Upward Pressures under Dams: Experiments by the United States Bureau of Reclamation

Uplift measurements have been made by the United States Bureau of Reclamation on three gravity diversion dams and one gravity storage dam. Two of the diversion dams, the Colorado River Dam, on the Grand Valley Project, in Colorado, and the Percha Dam, on the Rio Grande Project, in New Mexico, are founded on gravel. The other diversion dam, the Willwood Dam, on the Shoshone Project, in Wyoming, is founded on sandstone and shale, thoroughly grouted. The gravity storage dam, the recently completed American Falls Dam on the Minidoka Project of Idaho, is built on a columnar basalt foundation, also thoroughly grouted.The measurements at the Colorado River and Percha Dams indicate that the materials deposited above a dam built on porous foundation act somewhat as a filter. When the filter surface is thoroughly seasoned, it offers considerable resistance to the passage of water, and the uplift pressure is partly relieved. When the filter surface is disturbed, as in time of flood, the uplift pressures are greater. The results are too indefinite to permit an attempt to prove, or disprove, any of the theories of flow under dams of this type.The worst uplift conditions occur with a freshly disturbed filter surface, and experimental data taken under any other circumstances should be used with care. A limited a rea of escape at any point below the filter surface as at the Colorado River Dam, causes an increase in pressure above the constriction, and a decrease below.At the Willwood Dam there is, generally, no evidence of appreciable uplift for that portion of the dam founded on shale. For sandstone the uplift pressures vary with the up-stream and down-stream water levels, in a reasonably regular way, and the uplift appears to vary, roughly as a straight line, from full tail-water depth at the toe to full tail-water plus one-fourth the difference in tail-water and head-water elevations at the heel.The results at American Falls are gene rally similar to those at Willwood. Although there are no shale areas at this site there are a few apparently water-tight holes. The uplift pressure intensities are on the average slightly higher, relatively, than at Willwood.