A well-executed grouting program gives a much-desired measure of safety.
The most common design consists of a single line of grout holes located near the upstream face of the dam, drilled at 5-foot centers and to a depth ranging from four-tenths to six-tenths of the maximum hydrostatic head on the base of the dam. A corresponding line of drainage holes is drilled a few feet downstream from the grout curtain and to a depth roughly two-thirds to three-quarters that of the cutoff curtain. This grout curtain may be constructed by drilling and grouting from a gallery within the dam, from the top of a specified thickness of concrete, or from the top of foundation rock. If a gallery is provided, then a series of drain holes will be drilled from the gallery and located just downstream of the grout curtain. See Figure 1 for a layout and details of a grouting gallery with foundation grouting holes and foundation drain holes. It is essential to control the grouting pressures so that splitting and lifting of rock will not occur. Thin-bedded rocks are especially susceptible to damage by excessive grout pressures.
When a stilling basin (also referred to as an “apron” or “bucket”) is founded on rock, drain holes should be provided in the rock with a collector and discharge system at the founding level for partial relief of the pressure differential.