Rocker Pipe.pdf 73e2l

A MODEL STUDY OF THE ADVANTAGE OF ROCKER PIPES TO ALLEVIATE DIFFERENTIAL SETTLEMENT INDUCED PIPELINE FAILURES Wijeyesekera D.C. University of East London, Dockland, United Kingdom

Reginold, J.T. University of East London, Dockland, United Kingdom

ABSTRACT: Soil-pipe interaction studies can help in the evaluation of settlement of pipelines. However, pipeline failures still occur due to differential ground movements between a heavy yielding structure and a pipeline firmly connected to it. Such differential movements induce excessive stress concentrations in the pipeline. Often pipeline failures are a consequence of such movements, and the flexibility of plastic pipes can make them less vulnerable than rigid pipes. The magnitude and location of the maximum bending moments in pipelines arising from the yielding of the heavy structure can be determined by treating pipelines as beams on elastic foundation,. The provision of rocker pipe ts that entertain a permissible rotation helps to redistribute the bending moments to acceptable levels and thereby alleviate distress in the pipeline. This paper presents both a theoretical approach and a laboratory approach to the evaluation of the bending moment, shear force, vertical soil resistance at soil pipe interface due to differential settlement and also assess the benefits of rocker ts to alleviate the distress in pipelines. Innovative experimentation used in the laboratory research programme is presented. The paper further presents results from a laboratory investigation of the soil structure interaction of flexible strip foundations and articulated pipelines, with a view to establishing a method of assessing the distribution of soil sub grade reaction that is developed as a consequence of non-uniform settlement. The influence of the stiffness of the structure on the soil reaction distribution is also demonstrated and the effect of the various distributions on the bending moment distribution of the pipe is discussed. A few case histories of failures are summarised, demonstrating these effects, and pointing the way to possible solutions, which could be incorporated at the project design stage. Finally, the need for rational design procedures for pipeline foundations including rocker pipes to be incorporated into codes of practice such as EN 1295 is emphasised. Keywords: Differential Settlement, Displacement, Flexible ts, Soil sub grade reaction, Rocker pipes.

due to differential settlement and also assess the benefits of rocker ts to alleviate the distress in pipelines.

1. INTRODUCTION Both rigid and flexible pipelines are vulnerable to ground movements as a consequence of the significant levels of stress induced in them. Any form of unanticipated differential ground movements between a structure on a yielding foundation and a pipeline attached to it, can further exacerbate the stresses in the pipeline to unacceptable levels. Often such differential settlements that occur are either ignored or not allowed for in the design and the pipeline fails, subsequent to construction and even before it being fully commissioned.

When differential settlements occur between a structure and the connected buried pipeline the pipes will be subjected to longitudinal bending, and the ts to shear and angular rotation. Olliff et al, 1994 [2] raised the awareness for provision to be made for such differential settlements. The Materials Selection for Sewers, Pumping mains and Manholes (UK Water Industry Sewers and Water Mains Committee, 1996 [6]) suggested that the first t should be within 150 mm of the face of the structure. Authors of this paper suggested the adoption of rocker pipes in Olliff et al, 2000 [4]. Subsequently, section 4.6.6 of the Sewers for adoption, 5th edition, 2001 [8] recommended the

Both theoretical approach and laboratory approach to the evaluation of the bending moment, shear force, vertical soil resistance at soil pipe interface 1

need for a flexible t to be provided as close as feasible to the outside face of any structure in which a pipe is built. Furthermore, the next length of pipe (rocker pipe) away from the structure was recommended to be as shown in table 1.

described. The method can be applied to pipes of differing materials and different types of ts.

Table 1 - Recommended rocker pipe length (modified from Sewers for adoption; 2001 [8] )

When differential settlements occur between a structure and the connected buried pipeline the pipes will be subjected to longitudinal bending, and the ts to shear and angular rotation. The length of the pipe section immediately adjacent to the structure must be designed to keep all of these considerations within allowable limits. A method of determing this appropriate length of pipe section is described. The method can be applied to pipes of differing materials with different types of ts.

