Equation (3.30) can be developed further for application to multiple-
plane sliding surfaces within a complex foundation (Corns, Schrader and
Tarbox, 1988). Recommended minima for limit equilibrium factors of
safety against sliding are FLE2.0 in normal operation, i.e. with static load
maxima applied, and FLE1.3 under transient load conditions embracing
seismic activity.
(b) Comparative review of sliding stability factors
The expressions defining factors FSS,FSFandFLEdiffer in their concept of
sliding stability. They also differ in their relative rigour and sensitivity to
the shearing resistance parameters cand tan. The apparent margin of
safety against sliding failure demonstrated by a dam is therefore depend-
ent upon which stability expression is applied. Identification of the most
suitable expression requires considered assessment of the limitations of
each with respect to the prevailing conditions, notably with regard to foun-
dation complexity and integrity. Confidence in the choice of sliding
expression is contingent upon the quality of the foundation investigation
programme.
The shear friction stability factor, FSF, is very sensitive to the input
values of cand tanemployed. Confidence in the latter is in turn related
to the quality and quantity of test data, and hence to the adequacy of the
foundation investigations.
The limit equilibrium factor, FLE, is a concept of relatively recent
origin (USACE, 1981). In its logic it conforms to wider definitions of
stability from soil mechanics practice. FLEshares the input sensitivity of
the shear friction factor, and is considered to be more applicable to dams
resting on less competent foundations. The method is discussed fully in
Nicholson (1983).
It must be stressed that values for FSS,FSFandFLEcannot be directly
correlated. The stability factor and sliding criteria most appropriate to a
specific dam are determined by the designer’s understanding of the con-
ditions. An element of uncertainty must always persist with respect to con-
ditions below the base interface, irrespective of the extent of the
investigations carried out. Comprehensive studies are necessary to mini-
mize uncertainty regarding the presence of low-strength layers or inclu-
sions, or discontinuities containing undesirable infill material with low
shear resistance. Table 3.7 provides an illustrative comparison of calcu-
lated sliding stability factors for a hypothetical simple triangular gravity
profile.
142 CONCRETE DAM ENGINEERING
