9. 1 Introduction
Earth slopes may be found in nature or may be man-made. These are invariably required in
the construction of highways, railways, earth dams and river-training works. The stability of
these earth slopes is, therefore, of concern to the geotechnical engineer, since failure entails
loss of life and property.
The failure of an earth slope involves a ‘slide’. Gravitational forces and forces due to
seepage of water in the soil mass, progressive disintegration of the structure of the soil mass
and excavation near the base are among the chief reasons for the failure of earth slopes. Slides
and consequent failure of earth slopes can occur slowly or suddenly.
The slides that occurred during the construction of the Panama canal, connecting the
Atlantic and the Pacific Oceans and during the construction of railways in Sweden spurred the
geotechnical engineers all over the world into a lot of research on various aspects of the stabil-
ity of earth slopes. Swedish engineers were in the forefront in this regard.
Determination of the potential failure surface and the forces tending to cause slip and
those tending to restore or stabilise the mass of earth are the essential steps in the stability
analysis of earth slopes and the available margin of safety. The soil mass is assumed to be
homogeneous. It is also assumed that it is possible to compute the seepage forces from the flow
net and the shearing strength of the soil from the Mohr-Coulomb theory.
The slope may be an ‘infinite’ one or a ‘finite’ one. An infinite slope represents the sur-
face of a semi-infinite inclined soil mass; obviously, such a slope is rather hypothetical in
nature. A slope of a finite extent, bounded by a top surface is said to be finite. Slopes involving
a cohesive-frictional soil are most common; however, the case of purely cohesionless soils is
also treated as a useful introduction to the treatment of c – φ soils.
9.2 Infinite Slopes
An ‘infinite slope’ is one which represents the boundary surface of a semi-infinite soil mass
inclined to the horizontal. In practice, if the height of the slope is very large, one may consider
it as an infinite one. It is assumed that the soil is homogeneous in its properties. If different
Chapter 9
STABILITY OF EARTH SLOPES
318