Stability of slopes


A slope refers to the sloped boundary exterior amongst air and the body of a fortification like the highways, cut or fill, earth dams, levees and also the river training works. The constancy of the slope is a very critical aspect in the civil engineering practice. A properly common engineering breakdown of slope is gliding of a ridge or cutting. The feature leading to instability can normally be categorized into:

  1. Those causing augmented stress and
  2. Those leading to a decrease in strength.

Various factors leading to augmented stress are:

a)  Amplified unit mass of soil by dampening

b) Augmented extra loads (the shifting loads, buildings etc.)

c) Steepened slopes moreover by excavation or by the process of erosion

d) shock loads

loss of strength may happen because of

a) Vibration and tremors

b) Upsurge in humidity content

c) Cold and defrosting action

d) Upsurge in pore pressure

e) Forfeiture of cementing pressure

Thusly a sensible plan of the slant of these structures is exceptionally basic since an auxiliary failure may prompt to loss of human life and cash. The regular technique for a slant steadiness examination of characteristic inclines and inclines framed by cutting and filling depend on restricting the equilibrium position.

In this sort of investigation, the component of safety as to the slant dependability is assessed by analyzing the state of equilibrium when nascent failure is expected along a foreordained failure plane and afterward contrasting the quality important with keep up balance with the accessible quality of the dirt. Solidness investigation of slants depend on the accompanying suppositions:

1. The problems have to be two dimensional

2. Coulomb’s hypothesis can be utilized to register shear quality and shear quality is expected as uniform along the slip surface.

3. The flownet if there should arise an occurrence of drainage can be drawn and leakage powers assessed.

The dependability investigation could be part up into two classes:

  1. The dependability investigation of limitless slants
  2. Stability analysis in case of finite slopes

Stability analysis of the infinite slope: The limit equilibrium technique is basically utilized for the examination of finite slopes. Slopes ranging to infinity do not occurs in nature. For different practical reasons, any slope of higher extent with soil conditions fundamentally same for all equal stature below the surface are called the infinite slopes.

a) Infinite slopes in dry sand: The figure 1 demonstrates the failure circumstances for an infinite slope of cohesion less soil.

The issue of safety of an immeasurable slope is demarcated as the ratio of soil strength in the essential shear stress for balance. The issue of safety against sliding is expressed by:

clip_image002

Where  clip_image004 =shear strength

clip_image006 = mobilized shear strength due to gravity

clip_image008 = angle of internal friction

i = inclined angle of slope

Failure condition for an infinite slope of cohesionless soil

b) Infinite slope in clip_image012 soils: In  clip_image012[1]  soil, the slope is steady as long as the slope angle i is equivalent to or minimum the angle of internal friction clip_image008[1] . If the slope angle i, is higher than  clip_image008[2] , the slope could be steady only upto incomplete stature called critical height is shown by:

clip_image014

Where, c= cohesion

clip_image016 = unit weight

clip_image008[3] = angle of internal friction

i = slope angle

clip_image018 = critical height

If the factor of safety Fc is utilized in cohesion, the equipped cohesion at depth H, given by

image

Then the depth H computed by utilizing mobilized cohesion Cm will not be acute. the issue of safety against stature also signifies the feature of safety with respect to cohesion Fc.

Fc is shown by

image

A dimensionless parameter called a stability number is normally utilized for the examination of slopes of  clip_image023  soils and can be known by the subsequent computation

image

Where Sn= stability number (a dimensionless quantity)

The reciprocal of stability number is called stability factor.

Fig.2 demonstrates the failure form of an infinite slope of cohesive soil.

Failure condition of an infinite slope of cohesive soil

Figure 2- failure condition of infinite slope of cohesive soil

Stability analysis of finite slopes:

Failure of finite slopes happens alongside a bent surface. In stability examination of finite slopes, the real surface of rift is substituted by a curve of a round. As to the manner of breakdown, the slope might fail essentially in the following two categories:

a) The failure surface passing via the toe of the slope or beyond the toe of slope is called slope failure.

b) The rupture is deep seated and passes through the ridge retaining soil below the toe of the slant is called base failure.

Slope failure above toe (or face failure) a) Slope failure above toe (or face failure)

 Slope failure through toe (or toe failure) b) slope failure through toe

Base failure c) Base failure

The base failure normally happens specifically when the soil below the embankment is weaker and more elastic than the slant creating soil itself. There are different techniques easily available for the constancy analysis nonetheless the subsequent methods are simple and extensively apploed

1. Slip circle method (Swedish circle technique)

2. Friction circle technique

Swedish circle method or method of slices: The technique accepts the surface of sliding is an curve of a circle. This was recognized by learning the failure or embankments in Sweden. (fig.4)

Analysis of the purely cohesive soils ( clip_image035 = 0 analysis)

Consider a probable spherical slip surface AD (fig.4) with centre at O.

Slip circle: Cohesive soil

The disturbing moment of the cylinder of the soil about O= Wx

Where ‘x’ is the distance of the line of action of W from the vertical line passing through the center of rotation.

If Cu is the unit cohesion, L= length of the slip arc,

clip_image039

The shear resistance developed along the slip surface will be equal to Cu.L.

The resisting moment avoiding the soil from shifting is all because of friction along arc length AD which has a lever arm equivalent to radius r about O.

Resisting moment = cohesion x arc length AD x r = CuL.r

Factor of safety against sliding = (Resisting moment / Disturbing moment)

clip_image041

A sequence of slip circles is checked, and the lowest factor of safety is the most probable failure plane.

clip_image023[1] Soil (clip_image023[2] Analysis)

In order to study stability of the slope of a  clip_image023[3]  soil, a possible slip circle is selected and separated into bands of equivalent width as revealed in fig 5 (a) and (b).

Slip circle: Friction soil

One strip

Disturbing moment about centre O = T x r

Total driving moment =  clip_image047

Where  clip_image049 = algebraic sum of all tangential components

The resisting force on one strip is made up of cohesion and friction and is given by:

Resisting force=  clip_image051

Resisting moment on one strip=  clip_image051[1]

Total resisting moment =  clip_image053

clip_image055

Where  clip_image057 = sum of all normal components, L= arc length

Factor of safety

clip_image059