Geotechnical Engineering

DHARM

STABILITY OF EARTH SLOPES 325

9.3 Finite Slopes

A ‘finite slope’, as has already been defined, is one with a base and a top surface, the height
being limited. The inclined faces of earth dams, embankments, excavations and the like are all
finite slopes. Thus, the stability analysis of such slopes is of vital importance to the geotechnical
engineering profession.
Investigation of the stability of finite slopes involves the following steps according to the
commonly adopted procedure:
(a) Assuming a possible slip surface,
(b) studying the equilibrium of the forces acting on this surface, and
(c) repeating the process until worst slip surface, that is, the one with minimum mar-
gin of safety, is found.

Failure of finite slopes is cohesive or cohesive-frictional soils tends to occur by rotation,
the slip surface approximating to the arc of a circle as shown in Fig. 9.7.

Tension

crack

Slip

surface

Heave of

material at

toe

To e

(a) Slope in cohesive material (b) Slip surface of a slope in cohesive material

To e

Fig. 9.7 Typical characteristics of the rotational slip in a cohesive soil
The following important methods will be considered:
(i) Total stress analysis for purely cohesive soil:
(ii) Total stress analysis for cohesive-frictional soil–the Swedish method of slices
(iii) Effective stress analysis for conditions of steady seepage, rapid drawdown and im-
mediately after construction
(iv) Effective stress analysis by Bishop’s method
(v) Friction circle method
(vi) Taylor’s method.

9.3.1 Total Stress Analysis for a Purely Cohesive Soil

Analysis based on total stresses, also called ‘φ = 0 analysis’, gives the stability of an embank-

ment immediately after its construction. It is assumed that the soil has had no time to drain

and the shear strength parameters used relate to the undrained strength with respect to total

stresses. These may be obtained from either unconfined compression test or an undrained

triaxial test without pore pressure measurements.