Piezometers in or near toe drains would only be effective when a downstream blanket has been added and uplift pressures need to be measured. Drainage galleries and tunnels are used mostly in abutments in the United States to intercept and control seepage in fractured rock. Drainage tunnels along extensions of the dam's axis or in downstream abutment areas serve to collect seepage. Piezometers located near the drainage tunnels would indicate the effectiveness of the tunnel. Cut-slope drains can be used to intercept seep- age and collect drainage along abutment slopes while piezometers placed both upstream and downstream of the drains can determine effectiveness of the col- lection system, figure 13-9. d. Downstream Areas. Seepage can migrate beyond the embankment toe particularly in clay shale or fissured formations. Geologic site characteri- zation in most cases will determine the need for piezometers downstream of the toe but if there is any doubt they should be installed in the questionable formations 50 to 150 ft beyond the toe, figure 13-1. Piezometers should also be installed downstream of the outlet works, spillway, and stilling basin if they extend well beyond the toe and are not close to other piezometers. 13-7
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EM 1110-2-1901 30 Sep 86 e. Near Relief Wells. If a relief well system has been installed, pore pressures should be checked in the vicinity to evaluate the efficiency of the system. Piezometers should be located both upstream and downstream of the relief wells and should intercept the stratum being drained, figure 13-3. If there is a line of wells, piezometers should also be installed generally at the midpoint between the wells or at the point expected to have the highest pore pressures. f. Spillways, Stilling Basins, and Outlet Works. Underseepage control beneath the stilling basins of spillways and outlet structures founded on per- vious foundations is generally provided by drainage blankets supplemented in the case of stratified foundations by deep well systems (figures 13-10 and 13-11). Drainage blankets extend beneath the chute slabs, if necessary. As shown in figure 13-10, piexometers should be installed to check the effective- ness of the drainage blanket and relief wells, and to check pore pressure beneath the outlet channel and against the stilling basin walls. Piezometers are used to check pore pressures occurring below the relief well system (fig- ure 13-10). Generally, a sheet pile wall, with a minimum penetration of 15 ft, is installed along the downstream toe of the stilling basin to control piping and a piezometer installed downstream of the wall is needed to deter- mine the effectiveness of the wall. 13-3. Flow Measurements.
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