Nominal diameter mm 150 <600-150 675 <750-675 >750

Effective length mm 600 600 1000 1000 1250

2. PIPELINE FLEXIBILITY NEAR SETTLING STRUCTURE

Length to Diameter Ratio 4.0 1.0 1.5 1.3 1.7

The term ‘rocker pipe’ has caused much confusion over the years. Engineers who are not acquainted with the subject seem to assume that some sort of mystical qualities are somehow bestowed on pipes as soon as they are described as ‘rockers’. In fact, there is nothing mystical about ‘rocker pipes’. They are ordinary pipes, which because of their location, can rotate in the vertical plane, so that two ends are at different levels. This enables the pipe to accommodate differential settlements. It is only the location of the pipe, not its length, which dictates that it will function as a ‘rocker’ (Olliff et al, 2003 [5]; Wijeyesekera et al 2006 [9,10 &11] )

150 mm Maximum 600 mm

2.1 ANALYTICAL STUDY Failure to design pipelines to accommodate, or avoid differential settlements is one of the more common causes of structural failure, and a design analysis should therefore be carried out for an evaluation of permissible bending moments. A prismatic beam (figure 2) connected to a structure and ed continuously along its length by a foundation will experience elastic deformation. The resulting sub grade reaction can be assumed to be linearly proportional to the beam deflection at any point Wijeyesekera et al, 2006 [10&11]. Under such conditions the reaction per unit length of the beam can be represented by the expression ksy, where y is the deflection and ks is a constant usually called the modulus of the soil foundation. This constant denotes the reaction per unit length when deflection is equal to unity.

150 mm Maximum 600 mm Flexible t

Rocker Pipe

Rocker Pipe

This assumption helps in writing the stability equations that are amenable to solution. This represents an idealization closely approximating many real situations. Beam behaviour of pipeline is analysed according to the theory of beams on elastic foundations (Selvadurai, 1984 [7]), a theory validated by the results of many field studies and experiments (Olliff et al 2000, 2003 [3&4]).

Fig. 1. Typical t detail with flexible t at 150mm from the face of structure

A length of the pipe section immediately adjacent to the structure must be designed to keep all of these considerations within allowable limits. A method of identifying this appropriate length of pipe section is 2

M = 2 EI∆e − βx β 2 (cos β x − sin βx)

………(4)

Structure

Differentiating the equation 4 then gives the shear force at x; V = −4 EI∆β 3 e − βx cos βx ………(5)

X=∞ Soil Sub grade Reaction

Soil Foundation

In the analysis for the location of the first flexible t, the pipe length (AA1) is considered to be finite, see figure 3.

Y Fig. 2. Semi-infinite beam on elastic foundation

For this particular case the corresponding equations become;

In figure 2, x represents the location of the point from the settling structure, at which the bending moment is evaluated. The analysis presented here establishes the minimum length required to ensure that the allowable rotation of the flexible t is not exceeded. Knowledge of this length aids in determining the bending moments in the rocker pipe and the shear forces at its ends. If these are excessive, they must be reduced to levels below the allowable limits. This cannot of course, be done by reducing the length of the ‘rocker pipe’, otherwise the t rotation criteria would not be met.

150mm [Finite Pipe Length] Flexible t Heavy Structure

X=∞

Soil Sub grade Reaction [kr]

A Y

Soil Foundation

Fig. 3. Semi-infinite beam with flexible t on elastic

foundation

The deformed shape of a beam on elastic foundation (Selvadurai, 1984 [7]) is given by the equation (1)

y = e βx [A cos βx + B sin βx ] + e − βx [C cos βx + D sin βx ]

y = e βx [A cos βx + B sin βx ] + e − βx [C cos βx + D sin βx ]

y ' = βe − βx [− A(cos βx + sin β x) + B(cos β x − sin β x)]

. …….. .. (6) + β e βx [− C (sin β x − cos β x) + D (cos β x + sin β x)]

…….(1) For the particular problem illustrated in figure 1, the following boundary conditions apply; Reginold (2006) [5]

. …….. .. (7) y '' = 2 β 2 e − βx ( A sin β x − B cos β x) + 2 β 2 e βx (−C sin βx + D cos β x)

For a semi infinite pipe (when x >150mm) the deflection, y is zero. At the interface of the structure and the pipe ( x = 0 ) the pipe deflection will be the same as that of the settling structure (∆) and the slope of the pipe will be zero.

