Incorporating Risk Reduction Measures in the Design and Construction of the Seismic Rehabilitation of an Earthen Dam with a Nearly Full Reservoir

This paper discusses risk management steps taken during the design and construction of the seismic retrofit of a 1930’s era 90-foot-tall, 475-foot-long, high-hazard earth filled dam. The retrofit required removal of a large portion of the downstream slope to construct a 110-foot-high buttress, measured from the bottom of the foundation key to the dam crest, while maintaining a nearly full reservoir throughout the construction period. Starting with the design and working through various failure modes to identify construction risks, appropriate stabilization methods and construction procedures were developed to minimize the risk during construction. Risk reduction measures included evaluating, repairing, and upgrading the outlet works prior to the seismic rehabilitation of the dam to ensure the reservoir could be drawn down in case on emergency; dewatering of the downstream slope and monitoring of the effluent as construction progressed; monitoring the slope for movement; modification in the field to the buttress material and the testing requirements to reduce the amount of time the downstream slope was compromised; and close involvement of the approving agency to ensure timely approvals of construction or design changes. Dam construction projects can be complex and unique. Proactive and reactive risk reduction measures are often necessary during the construction phase as conditions often change from what was assumed during the design. This project was no exception and had its challenges during construction that needed to be evaluated and mitigated to ensure the project was completed safely.