Transient Seepage Analyses of Soil-Cement Uplift Pressures during Reservoir Drawdown

Abstract

The south Denver metro area is experiencing significant growth and development, which has resulted in an increased demand for residential and commercial water. A new 61-meter-high (200-foot-high) zoned earthen dam is currently being constructed to provide an additional 9.2 x 10 7 m 3 (75,000 acre-feet) of water storage. Upstream slope protection will consist of soil-cement, which is not free-draining like riprap. During periods of sustained reservoir drawdown, uplift pressures could occur on the soil-cement if the reservoir is lowered faster than water within the upstream shell can drain. