SMALL DAMS

G eological and geotechnical studies
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!" structural indications, depending on the nature of the ground, e.g. stability of the
wall, intense cracking or crushing, percentage recovery or rate of undisturbed sampling
(total length of core over footage drilled), cracking factor (median of the length of the
samples within a sampling pass), RQD 1 if the nature of the ground is suitable (RQD is
not very meaningful in a very anisotropic rock, schistosised rock in particular);
!"hydrogeological comments such as the water level during and at the end of drilling,
fluid losses during drilling, ingress of water, artesian springs, water tests, etc.;
!" any information about how the drilling went: start and finish dates, nature and
dimension of drilling tools and temporary tubing, any final fittings (piezometer, etc.),
position of the limits between core sampling passes, miscellaneous incidents (collapse,
falling tools),water levels at the beginning and end of each shift or day.
The number, spacing and depth of the drillholes cannot be set in absolute figures but
must be defined during the previous investigations, taking into account the specific
features of each site, how heterogeneous the ground foundation is, and the spatial
scale of lateral variations in facies, as well as any problems that may crop up.
The drillholes should be set out as far as possible in harmony with other investigation
techniques (either by filling in any gaps in the observations or by permitting mutual
calibration or by bringing a response to questions posed by those techniques). Drilling
that only goes through rock may in general be oriented in any direction with no major
difficulty. In loose overburden, the drillholes should be sloped at most 30° from the
vertical.
It is common to drill at least three holes along the axis of the dam (one at the valley
bottom and one at the top of each abutment), but more if the dam's crest length is
greater than 100 metres. A horizontal spacing of 50 metres and vertical spacing of
10 metres between consecutive drillholes is recommended but those figures may be
too high in some cases.
If the base of the dam extends more than about 100 metres on either side of the dam
axis, two additional lines of drilling (at the dam's upstream and downstream limits)
may be required, especially when the foundation is very heterogeneous or of mediocre
quality.
Core samples may be accompanied by water tests (LUGEON, under pressure, in
rock; LEFRANC, by natural flow, in loose overburden), especially on the axis of any
watertight elements. Getting meaningful results from water tests requires the use of
suitable equipment complying with standardised operating conditions and carrying
out tests in a rational manner:
!"drilling must be exclusively with clear water (no bentonite or biodegradable slurry)
and before each test the drillhole wall must be cleaned (by successive passes of the
tool with water injected in until clear water comes out at the drill head), in order to
remove any fine deposits ("cake") that could clog the pores and cracks responsible
for permeability and thereby distort measurements;
1. Rock Quality Designation = total of the lengths of the core samples greater than 10 cm / length of the
corresponding drilling.