Piles driven or bore to firm ground

Figure 6.4 Piled underpinning


Chapter Seven : Excavation , bracing, techniques.

Syllabus:

7.1

ground water, dewatering

Excavation, bracing ground water, dewatering techniques

Excavation and bracing

Ordinarily excavations in most cases will proceed without . However in deep excavations it will be necessary to the sides in order to protect the workers Bracing is usually done by installing a of struts and piles. In very soft and loose soils the piling is done first. This is then followed by installation of struts as the excavation is done. As the depth increases the soil starts to yield before the strut is installed. Because of the being granted by the s the Rankine conditions are not met in the force generation. Figure 7 shows a braced excavation. The pressure on the struts for design purposes is empirically determined from the empirical formulas shown on the Figure Figure a) is the strutted excavation Figure b) shows the measured loads in sand excavations Figure c) shows the Estimate of lateral load in sand excavations. The pressure is rectangular with the maximum value being 0.65*Ka*ɣH. Figure d) Estimate of lateral load in clay excavations where the stability number ɣH/Cu is less than 4. Where the pressure varies between 0.2 ɣH and 0.4 ɣH. Note the large variation Figure e) Estimate of lateral load in clay excavations where the stability number ɣH/Cu is greater than 4. The pressure is rectangular with the maximum value being 1.0*Ka*ɣH.

M is usually taken as 1 but may be taken as low as 0.4 for the very soft clays Note the large variation


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Figure 7. 1 Strutted excavation

7.2

Ground water and dewatering techniques

Dewatering involves controlling groundwater by pumping, to locally lower groundwater levels in the vicinity of the excavation. Sump pumping The most common and simple form of dewatering is sump pumping. In this case groundwater is allowed to enter the excavation where it is then collected in a sump and pumped away by robust solids handling pumps. Sump pumping can be effective in many circumstances, but seepage into the excavation can create the risk of instability and other construction problems. Wellpoints Wellpoint dewatering is widely used for excavations of shallow depths, especially for pipeline trench excavations. A typical wellpoint system consists of a series of small diameters wells (known as wellpoints) connected via a header pipe, to the suction side of a suitable wellpoint pump. The pump creates a vacuum in the header pipe, drawing water up out of the ground. For long pipeline trenches, horizontal wellpoints may be installed by special trenching machines. Wellpoints are typically installed in lines around the excavation, and are pumped by diesel or electrically powered pumps, with associated header mains, water discharge pipes, power supply generators, electrical controls and monitoring systems.


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Figure 7.2 Dewatering from wellpoints Source (www.groundwaterenginneering.com Shallow Wells Shallow wells comprise surface pumps which draw water through suction pipes installed in bored wells drilled by the most appropriate well drilling and or bored piling equipment. The limiting depth to which this method is employed is about 8 m. Because wells are prebored,this method is used when hard or variable soil conditions preclude the use of a wellpoint system. Since the initial cost of installation is more compared to wellpoints it is preferred in cases where dewatering lasts several months or more. Another field of application is the silty soils where correct filtering is important. Deep wells A deep well system consists of an array of bored wells pumped by submersible pumps. Pumping from each well lowers the groundwater level and creates a cone of depression or drawdown around itself. Several wells acting in combination can lower groundwater level over a wide area beneath an excavation. Because the technique does not operate on a suction principle, large drawdowns can be achieved, limited only by the depth of the wells, and the hydrogeological conditions. The wells are generally sited just outside the area of proposed excavation, and are pumped by electric submersible pumps near the base of each well. Water collection pipes, power supply generators, electrical controls and monitoring systems are located at the surface.


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Figure 7.3 Dewatering for a rectangular foundation from deep wells Source (www.groundwaterenginneering.com)


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