Seismic Rehabilitation of River Mill Dams

The River Mill hydroelectric project consists of two large Ambursen (buttress type) dams constructed in 1922 on the Clackamas River in Oregon. The first of these dams is an intake/powerhouse section located adjacent to and contiguous with the powerhouse. The second of these dams is a spillway section located in an adjacent channel next to the powerhouse dam. Three dimensional finite element dynamic analysis showed that the dams were incapable of withstanding the effects of loads from large earthquakes. Specific problems included out of plane bending of the buttresses, and also excessive shear loads in the corbels that support the upstream face slabs. Two methods of remediating the dam were evaluated in detail: concrete infill and strengthening of the structure. Infilling the dam with concrete was considered as complete infill using roller- compacted concrete or as a partial infill using formed pumpcrete. The latter method was determined to be the less costly variant of the infill scheme. However, strengthening the structure was initially found to be both technically feasible and considerably less costly than either type of concrete infill for implementation at each of the dams. The preferred method of strengthening the buttress walls consisted of constructing reinforced diaphragm walls running parallel to the dam axis between the buttresses in alternating bays to eliminate the out-of-plane bending problem, and using drilled-in post-tensioned anchors to solve the corbel shear problem. However, two events influenced the options ultimately selected for final design. The first was the discovery of extreme low strength concrete in the powerhouse dam shortly after the beginning of the feasibility study. The second was a significant increase in the regional seismicity above that upon which the original dynamic analyses and feasibility study had been based. The combined effect of these two events increased the cost of strengthening the powerhouse dam to the point where the infill option became more economical and was implemented as the final design for that dam. Strengthening of the spillway dam with the diaphragm walls and post- tensioned anchors remained the most economical solution and was implemented as the final design for that dam.