GE
Stability of slopes plays an important role in the earth embankments or in cuttings required for railways roadways, earthen dams, river training, levees, highways Slopes may be artificial or may also natural as in Hillside and valleys, coastal, river clifts etc. Hence in case of slopes may be artificial or natural, the force exist which tend to cause the soil to move from high points to low points. If actual movement of soil mass occurs, it is a slop failure Force of gravity and seepage force are mainly responsible to cause the instability of Lipes in the areas of seismic activity, the earthquake forces is also an important factor causing instability of slope. Stability analysis is made commonly by using a Lit equilibrium approach. In this method, the shearing resistance required to maintain a limiting equilibrium condition is compared with the available shearing strength of the soil. The forces causing the instability induces the shearing stresses throughout the soil muss is termed as actualing force. Gravity force, seepage force and earthquake force are actuating forces. The surface along which the failure occurs in the form of mass movement of soil due to larger shearing stress is called as slip surface. Analysis of stability of slopes requires the determination of the actuating forces such a gravity forces, seepage forces and earthquake forces
Infinite slopes Finite slopes Homogeneous slopes Non-homogeneous slopes Slopes can be a natural slopes or man-made slopes. When the stope is formed with a continuous process of erosion and deposition of soil caused by natural agencies, then it is called as natural slopes. Examples of natural slopes: River banks, hill sides or mountain sides are the examples of natural slopes. When the slope of earth structures is made by or constructed by human, then it is called as manmade slopes. Example of man-made slopes: Slopes of embankment of dam, canals, roads, highways, railways, cutting for roads and rails, filling for reclamation trench excavations etc are the examples of manmade slopes. The boundary surface of a semu-infinite soil mass having the constant soil properties for all identical depths below the surface is called as infinite slope. The Fig. shows an infinite slope with its traverse extent large as compared to the depth of its failure zone. Slopes extending to infinity do not exist in nature. When the slope is having the limited extent, then it is called as finite slopes. Most man-made embankments and road or railway, cuts etc. are finite in extent. Examples of finite slopes: Inclined faces of earth dam, embankments, cuts etc are the examples of finite slopes. Fig.shows the finite slopes. When the slopes are made up of same materi: I more or less within the failure zone, then it is termed as homogeneous slopes. When the slopes are made up of different earth material or when the failure surface goes through two or more zones of different soil properties, then it is termed as non-homogenous slopes. Infinite slopes Finite slopes Homogeneous slopes Non-homogeneous slopes
There are various failure types of infinite and finite slopes. The failure of the slopes can be in the form of plane slide (i.e., translational slide) or rotational slickfailure along essentially planer surface is called translational slides: For an infinite slope in a homogeneous soil, failure of slope take place due to sliding of the soil mass along a plane parallel to the ground surface at a certain depth. In short, for long steep slope, failure of slope will be along a surface parallel to the ground surface and in such case sliding surface is plane as shown in Fig. There are three basic modes of failure in case of finite slopes. Face failure or slope surface Toe failure Base failure When the failure occurs along a surface of sliding interests the slope above the toe, then slide is termed as slope failure or face failure. This type of failure occurs due to very high slope angle (i) and soil close to the toe is quite strong or the soil the upper part of the slope is comparatively weak. Fig. shows plane slide and rotational slide respectively in case of slope failure, face failure. When the failure surface passes through the toe of slope, then it is termed as toe failure. Toe failure is most common mode of failure and occurs in steep slope consisting of the homogeneous solid mass above and below the base. When the failure surface passes below the toe of slope, then it is termed as base failure. This type of failure occurs due to the weaker material or relatively weak and soft soil at the base than that of slope. Fig. shows the base failure. Face failure or slope surface Toe failure Base failure
The slipping along the slip is at critical equilibrium is resisted by the total cohesive force (CL). This force is proportional to the cohesion C and the height H of the slope. The force responsible for instability of i se slope is the weight of the wedge which is equal to the unit weight 
. Let (F.S) c be the factored of safety for cohesion. Here
is the dimensionless quantity which is called as Tayler's stability number (Sn). Consider Cm be mobilised unit cohesion which is essential so as to have the equilibrium of a slope having height H. Also, for critical height H the factor of safety with respect to height (H) is equal to the factor of safety with respect to cohesion (C) Taylor analysed the friction circle method and determined the values of stability number (Sn) for finite slopes and prepared the table as shown in Table gives the values of Sn for the various values of and slope angle (i). When a dangerous circle passing below the toe occurs, then stability numbers such as (0.145). (0.068) and (0.023) mentioned in Table indicates the most dangerous circle through toe. The Fig. shows Taylor's stability number charts for 
