CS4C - Concurrent Session 04C: 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 high-hazard earth fill dam that maintained a full reservoir capacity 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. The need to buttress the dam required removal of a large portion of the downstream slope during construction. During construction, the excavation of the downstream slope was steepened to 1.25H:1V to minimize the impact to the dam crest and service utilities in the crest of the dam. The main risk reduction safety measure used to stabilize the downstream slope during construction was accomplished by installing a series of dewatering wells 15 feet below the excavated surface on the downstream slope.

Discharge from the wells was monitored for water level, pumped volume, and turbidity. Additionally, potential slope movement was also monitored using buried Time Domain Reflectometry monitoring along the crest and an inclinometer near the maximum section. Threshold values and action levels were established and procedures were put in place to lower the reservoir in case of adverse conditions.

Dam construction projects can be complex and unique. Proactive and reactive risk reduction measures are often necessary during the construction phase as conditions can 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.