Uplift Reduction at Upriver Dam, Spokane, Washington

The reinforced concrete Upriver Dam is an eight-bay, radial-gated spillway, 35 feet high by 230 feet wide, on the Spokane River. This dam was originally constructed in 1895 and extensively modified in 1936. Drains were installed in the spillway dam rollway and apron during the 1936 modification to relieve uplift; nevertheless, during an August 1989 Part 12 FERC safety inspection, these drains were observed to be largely plugged. The one apron that was working on the side of the left abutment was piping sand and gravel. Screens were added to the drains, and additional holes were drilled in the apron to enhance drainage capabilities. The holes added as part of this effort soon plugged, which was believed to be caused by insufficient screens and insufficient capacity to match the large flows. An engineering investigation was then commissioned, which, in turn, led to the preparation of construction documents to correct the drainage deficiency and high uplift. During the bidding process, however, an emergency incident occurred beneath the apron near the left (south) abutment that involved very high uplift pressures. The City of Spokane responded by lowering the reservoir 10 feet to reduce the pressure. As part of the repairs, an upstream geomembrane liner was installed and new drains were installed in the left abutment. This reduced piezometric levels for onIy about four months, upon which time they inexplicably increased to near prerepair levels. Additional piezometers were installed beneath the spillway and in the abutments, and an extensive underwater survey was conducted. Based on the information gathered from the survey and the piezometer readings, a decision was made to chemically grout the left abutment where piezometric levels were the highest. This process only slightly reduced piezometric levels. By January 1992, despite significant repair work to reduce uplift pressure at the Upriver Dam, it became clear that while such pressures were relieved in several peripheral areas, one target area had not improved-and had, in fact, become slightly worse. The following account provides an overview of the dam structure, reviews the initial repair attempts, examines the reasons why these repairs could not permanently reduce uplift, and describes the solution that finally succeeded in reducing uplift and eliminating piping beneath the Upriver Dam.