. …….. .. (8) y ''' = 2 β 3 e − βx [ A(cos β x − sin βx) + B (sin βx + cos βx)]

+ 2 β 3 e βx [− C (cos β x + sin β x) − D(sin βx − cos β x)]

. …….. ..

The equation 1 then reduces to y = ∆e − βx [cos βx + sin βx]

(9)

For a 40mm diameter pipe, the solutions for the equations 6 to 9 are presented graphically in figures 3 and 4. The vertical displacement variations in figure 5 for the three pipe lengths of 1.5 and 3.0m are coincident. The maximum uplift (1.60 to 1.75mm) of the pipes occur at x= 278±2 mm (x/D of 2.2 to 5.5). These variations are very coincident and this is illustrated in figures 4 and 5.

……. (2)

Differentiation of equation 2 gives the slope at x to be y ' = −2∆β e − βx sin β x …….. (3) Differentiation of equation 3 gives the bending moment, M, at x; 3

Vertical Displacement "mm"

Vertical Displacement along the length of Pipe -10

0

500

1000

1500

2000

2500

3000

0 Length of Pipe

10

"mm"

10mm Settlement for L=3m 20mm Settlement for L=3m 40mm Settlement for L=3m 10mm Settlement for L=1.5m 20mm Settlement for L=1.5m 40mm Settlement for L=1.5m

20 30 40 50

Fig.4.Vertical displacement variations for pipe lengths of 3m and 1.5m

Fig. 6. Flexible t at structure pipe interface

Longitudinal Bending Moment along the length of Pipe

Longitudinal Bending moment "Nmm"

-8.E-03 -3.E-03 0 3.E-03 8.E-03 1.E-02 2.E-02

500

1000

1500

2000

2500

3000

Length of Pipe "mm" 10mm Settlement for L=3m 20mm Settlement for L=3m 40mm Settlement for L=3m 10mm Settlement for L=1.5m 20mm Settlement for L=1.5m 40mm Settlement for L=1.5m

Fig.5.Variation of bending moment in pipe lengths of 3m and 1.5m

As sometimes adopted in practice, it is prudent to provide a flexible t ( /rocker) at a distance no greater than 150mm from the face of the structure rather than providing one at at x=0, (Figure1).

Flexible t

This figure further illustrates the provision of a permissible 2º rotation at this first flexible t. Figure 6 shows the adoption of this concept. Figure 7 and 8 also illustrates the adoption of these in manholes both constructed in situ and prefabricated.

Rocker pipe with Flexible t

Flexible t with Allowable angle of Rotation

Figure 8 illustrates an example where the bending moment at the structure is so severe that failure occurred at 36.28m (13xD) from the face of the structure. Failure can be alleviated by providing rocker pipes ie further (2nd and 3rd) flexible ts away from the structure. Following from the discussions above the author considers the analysis of a second flexible t, with the first t at a distance of x =150mm from the face of the structure.

Fig 7. Rocker pipe location for 100mm diameter pipe connected to cast insitu manhole

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Longitudinal Bending Moment for Fully Fixed and Flexible t Condition for 10mm Settlement

CROSS SECTION THROUGH PUMPHOUSE 3 GRP intake pipelines on compressible marine foundation 36.28m

L o ng itudina l B ending M o m ent "kN .m "

-2.E-03

Position of Fracture

-1.E-03

0

200

400

600

800

1000

0.E+00

Distance along the Pipe "mm"

1.E-03

Fully Fixed Condition - 10mm Settlement One t Condition - 10mm Settlement Two t Condition - 10mm Settlement

2.E-03 3.E-03 4.E-03

Fig.10. Influence of flexible rocker ts on bending moment for varying settlement of 10mm. Fractured GRP pipes ( 3 No.) at site due to differential settlement

Figure 11 is a design chart developed to facilitate the evaluation of the number of rocker pipes that need to be provided to meet an anticipated differential ground movement of ∆.

Fig 8. Photographic evidence of offshore GRP pipeline failure at pipe structure interface.

1.4

MCRITICAL / MFAILURE

1.2

3. ROCKER PIPE DESIGN The analysis described above established the minimum length required to ensure that the allowable t rotation is not exceeded, and knowing this length, the bending moments in the rocker pipe, and the shear forces at its ends, can be calculated. If these are excessive, they must be reduced by increasing the number of rocker pipes. Figures 9 and 10 compare the influence of one / two ts on the vertical displacement and bending moment profile respectively.

Full length pipe-No Rocker Pipe t One Flexible t Rocker Pipe Two Flexible t Rocker Pipe

1.0 0.8 0.6 0.4 0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

∆/D

Fig.11. Rocker pipe t design chart M CRITICAL: Maximum bending moment M FAILURE: Bending moment at failure

Vertical displacement"mm"

Comparision of Results for Fully Fixed and Flexible t Condition for 10mm Settlement

-2 0

0

50

100

150

200

250

300

350

400

450

From the information available from the pipe/flexible t manufactures and soil investigation for structural foundation, the design engineer can easily estimate the anticipated differential settlement. And the required number of flexible rocker pipe ts to accommodate the distress induced on connected pipeline due to differential settlement within the transition zone, see figure 12.

500

Distance along the Pipe "mm"

2 4 6 8 10

Fully Fixed Condition - 10mm Settlement One t Condition - 10mm Settlement Two t Condition - 10mm Settlement

12

Fig.9. Influence of flexible rocker t for varying settlement of 10mm.

5

Intermediate length of the pipeline

Flexible pipeline t

Original Position of the Pipeline

Differential Settlement

Position of the pipeline Settled profile of the after differential pipeline after differential settlement

l

Rocker Pipe

Transition Zone Unsettled ground profile

Flexible t

Fig.14. Rocker pipe location for 100mm diameter pipe connected to prefabricated manhole

Fig.12. Influence of flexible rocker t for varying settlement of 10mm.

Other options for dealing with this conflict include the following: •

Reducing the backfill load by replacing some of the fill by expanded polystyrene. This will reduce the settlement, and also reduce the bending moment and shear forces.

•

Increase the effective bending and shear strengths of the ‘rocker pipe’, by ing it on a reinforced concrete beam. (Note: The use of reinforced concrete beams can also serve to increase the effective length of the ‘rocker pipe’. Two or more standard length pipes laid on the same beam will ‘rock’ as if they were a single pipe).

The above description of the problems of deg pipelines with rigid pipes to accommodate differential settlements highlights the potential advantage of continuous, fusion ted polyethylene or polypropylene pipelines. These will conform to a settled soil profile by bending, and only if the bending is very severe, will there be a risk of failure, by buckling.

4. EXPERIMENTAL STUDY In view of the dearth of information from the reviewed papers that deal with the soil interaction with buried rigid/flexible conduits subjected to a vertical movement/settlement, a laboratory research program was performed.

Inspection Chamber

Prototype field experiments to investigate the soil structure interaction can be very expensive. In this research programme, a series of laboratory soil box test with specially design and built loading frame is used to induce settlement of the structure relative to the connecting pipeline (see figure 15). The objective of the laboratory research programme is to observe, evaluate and compare the mathematical predictions for the stress strain regimes around a pipe subjected to differential settlement.

Rocker Pipe Flexible t

The laboratory tests were carried out with small diameter plastic pipe generally used in the residential drainage connections. Literature research reviewed that such similar works are not carried out

Fig.13. Rocker pipe location for large diameter pipe connected to insitu inspection chamber

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in the past to practically design the rocker pipe length.

sensors (PS), eleven linear displacement transducers (DS), one load cell (LC) and eight strain gauges (S). Following are the test assumptions made during the testing, observation and analysis: Fixed Boundary conditions during soil box investigation. The pipe used in the soil box test is very flexible, and is not stiff enough to elongate laterally to exert horizontal thrust on the soil mass with decreasing vertical diameter. A mathematical model for defining the soil pipeline interaction in response to differential settlement was described in section 2.1. The results of the physical full scale analysis described in this paper was compared further with the mathematical modelling outlined and referring to displacement from differential settlement and pipeline t rotation/Rocker pipe is proposed , see figure 17.

View: A

View: B 45

Fig.15. Soil Box used and instrumentation setup

Test No: SB-FL1-h0 -J0-Y1

Predicted values and Observed values for a vertical end settlement of 40mm 1

40

1

35 Predicted Value

100% Perfect Match

2 2

30 25

1st Flexible t @ X=150m m

20

1500mm

11

h=0mm

15

Fully 10

9

8

7

Restrained End

10 3 3 0 4-11 0 4-11

CH4-FF-Analy-40mm 5

10

15

20

5

4

h=0mm

Settlement End

h=0mm

SB-FL1-h0 -J2-Y1-40mm

5

6

2 1 3 Imposed

2nd Flexible t @ X=250m m

Section

25

30

35

40

45

Observed Value

Fig.17. Compression of observed and predicted pipe deformation for a differential settlement of 40mm with two flexible rocker ts

5. CONCLUSIONS The following conclusions can be drawn from the study Fig.16. Detailed Instrumentation along the length of the pipe

•

Thirty observations were monitored in the soil box experiments (see figure 9). Data logging was carried out using the programmable data logging device to record observations from ten flexi force pressure

• •

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Established pipeline design procedures frequently ignore or underestimate the settlements of soil masses, pipelines and structures. Analysis of pipelines as strip foundations can provide a useful estimate of likely settlements. Pipeline design should include analysis of settlements, and the provision of measures to

• •

• •

5. Reginold J.T., (2006) Rocker pipe solution to alleviate differential settlement induced distress in flexible pipes. Ph.D Thesis, University of East London, London, United Kingdom.

limit them and/or enable the pipelines to accommodate their effects. The ability to accommodate settlements should be considered during the pipe material selection process. The effective modulus of a pipeline foundation will vary from place to place, reflecting inconsistencies in the placing and compaction of bedding material, variations in bedding thickness, and in sub-grade properties. The first flexible t or rocker pipe needs to be within the first 150 mm from the yielding structure. If there is no provision in the form of rocker pipes made, a failure of the pipe can occur at a distance of 10 – 15 diameters from the face of the yielding structure.

6. Robert S.,(1996) Material selection for sewers, pumping mains and manholes. UK water industry sewers and water mains committee. 7. Selvadurai A.P.S., (1984) The flexure of an infinite strip of finite width embedded in an isotropic Elastic Medium of finite Extent ,International Journal of Numerical and Analytical Methods in Geomechanics,Vol.8. 8. Sewers for adoption manual, 5th edition, UK, 2001.

ACKNOWLEDGEMENTS I am highly obliged and owe a great debt of gratitude to my supervisor Professor D.C. Wijeyesekera. Jonathan Olliff of Montgomery Watson, with their expertise in buried pipeline design. Ralph Potter for allowing me to use the Pipeline Technology laboratory. I wish to especially thank my father for sharing his worldly expertise in the field of civil engineering.

9. Wijeyesekera D.C.,Reginold J.T.,(2006) Rocker pipe- A solution for differential settlement induced distress in pipeline. ASME 6th International Pipeline Conference Proceeding IP2006, Calgary, Canada. 10. Wijeyesekera D.C.,Reginold J.T.,(2006) Rocker pipe solution to alleviate differential settlement induced distress in flexible pipes. Advances in computing and technology Conference Proceeding AC&T2006, London, United Kingdom.

REFERENCES 1. Civil Engineering Specification for the Water Industry, 5th Edition, 2003.

11. Wijeyesekera

D.C.,Reginold J.T.,(2006) Study of the use of rocker pipes to allow for differential ground movement in pipelines. Plastic Pipes conference- xiii, Washington DC, USA.

2. Olliff J.L.,(1994) Pipeline Foundation Design, Document C164/165/JWG1, CEN. 3. Olliff J.L., Rolfe S., Wijeyesekera D.C.,Reginold J.T., (2000) Soil Structure Pipe interaction with particular reference to ground movement induced failures, Plastic Pipes conference- xi, , pp 941-950.

12. Wijeyesekera D.C.,Reginold J.T.,(2007) Mathematical and Physical study of pipelines subjected to differential settlement. Advances in computing and technology Conference Proceeding AC&T2007, London, United Kingdom.

4. Olliff J.L., Rolfe S., Wijeyesekera D.C.,Reginold J.T., (2003) Settlement induced failures of plastic and other pipes, Plastic Pipes conference- xii, Italy.

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