The soft consistency of the clays may necessitate the use of special
sampling techniques. In such situations continuous sampling or in situcone
penetrometer testing techniques offer advantages. Stability and settlement
considerations will require the determination of drained shear strength
and consolidation parameters for the clay.
(c) Dams on pervious foundations
Seepage-associated problems are normally dominant where a dam is to be
founded on a relatively pervious foundation. In a high proportion of such
instances the soil conditions are very complex, with permeable and much
less permeable horizons present and closely interbedded.
(d) Dams on rock foundations
The nature of the investigation is dependent upon whether an embank-
ment or a concrete dam is proposed. Where the decision is still open, the
investigation must cover either option; both require a full understanding
of the site geology.
CONCRETE DAMS
Concrete dam foundation stability requires careful assessment of the fre-
quency, orientation and nature of the rock discontinuities, including the
characteristics of infill material, e.g. clays etc. Foundation deformability
will be largely dependent upon rock load response characteristics and on
discontinuity structure. Rotary coring is widely employed, but to assess the
rock structure reliably on the macroscale it is also advisable to expose and
examine it in trial excavations and, wherever justifiable, by driving
exploratory adits. The latter can be used subsequently for grouting or as
permanent drainage galleries. Abutment stability and deformability are
very important to all types of concrete dam in narrow steep-sided valleys,
and most particularly if the design relies on some degree of arch action.
Detailed investigations should, therefore, extend to the abutments, with
particular regard to the possibility of large-scale wedge or block instability
or excessive deformation and yielding. Large-scale in situloading tests to
evaluate the strength and load-response characteristics of the rock, while
costly, should be conducted in parallel with laboratory testing whenever
practicable.In situtests of this nature can be carried out in exploratory or
drainage adits, or at suitably prepared exposures, e.g. in excavations.
In situpermeability testing is generally conducted through use of
borehole packer tests, but the proper interpretation of field permeability
data can prove difficult.