EMBANKMENT DAMS
Foundation seepage is less critical than for the concrete dam, as seepage
paths are much longer. Discontinuity shear strength is generally of less
importance, but deformability and settlement involving determination of
elastic moduli etc. may be a significant consideration if a decked, i.e.
upstream membrane, embankment is contemplated.
As with the concrete dam, discontinuity shear strength, elastic
moduli and related rock parameters are best determined from large-scale
in situtests wherever practicable and supplemented by appropriate labora-
tory tests. Moduli can be determined in the laboratory using cylindrical
specimens of intact rock recovered from boreholes, as can uniaxial com-
pressive strength, e.g. by direct compression or by point load tests etc.
Intact moduli obtained from such small-scale specimens must be adjusted
to values appropriate to the prototype rock mass, as governed by the dis-
continuity pattern, by application of a reduction factor jc, thus:
EmassjcElab.
The reduction factor is related to fracture spacing and other geological
and physical characteristics, including degree of weathering. It may be as
low as 0.1 in some circumstances, ranging up to 0.5 or more in others.
Durability of rock for use in rockfill and/or as upstream face protec-
tion may also require to be assessed where an embankment dam is
contemplated. This will necessitate chemical, attrition and accelerated
weathering tests to study longer-term degradation.
(e) Dams on karst foundations (carbonate rocks, e.g. limestones etc.)
The presence of extensive solution cavities and fissures renders all such
sites peculiarly difficult. It is essential that the extent of the karstic fea-
tures and, most importantly, their configuration in terms of void con-
tinuity be established. Geological studies can be useful for initial
interpretation of the karstic landforms as a guide to the planning of
detailed investigation. Aerial survey often reveals shallow karstic cavities,
and geophysical methods can also be of value. It will be necessary to
confirm the size and nature of all features initially identified by geophysi-
cal or other indirect techniques through drilling and other direct methods
of investigation.
30 ELEMENTS OF DAM ENGINEERING
