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Many factors are known to cooperate in causing a mass of material to flow or fail or fall. Some of them play a direct role and are easily understood whereas others are indirectly responsible for the instability of the land mass.
All such factors that facilitate mass failure in one way or another are generally grouped in two headings:
(A) Internal Factors and
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(B) External Factors.
(A) Internal Factors:
These include such causes, which tend to reduce the shearing strength of the soil or rock mass by virtue of which it had remained stable at a given position on the ground. Among these factors, the nature of the slope and the water content are very crucial in defining the stability of ground anywhere.
The composition of the ground material and the geological character of the area are other two internal factors that determine to a great extent if a land mass in a given area would be stable or not.
The role these natural factors play in the stability of the land masses are discussed below only very briefly:
(1) The Nature of Slope:
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Some slopes are very stable even when very steep (60°-90°) whereas others are inherently unstable, and may fail repeatedly even at very gentle angles. Since a great majority of mass failures are confined to slopes only, it is reasonable to conclude that nature of a slope may be a deciding factor in defining the stability or otherwise of a land mass. By nature of slope is meant here the type of material of which the land mass is made up (soil or rock) and the angle at which this particular mass is inclined with the horizontal (the slope angle).
Any mass forming a slope is subject to two types of forces:
First, those by virtue of which it can retain its position in space, and
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Second, those which tend to induce failure into it.
The most important force among the first category is undoubtedly the shearing resistance of the mass, which in its simplest case is given by the relationship:
τ = c + σ tan φ … (1)
where τ = shearing strength,
c = cohesion,
σ = normal stress and
φ = angle of internal friction of the mass.
Among the forces that tend to induce failure in a mass, the most important is the pull due to gravity which acts through the weight of the material. Any mass at a given place remains stable so long the set of forces of category first are dominant or at least in equilibrium with the set of forces of category second.
As, when and where the forces tending to induce failure dominate over those tending to resist failure, the mass becomes unstable.
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This may be explained in a simple case by assuming a mass M forming a slope angle (3 and resting over a possible surface of planar failure (Fig. 20.8).
Forces resisting failure, RF are:
RF = cA + W cos tan φ … (2)
and forces inducing failure, IF are:
IF = W sin … (3)
where c is the cohesion of the mass, W = force of mass acting through it e.g. is the angle of slope, φ is the angle of internal friction and A is the area of block at the contact.
The condition of limiting equilibrium is satisfied when:
IF = RF … (4)
In the analysis of stability conditions for slopes, a Factor of safety, FS, is sometimes desired, which is obtained from the above relationship and is expressed as:
FS = Sum of forces available to resist failure / Sum of forces tending to induce failure
= cA + W cos tan φ / W sin
Angle of Repose:
It has been observed that most materials are stable upto a certain angle of slope. This is called the critical angle of slope (sometimes called angle of repose) and varies from 35° for unconsolidated sediments to 90° for perfectly crystalline unjointed rocks. For partially jointed rocks it may range from 60°- 90°. There are, however, many factors that may combine together to make the stability analysis most complicated problem and the slope may fail despite being at gentler angles than those mentioned above.
In fact much importance has been attached in recent times to the height of the slope in addition to the angle of slope for determining the factor of safety. The presence of water behind and below and within the mass and nature, intensity and inclination of joints, fissures and cracks in it are known to add to the forces tending to induce failure, the slope angle remaining the same.
Natural situations which offer such conditions for combination and cooperation of gravitative forces with the above mentioned factors in reducing the frictional forces of masses are numerous and varied. Hence slope failure is a very common natural geological phenomenon that remains as yet a challenge for a civil engineer.
(2) The Role of Water:
Much importance is attached to the role of water – both surface and subsurface, in causing mass movement. Water may act directly to reduce the shearing strength of a rock or soil mass in a number of ways. It may also play an indirect role in promoting the instability conditions for land masses.
Some general cases are discussed below only briefly:
(i) Water that penetrates the soil and rocks through seepage and moves into the pores of the mass may be the cause of an uplift or pore-pressure within the mass under consideration. This pore pressure, p, may approach considerable magnitude in most cases and reduce the normal stress σ of the mass effecting its shearing strength adversely.
The net normal stress available in the mass for stability then becomes σ-p; this is sometimes called the effective (normal) stress and is used for calculating the shearing strength according to the empirical relationship given by Coulomb:
τ = c + (σ – p) tan φ … (6)
(ii) Water that accumulates at the back of a mass may exert a pressure, parallel to the direction of flow and add to the shearing forces causing instability. Such directed water pressure, say P, with or without pore-pressure, p, may cause a great reduction in the factor of safety of a mass.
