3.2.3 Sliding stability
Sliding stability is a function of loading pattern and of the resistance to
translational displacement which can be mobilized on any plane. It is con-
ventionally expressed in terms of a factor of safety or stability factor
against sliding, FS, estimated using one or other of three definitions:
- sliding factor, FSS;
- shear friction factor, FSF;
- limit equilibrium factor, FLE.
Irrespective of the definition employed, the resistance to sliding on any
plane within a dam will be a function of the shear resistance mobilized in
the mass concrete. The horizontal construction joints (Section 3.5.4) will
generally be the critical internal planes. At the base, concrete–rock bond
and the resulting interface shear strength are the critical factors. Below the
base interface the geological structure and shear strength parameters
within the rock mass are interdependent, and collectively govern sliding
stability.
Sliding resistance in the rock is a function of the surface or path
investigated. It is controlled by geological discontinuities such as faulting,
joints or surfaces with reduced shear resistance, etc. The geological struc-
ture of the rock foundations must therefore be thoroughly investigated,
and the presence, nature, frequency and orientation of all significant dis-
continuities including critical intersections established. Extensive in situ
and laboratory testing is necessary to confirm design parameters.
(a) Sliding resistance: parameters and stability factors
The resistance to sliding or shearing which can be mobilized across a plane
is expressed through the twin parameters cand tan.
Cohesion,c, represents the unit shearing strength of concrete or rock
under conditions of zero normal stress. The coefficient tanrepresents
frictional resistance to shearing, where is the angle of shearing resistance
or of sliding friction, as appropriate (cf. shear strength parameters cand
in soil mechanics usage; Section 2.3.2).
Envelope values for cand tanrecorded within mass concrete, rock,
and at a concrete–rock interface are presented in Table 3.3. The great
range of values encountered within foundation rocks will be noted.
Resistance to shearing within the foundation zone of a dam is deter-
mined by geological structures, rock type and rock integrity. The shearing
or sliding parameters for examples of sound and inferior foundation con-
ditions are set out in Table 3.4. The table also shows values for very low
strength and potentially dangerous geological features, e.g. clay layers or
136 CONCRETE DAM ENGINEERING
