1.6.5 Materials for dam constructionInitial exploration for sources of materials is conducted by a combination
of surface and aerial reconnaissance in conjunction with extensive geologi-
cal surveys. Potential borrow areas or aggregate sources so revealed must
be thoroughly evaluated in terms of the suitability of the materials they
contain for different purposes, e.g. core material, material for shoulder
zones and filters etc. or for aggregates. The quantities which can be real-
ized from a source must also be estimated. Geophysical methods can play
a useful part in this latter process, but they must always be correlated with
hard evidence from natural or trial exposures or from conventional drilling
programmes. Investigation of the suitability of an earthfill or rockfill
material will invariably justify an instrumented trial fill, which can provide
much invaluable data on physical characteristics, compaction character-
istics and plant performance, and on geotechnical design parameters.
The evaluation of earthfill borrow sources is notoriously prone to
overestimation of the available yield of suitable materials owing to unde-
tected variations in soil type or quality. It is therefore essential to prove
quantities of individual fill materials very substantially in excess of the esti-
mated requirement for each, and figures as high as three and four times the
nominal requirement have been suggested as prudent. The percentage of
each material likely to prove unusable or anticipated extraction difficulties
may be critical factors in relation to relative costs for alternative sources.
The proving of sources for rockfill is superficially more straight-
forward than for earthfill. The essential requirement is a source of sound
durable rock, the location of which is generally apparent from the initial
geological appraisal. Investigation of the suitability of the rockfill will nor-
mally require a trial fill and, in the case of excavated or quarried rocks, it
will also be necessary to conduct blasting or ripping trials to determine
rock fragment size, grading and shape, etc. The percentage of oversize or
fine material or the excavation cost for a very dense hard rock will be criti-
cal economic factors.
Aggregate sources for concrete dams include natural borrow areas
and the use of crushed aggregates derived from quarries or excavations. In
checking the quality of aggregates, physical and mechanical properties and
long-term chemical stability, e.g. with regard to risk of alkali–silica reac-
tion (ASR), will require investigation.
1.6.6 Selection of type of damThe optimum type of dam for a specific site is determined by estimates of
cost and construction programme for all design solutions which are techni-
cally valid. Where site circumstances are such that viable alternatives exist