The factor of safety for the mass M (Fig. 20.9) which was simply-
F = cA + W . cos tan φ / W sin
in dry state, may now be expressed by the following relation-
F = cA + (W . cos – p) tan φ / W sin + P
where p and P are pore pressure and directed water pressure respectively. This reduction in factor of safety in the stability of mass becomes considerable with increasing water content. In submerged conditions, the factor of safety for a given mass may be 100 percent or more lower than for the same mass under dry conditions.
(iii) A still different way in which water may weaken the soil or rock mass is through its repeated change of state with climate changes. Water freezing within the pores and other open spaces during extremely cold weather expands and exerts considerable pressure. This may be followed by melting or thawing of ice crystal in following summers. The water so produced may saturate the mass and is available for other actions.
This process of frost action may be repeated again and again in cold humid regions resulting in disintegration of superficial layers of soils and rocks. As a consequence formerly stable slopes of massive nature may change gradually into unstable slopes of virtually incoherent nature. Flowage, whether slow or rapid, invariably involves presence of water. In cold regions, frost action has been found to be the primary cause of frequent rock falls from high steep slopes.
In addition to disintegrating the mass by its freezing, thawing cycles, ground water may cause disintegration of rocks by virtue of its flow-velocity within the cracks, joints and pore spaces of secondary origin. This may be particularly prominent, in weakly cemented rocks. The solvent action of water should also not be underestimated. In rocks like gypsum and limestones, or in those rocks which have soluble minerals as their constituents, water may gradually remove the soluble components reducing the shearing strength of the mass.
(iv) Water also facilitates mass failure through its lubricating action. Thus, when groundwater happens to move along a plane of weakness (e.g. a joint set, a fault pane, shear zone and a bedding plane etc.) within a mass, that plane gets lubricated, thereby decreasing the friction forces.
In addition to its direct contribution towards conditions favouring mass failures, water is also considered a powerful agent in promoting instability through its transportation action. At the base of many slopes, a streamlet, rivulet, stream or river may be available for carrying away all the waste material that has been moved downslope through failure of one type or another.
In the absence of such a moving surface body of water, the waste material may accumulate at the base of the slope to create a natural barrier for future failures. But where running water is available at the base the slope failure process is greatly accelerated. Similarly, many slopes along the river banks fail due to undercutting by water at the banks.
(3) Composition of the Mass:
Some materials are stable in a given set of conditions of slope and water content whereas others may be practically unstable under those very conditions. Crystalline igneous rocks like granites and gabbros and massive metamorphic rocks like marbles, quartzites and gneisses may be stable even with vertical slopes, whereas the same cannot be said about chalk – a soft variety of limestone, or shale or claystone or soils. It is, therefore, obvious that a fundamental character, that is responsible for the stability of mass, is its composition, which has both chemical and physical implications.
By composition of the mass is meant in the present study:
(i) Whether the mass is in the form of soil or a rock;
(ii) If soil, whether it is cohesive or non-cohesive, and also, if it is sandy, silty or clayey or a mixture of two or more of these components;
(iii) If rock, whether it is an igneous or sedimentary or metamorphic rock, and also, within each group, to which particular class it belongs.
The role of composition of mass in its stability becomes important because a broad assessment can be made about the stability of a mass if its exact composition (in both physical and chemical sense) is known. Thus, sandstones, for instance, occur in a great variety of types. Fine textured, dense and massive sandstones with siliceous cements may be very stable even at vertical slopes whereas the same rock with ferruginous, calcareous or clayey cements may become unstable at angle of 60° or even less.
Along with composition, the texture of rocks and soil masses play an important role in their stability. Texture indicates the size, degree and manner of packing of the constituent crystals or grains in a rock or soil. As such it controls the porosity and permeability of the mass, two very important characters controlling the infiltration and percolation of water through the mass. Thus, other things being same, compact, dense and impervious rocks are more stable than loosely packed, porous and permeable masses under similar conditions.
(4) Geological Structures:
Geological structures are of great significance in defining the stability of mass, especially in rocks.
These structures may be divided into three classes:
(a) The bedding planes in stratified (sedimentary) rocks.
(b) The schistosity, foliation and cleavage in metamorphic rocks.
(c) The jointing structures, faults and shear zones in all types of rocks and fissures in the consolidated clays.
Relationship of each class of these structures with the stability of mass has been discussed below:
(a) The Bedding Planes:
Many sedimentary rocks are layered or stratified and thickness of layers may range from a few centimeters to many meters. The bedding plane (the surface between any two adjacent layers) is a plane with least cohesion in such a layered mass. In their natural disposition, these layers may be horizontal, inclined at various angles with the horizontal (dipping) or even vertical. The dip of the stratified rocks exerts very important influence on the stability of slopes.