=0 The Swedish Slip Circle approach assumes that the friction perspective of the soil or rock is same to zero, i.e., tau =c. In different words, while friction perspective is taken into consideration to be zero, the powerful strain time period is going to zero, hence equating the shear power to the brotherly love parameter of the given soil. The Swedish slip circle approach assumes a round failure interface, and analyzes strain and power parameters the use of round geometry and statics. The second due to the inner riding forces of a slope is as compared to the instant due to forces resisting slope failure. If resisting forces are more than riding forces, the slope is thought stable. The balance of earth slopes in keeping with the friction circle approach, as generally employed, is primarily based totally at the circumstance of equilibrium of forces, and at the circumstance that at restriction equilibrium the actuating and resisting forces are concurrent at one factor. This specific circumstance, assumed in area of the overall one in all equilibrium of moments on the stated balance restriction, isn't according with the essential hypotheses of the kinematics of motion; it offers upward thrust as nicely to the indetermination of the couple of values of the soil resistance parameters c (cohesion) and ϕ (friction) at restriction equilibrium. Therefore, the trouble is taken up once more and resolved on the premise of rational equilibrium situations and assuming that at failure in any factor alongside the sliding circle the maximal electricity of the cloth is evolved and consequently restriction values of c and ϕ right to failure of the specific soil are uniformly reached alongside the circle itself. On this foundation the circumstance of equilibrium of forces determines one parameter of the distribution sample of the everyday additives of stresses alongside the circle; the price of a 2d parameter of the stated distribution, concerned withinside the circumstance of equilibrium of moments, may be without problems decided in an approximate way. It is consequently viable to decide without problems and swiftly the arm of the whole actuating pressure which realizes the restriction circumstance of the intended sliding motion alongside the taken into consideration circle. The contrast of this arm with the actual one of the aforesaid pressures suggests the diploma of balance for any taken into consideration circle. The extension of the approach to an embankment made of zones of various substances is likewise examined. The Modified Bishops approach is barely distinct from the regular approach of slices in that everyday interplay forces among adjoining slices are assumed to be collinear and the consequent interslice shear pressure is zero. The technique become proposed with the aid of using Alan W. Bishop of Imperial College. The constraint delivered with the aid of using the everyday forces among slices makes the hassle statically indeterminate. As a result, iterative techniques need to be used to clear up for the thing of protection. The approach has been proven to provide thing of protection values inside some percentage of the "correct" values. Taylor’s Stability Number Swedish slip circle method Friction circle method Bishop’s method
When a slope shows the boundary surface Q of a semi-infinite soil mass and the soil properties remains same or constant below the surface for all identical depths, then it is termed as infinite slope. Fig. shows an infinite slope 'PQ" makes an angle T with the horizontal. Note that soil properties and the soil stresses on any plane parallel to the slope surface remains same for an infinite slope. Due to this, the failure of the slope is a sliding of a soil mass along a plane parallel to the slope at some depth. In short for an infinite slope, Soil properties and soil stress on any plane parallel to the slope surface are identical. Sliding of a soil mass along a plane parallel to slope is the failure of slope due to identical soil. Consider a failure plane 'RS' at depth 'Z' below the slope surface PQ. properties and soil stress. let us consider a prism of soil with critical surface or failure plane. inclined length (1) along the slope and depth "Z upto the critical surface. Because of shear, the factor of safety (F-S) of the slope against sliding is as follows Note that, in general shear strength consists of both cohesion as well as internal friction, hence two cases of soil like (1) cohesionless soil and (2) cohesive soll are to be considered separately.
Although many types of mass movements are included in the general term "landslide, the more restrictive use of the term refers only to mass movements, where there is a distinct zone of weakness that separates the slide material from more stable underlying material. The two major types of slides are rotational slides and translational slides. Rotational slide: This is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide. Translational slide: In this type of slide, the landslide mass moves along a roughly planar surface with little rotation or backward tilting. A block slide is a translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a relatively coherent mass. Falls are abrupt movements of masses of geological materials, such as rocks and boulders, that become detached from steep slopes or cliffs. Separation occurs along discontinuities such as fractures, joints, and bedding planes, and movement occurs by free-fall, bouncing, and rolling. Falls are strongly influenced by gravity, mechanical weathering, and the presence of interstitial water. Due to seepage pressure of percolating ground water. During rainy season water content increases which may lead to landslides. Because of alternative