DESIGN FEATURES AND CONSTRUCTION 165
3.5.2 Cut-offs and foundation groutingThe cut-offs under modern concrete dams are invariably formed by grout-
ing. The relatively shallow key trench constructed under the heel of some
dams contributes little to seepage control. Grouting below concrete dams
falls into two categories, as shown in Fig. 3.17.
- Curtain grouting.The purpose of curtain grouting is to form a partial
cut-off to limit seepage and, in theory, to modify the downstream pres-
sure régime. The primary grout screen or curtain is formed by drilling a
regular series of holes, typically at intervals of 2–3 m, from a narrow
scarcement or platform extending upstream from the heel and injecting
a grout, generally cement based, under pressure. The curtain depth is
frequently comparable to the height of the dam and extends beyond
either abutment as required. Additional grouting may be done through
intermediate or additional secondary holes introduced as necessary. - Consolidation grouting.The primary aim of consolidation grouting is
to ‘stiffen’ and consolidate the rock in the critical contact zone imme-
diately under the dam. It also assists in reducing seepage in the
contact zone, where the rock may be more fissured or weathered
than at greater depths. Care is required over the grout injection pres-
sures employed to avoid disruption, fracturing and the opening up of
horizontal fissures.
Examples of grouting practice are discussed in Thomas (1976) and Bruce
and George (1982).
3.5.3 Uplift relief drainsFoundation uplift relief is effected by a line of drainage holes close down-
stream of the grout curtain (Fig. 3.17). The holes are generally about
75–100 mm diameter spaced at 3–5 m centres, and are drilled from the
inspection gallery (see below).
Uplift within the dam is relieved by formed holes running the full
height of the structure and located close to the upstream face. They should
be at least 200 mm in diameter to inhibit blocking by leached out calcareous
deposits, spaced at about 3 m centres, and relieve into an outlet drainage
channel on the gallery invert. It is important to design the relief drain system
such that drains may be reamed out or re-drilled in the event of blocking.
Relief drain efficiency is a function of drain geometry, i.e. diameter,
spacing and distance to the upstream face. A comparative estimate of effi-
ciency for different drain configurations may be obtained from Fig. 3.18
(Moffat, 1984).