This may be explained in different cases as follows:
Case I:
The layers are horizontal (Dip = 0°). Such rocks forming the slopes of the natural valleys and artificial cuts (Fig. 20.11) are stable at all the angles upto 90°. When they fail, it may be due to presence of secondary jointing or related fractures.
Case II:
The layers are Inclined. In such a situation, assuming that the rock is free from any other type of discontinuities (joints shear and fault zones), the stability of a slope (natural or artificial) will depend primarily on the condition whether the layers are dipping backwards into the mountain or forward into the valley or the cut (Fig. 20.11B).
When dipping into the mountain, (i) the tendency of layers to slide along the bedding planes is resisted by enormous weight of the mountain resting against them. Hence, in such a situation, even a nearly vertical slope should be stable.
When dipping into the valley, (ii) however, the tendency of the layers to slide gets a free play, and sliding may take place where the dip of the layers is greater than angle of internal friction, φ = 30o and the slope angle exceeds the dip angle (Fig. 20.12(B)).
The safe height for a vertical cut on this type of face can be fixed only after proper evaluation of the forces tending to produce failure and those available for resisting the failure.
The dip factor has to be kept in mind while giving cuts in stratified rocks. It may be possible to explain now that cuts parallel to the dip of the rocks are more safe and stable compared to those parallel to the strike of the layers. It is because in the first case, the layers do not dip into the cut whereas in the second case (Fig. 20.12B) rocks do dip into the cut on one face.
It is this face which is more prone to sliding. The condition of stability is further endangered when rocks dipping towards the valleys are interstratified with layers of weaker material such as clay or shale. These weaker layers provide potential plains for failure due to sliding for the entire mass resting over them.
(b) The Metamorphic Structures:
Schistosity, foliation and cleavage structures as found in metamorphic rocks like schists, gneisses and slates respectively, all behave as surfaces of weakness and promote the failure. This is primarily due to the fact that in most cases weathering (deterioration) in these rocks takes place along these planes, making the contacts quite vulnerable.
In metamorphic rocks derived from stratified sedimentary rocks (e.g. slates), the direction of cleavage with respect to the original bedding may cause complex system of weak planes. As such slips may be of common occurrence in the slopes made of the above metamorphic rocks and especially so when these structures are inclined towards the free side of the slope.
(c) The Jointing Structures:
Joints of any type are always to be studied with great caution in rocks making slopes for two reasons:
Firstly, very few rocks are free from these structures which may occur due to tension, compression or shear to which these rocks have been subjected since their formation, and
Secondly, they occur in sets or groups effecting the rock from the surface to considerable depth on macro- or micro-scales, but eventually reducing the shear strength of the mass considerably. While studying the influence of jointing on the stability of a rock slope, it is their geometry, spacing, grouping and above all inclination with respect to the free face of the slope that is given great importance.
Joint sets profusely developed in a mutually intersecting fashion and one set being inclined (Fig. 20.13A-D) towards the free side of the slope are given first importance in the stability analysis in all types of rock slopes. Faults behave similar to joints and bedding planes except that they are generally on macro-scales and amenable to rock treatment solutions. Shear zones are most potential surfaces for rock slips, especially when lubricated with water due to the soft character of the shear zone material.
(B) External Factors:
An analysis of many slope failures makes it clear that in quite a few cases an external factor might have triggered the slide whereas in other cases it might have been one of the causes contributing towards the failure. Among the first category of factors are included vibrations from artificial and natural phenomena.
A mass perched on the slope may be stable but critically – the gravitative forces have not yet overcome its frictional resistance. A slight vibration or jerk to the mass may be sufficient to disturb this equilibrium and the mass becomes unstable. Such vibrations are easily available from heavy blasting (which may also cause fracturing much beyond the rock volume to be blasted out) and heavy traffic on hill roads.
Repeated failure of many hill slopes may partly be due to the cumulative effect of such vibrations. Vibrations due to natural causes such as earthquakes often initiate mass failures on a large scale that may continue much after the quake. The Assam earthquakes of 1897 produced some of the gigantic landslips ever observed in the region.
Another important external factor, often overlooked, is the removal of the support at the foot of the slope, as during excavation for road widening, without due regards to the critical conditions of the stability. The slope that might have been previously stable becomes hazardous after such an excavation. (Fig. 20.13 C). A similar cause for slope failure is loading a critical region of slope from above. It is also a common exercise during highway construction in hilly regions.
The stresses due to this additional load add to the stresses due to the mass of the slope. The resulting stresses may have a different radial dispersion intersecting with the free slope and facilitating failure (Fig. 20.13 D). Removal of vegetable cover especially trees (deforestation) is a third external factor that has been a contributive factor in causing slope failures in a large number of cases, including the notorious Nashri Slide near Ramban on National Highway-1 in Jammu Kashmir.