swelling and shrinkages of soil mass hair cracks are developed then it may lead to landslides. Due increase load of traffic or accumulated snow on the road surface. Undermining caused by erosion. Due to earthquakes. Due to vibration, faults are formed in bedding planes of the strata. Fissuring of pre-consolidated mass due to release of lateral pressure while doing cutting of rocks. Due to failure of a retaining wall or breast wall. Geological Causes Weak or sensitive materials Weathered materials Sheared, jointed, or fissured materials Adversely oriented discontinuity (bedding, schistosity, fault, unconformity, contact, and so forth) Contrast in permeability and/or stiffness of materials. Tectonic or volcanic uplift Glacial rebound Fluvial, wave, or glacial erosion of slope toe or lateral margins Subterranean erosion (solution, piping) Deposition loading slope or its crest Vegetation removal (by fire, drought) Thawing Freeze-and-thaw weathering Shrink-and-swell weathering Excavation of slope or its toe Loading of slope or its crest Drawdown (of reservoirs) Deforestation Irrigation Mining Artificial vibration Vulnerability to landslide hazards is a function of location, type of human activity, use, and frequency of landslide events. The effects of landslides on people and structures can be lessened by total avoidance of landslide hazard areas or by restricting, prohibiting, or imposing conditions on hazard-zone activity. In order to take preventive measures against landslides, study of causes of landslides is required. We cannot prevent landslides due to earthquakes. But we can prevent or control landslides due to any other cause by following preventive measure: By cutting hill sides in steps i.e., benching of soil slope. By proper surface and cross drainage. By reducing angle of slope. The hazard from landslides can be reduced by avo ding construction on steep slopes and existing landslides, or by stabilizing the slopes. Stability increases when ground water is prevented from rising in the landslide mass by Covering the landslide with an impermeable membrane, Directing surface water away from the landslide, Draining ground water away from the landslide, and Minimizing surface irrigation. Slope stability is also increased when a retaining structure and/or the weight of a soil/rock beam are placed at the toe of the landslide or when mass is removed from the top of the slope. By construction of breast walls and retaining walls. By stone pitching, cement grounding to the hill slopes, which will minimize the erosion und improve the stability. Soil Mechanics and Foundation Engineering by Dr.B.C. Punmia, Laxmi Publication Geotechnical Engineering by T.N. Ranamurthy & T G Sitharam, S Chand Publications. Principles of Geotechnical Engineering by Braj M. Das, Cengage Learning. Geotechnical Engineering by P. Purushothma Raj, Tata Mc Grawhill.
Unit - 6
Stability of Slopes
Actuating force
Slip surface
Key Takeaways:
In case of slopes may be artificial or natural, the force exist which tend to cause the soil to move from high points to low points then it is termed as stability of slopes.
Slopes are classified as follows:
1. Infinite slope:
Fig.6.1: An infinite slope
2. Finite slopes:
Fig.6.2: Embankment
Fig.6.3: Cut slope
3. Homogeneous slopes:
4.Non-Homogenous slopes:
Key Takeaways:
Slopes are classified as follows:
Modes of failure in Infinite slopes
Fig.6.4: failure in infinite slope
Modes of failure in Finite slopes
1. Face failure or slope surface
Fig.6.5: Plane side of Face failure
Fig.6.6: Rotational side of face failure
2. Toe failure
Fig.6.7: Plane side of Toe failure
Fig.6.8: Rotational side of toe failure
3. Base failure
Fig.6.9: Modes of base failure
Key Takeaways:
1.Modes of filure in infinite slopes
2.Modes of failure in Finite slopes:
There are three basic modes of failure in case of finite slopes.
Stability of slopes is determined by following methods:
6.4.1 Taylor’s Stability Number:
∴Weight of the wedge 
H2
We know,
=
=
=Sn …. (1)
Cm=
… (2)
Where C= Cohesion:
(F.S) c= Factor of safety with respect to cohesion
Hence the Equation (1) can be written as follows:
Sn =
=
…. (3)
∴ (F.C) c =Hc/H… (4)
Where (F.C) c = Factor of safety with respect to cobesion
There Equation (3) becomes as
Sn =
=
… (5)
Fig.6.10: Taylor’s stability number charts,
=0 with respect to depth factor
6.4.2 Swedish Slip Circle method:
6.4.3 Friction Circle Method:
6.4.4 Bishop’s Method:
Key Takeaways:
Stability of slopes is determined by following methods:
Fig.6.11: Infinite slope with a prism of soil
Horizontal length of prism 'abcd' of soil = 
Volume per unit length of prism=
Weight of prism (W) =
Vertical stress acting on the surface RS=w/l=
/l
Vertical stress is resolved into two stress components which are normal components and tangential components to the surface 'RS'.
Normal components =
and Tangential components=
t=
Here tangential components are termed as shear stress which develops the failors along "RS" and resisted by the shear strength of the soil.
F. S=
Where, F.S =Factor of safety of slope
=Shear strength
= Shear stress
Key Takeaways:
For an infinite slope, Soil properties and soil stress on any plane parallel to the slope surface are identical. Sliding of a soil mass along a plane parallel to slope is the failure of slope due to identical soil.
Definition:
The term "landslide" describes a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. The materials may move by falling, toppling, sliding, spreading, or flowing.
Types of Landslides:
Slides:
Falls:
6.6.1 Causes of Land Slides:
11. Morphological Causes
12. Human Causes
6.6.2 Remedial of Landslides:
Key Takeaways:
Landslides can be termed as a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these.
References:
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