Seepage and Stability of a Dam in an Artesian Setting

Willow Creek Dam is located in a unique region on the boundary of a confined aquifer, where artesian conditions exist. Recently, groundwater levels have exceeded the elevation of the downstream berm well above anticipated piezometric elevations. It was presumed from limited evidence that these high piezometric levels resulted from pressure in the aquifer rather than from seepage out of the reservoir. The purpose of this investigation was to determine the relative contributions of the reservoir and the regional groundwater to observed piezometric levels and how these levels affect seepage exit gradients and slope stability of the dam. Comparing the head levels in shallow and deep piezometers at the dam, the seepage is shown to originate from the regional aquifer and to flow upwards. The head in the groundwater exceeds the reservoir elevation, and the reservoir elevation has little influence on local phreatic elevations. The largest annual fluctuations in the groundwater levels result from local irrigation pumping. Influences of irrigation wells were quantified using a well influence statistic derived for this study. This statistic was found to be the best predictor of variation in groundwater elevations at the dam. Using a finite element analysis program, it was shown that the regional aquifer provides all the flow into the relief well system and solely contributes to the exit gradients long the downstream toe. Flow rates in the dam’s 27 pressure relief wells were found to be less than the flow rates when the reservoir was first filled, even though piezometer levels are much higher now. This points to a need to redevelop the pressure relief wells. Resulting higher relief well flow will decrease the piezometric levels at the dam and will result in reduced exit gradients. Local and global slope stability was not found to be in jeopardy with local seepage failure occurring well before stability failures. Advisor: John R. Rohd