Folds Faults And ts 1i6758

Sedimentary strata may lie horizontal as originally deposited or they may be bent into more or less regular folds, they may be displaced along fractures, or they may be traversed by planes or parting known as ts. Geologic structures influence engineering projects in many ways. Folds and faults obviously have much to do with the selection of dam sites and even such seemingly unimportant matters as the spacing of ts may have vital bearing on uplifting pressure and safety of dams. Gushed and chemically altered rocks contiguous to originating along faults may damage or destroy engineering structures. The design of deep cuts in rocks is greatly influenced of geologic structures on circulation of the ground water. Some of the most common that are involved in the study of geologic structures are as follows; 

Bedding Planes: The planes or surfaces which divide on bed from the other are called bedding Plane.

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Dip: The dip of a bed in the angle between the bedding and the horizontal plane.

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Strike: It may be defined as the direction of line formed by the intersection of bedding and horizontal plane.

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Outcrop: The area of exposure of bed on the earth’s surface is called outcrop.

1. FOLDS Perhaps the most common type of deformation is folding. As the name implies, folds are undulation, flexures waves which resembles to ocean waves. They are best displayed in stratified formation i.e. sedimentary rocks. But any layered or foliated rock such as banded gabbros or granite gnessis may display folds. Some folds are few miles away, the width of others to be measured in feet or inches or even in fraction of an inch. 1.1. Parts of Folds The axial plane or axial surface of a fold is the plane or surface that divides the folds as symmetrical as possible. In some folds the axial plane is vertical in other it is inclined and in still other it is horizontal. The sides of a folder are called the limbs or flanks. A limb extends from the axial plane in one fold of the axial plane in the next. The highest point of a fold is called the crest and the lower portion is called trough. 1.2. Nomenclature of Folds On the basis of dip relationships two major types of folds can be distinguished;

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Anticline: In which the strata on opposite flanks dip towards the axis in other words the folds that concave upwards.

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Monoclines: Folds in which horizontal or gently dipping beds are modified by simple step like bends.

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Over turned Fold: The axial plane is inclined and both limbs dip in the same direction but usually at different angles.

2. FAULTS Faults are fractures in the earth’s crust along which slippage or displacement has occurred. As a result, formerly continuous beds have been dislocated in a direction parallel to fault’s surface. The displacement may vary from a few inches or less, to many miles. When subjected to great pressure, the earth’s crust may have to withstand shear force in addition to direct compression. If the shear forces so induced become excessive, failure will result, movement will take place along the plane of failure until the unbalanced forces are equalized and a fault will be the result. 2.1. Terminology The vertical component of the displacement between two originally adjacent points is called Throw of the faults. The block above the fault is called Hanging Wall and the underlying block is termed as Foot Wall. The horizontal component of displacement is called the Heave and the angle of inclination to the vertical is called the Hade of the fault. 2.2. Types of Faults The two common types of faults are normal faults and reversed fault. In a normal fault the hanging wall is displaced downward relative to the footwall. In the reversed faults the hanging wall is displaced upwards relative to footwall. If the faults dip at angles less than 45 degree the term high thrust fault is applied. Strike slip faults are the high angle fractures in which displacement is horizontal, parallel to the strike of the fault plane. There is little or no vertical movement. Normal faults rarely are isolated fractures. Typically, a group of parallel normal faults develops a step like arrangement, or a series of fault blocks. A narrow block dropped down between two normal faults is called graben, and an upraised block is called a horst. 2.3. Active and Inactive Faults Fractures that are known to have experienced dislocation within historic time are known as active faults. As they present hazard to construction, the differentiation of active and inactive faults is matter

of considerable engineering importance, and it is quite unfortunate that frequently no very reliable decision can be made. The most direct and best evidence of activity is that furnished by seismographs and benchmarks. If the seismograph records show that earthquakes ocean along a fault it should, of course be regarded as active. Similarly accurately located bench mark exhibit horizontal in vertical displacement, and fault known to exist in the area should be regarded as active. If a fault is known to be overlain by younger strata that are not displaced, it is permissible to regard it as inactive.

3. TS ts are planes or surface which intersect rocks, but along which there has been no appreciable displacement parallel to the t surface. When displacement parallel to the fracture is measureable, the fracture is known as a fault. ts result either from tension or shear stress acting on rock mass. The cause of stresses may be due to contraction, compression, unequal lift, subsidence, earthquake or other earth phenomena. Tension ts arise, for instance by drying and resultant shrinkage of sedimentary deposits, or igneous rocks by contraction and cooling. Shear ts may arise from compression of sedimentary or igneous rocks. 3.1. Engineering Significance Because of their almost universal presence, ts are of engineering importance, especially in excavation operations. It is desire able for ts to be spaced closely enough to reduce secondary plugging and blasting requirement to a minimum, but not so closely spaced as to impair stability of excavation slopes or increase breakage in tunnels. Needless to say, the ideal conditions are seldom encountered. ts oriented approximately at right angles to the working face present the most unfavorable conditions, whereas ts oriented approximately parallel to the working face greatly facilitate blasting operations and ensure a fairly even and smooth break parallel to the face. t offer channels for underground water circulation and in working below the ground water table may greatly increase water problems. They also may exert an important influence on weathering.