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.
For dams founded on soil or piling, the soil or pile foundations should have both upstream and downstream cutoff walls (usually steel sheet piling) under the structure proper, plus a cutoff wall underneath the downstream end of the stilling basin. These cutoff walls prevent piping of the foundation material due to seepage pressure. Every effort should be made to provide a reliable drainage system underneath the dam proper and the stilling basin to ensure pressure relief between the cutoffs. A typical system uses 6-inch-diameter stainless steel well screen encased in a 2-foot-thick filter material, combined with access manholes, for relief of excess foundation uplift pressures that build up in the foundation. PVC (polyvinyl chloride) pipes connect the manholes to equalize the pressure in the system, and gate valves are provided so that the manholes can be unwatered to facilitate routine maintenance of the system.