Engineering Geology

E n g i n e e r i n g G e o l o g y
where c 1 = cohesion intercept, f 1 = angle of shearing resistance (these are average values
around the slip surface and are expressed in terms of effective stress), s = total overburden
pressure and u = pore water pressure. In a stable slope, only part of the total available shear
resistance along a potential slip surface is mobilized to balance the total shear force, t, hence:
St = Sc 1 /F + S(s - u) tan f 1 /F (3.6)
where F is the factor of safety. If the total shear force equals the total shear strength, then
a slip is likely to occur (i.e. F = 1.0).
Clay soils, especially in short-term conditions, may exhibit relatively uniform strength with
increasing depth. As a result, slope failures, particularly short-term failures, may be comparatively
deep-seated, with roughly circular slip surfaces. This type of failure is typical of relatively
small slopes. Landslides on larger slopes are often noncircular failure surfaces
following bedding planes or other weak horizons.
The factors that determine the degree of stability of steep slopes in hard unweathered crystalline
rocks (defined as rocks with unconfined strengths of 35 MPa and over) have been
examined by Terzaghi (1962). Terzaghi contended that landsliding in such rocks is largely
dependent on the incidence, orientation and nature of the discontinuities present. The value
of the angle of shearing resistance required for a stability analysis, f, depends on the type
and degree of interlock between the blocks on either side of the surface of sliding. Terzaghi
concluded that the critical slope angles for slopes underlain by strong massive rocks with a
random joint pattern is about 70∞, provided the walls of the joints are not acted on by seepage
pressures.
In a bedded and jointed rock mass, if the bedding planes are inclined, the critical slope angle
depends on their orientation in relation to the slope and the orientation of the joints (Hoek and
Bray, 1981). The relation between the angle of shearing resistance, f, along a discontinuity,
at which sliding will occur under gravity, and the inclination of the discontinuity, a, is important.
If a < f, the slope is stable at any angle, whereas if f < a, then gravity will induce movement
along the discontinuity surface, and the slope cannot exceed the critical angle, which
has a maximum value equal to the inclination of the discontinuities. It must be borne in mind,
however, that rock masses are generally interrupted by more than one set of discontinuities.
Classification of Landslides
Varnes (1978) classified landslides according to the type of materials involved on the one
hand and the type of movement undergone on the other (Fig. 3.8). The materials concerned
were grouped as rocks and soils. The types of movement were grouped into falls, slides and
94