where the coupled effects of the dam-foundation-reservoir system are
addressed by a substructure analysis procedure.
In this approach the dam is idealized as a two-dimensional plane-
strain or plane-stress finite element system, the reservoir being regarded as
a continuum. The foundation zone is generally idealized as a finite element
system equivalent to a viscoelastic half-space. The complexities of such an
approach are evident, and take it outside the scope of this text.
It has been suggested that the application of more rigorous methods,
such as dynamic analysis, in preference to the simplistic pseudostatic
approach is appropriate if the anticipated PGA will exceed 0.15 g (Charles
et al., 1991). This may be compared with the suggestion of an anticipated
PGA of over 0.25 g made in ICOLD (1989).
Dynamic response analysis is fully discussed by Chopra and also by
Idriss and Duncan, both within Jansen (ed.) (1988). The seismic assess-
ment of two dams in the UK, one an earthfill embankment, the other a
concrete gravity dam, is presented in Taylor et al.(1985).
SEISMIC DAMAGE TO EMBANKMENT DAMS
Earthquake damage to embankments, which could potentially lead to
breachings and catastrophic failure, can take the following forms:
- shear displacement and disruption by fault reactivation;
- instability of the face slopes;
- deep instability involving the foundation;
- liquefaction and flow slides;
- volume reduction and slumping of the embankment fill, leading to
differential displacements and internal cracking (a vulnerable foun-
dation may similarly be at risk);
- damage to ancillary structures, including spillways, outlet tunnels
and culverts;
- instability of valley slopes leading to displacement, overtopping and
instability, or to obstruction of the overflow.
The risk of liquefaction and a flow slide as a result of pore pressure gener-
ation and soil densification is effectively confined to loose, coarser soils
such as sands, typically composed of particles in the 0.06 mm to 0.6 mm
range.
Pseudostatic seismic analysis, described in Section 3.1.1, involves a
major simplification. It assumes that the dynamic effects of seismic distur-
bance can be substituted by static forces derived from assumed peak accel-
erations,, expressed as a proportion of the acceleration due to gravity, g.
Applied to an embankment dam, pseudostatic analysis in its simplest form
introduces an additional horizontal inertia load. This is similar in its influ-
ence to conducting a conventional stability analysis with the embankment
96 EMBANKMENT DAM ENGINEERING
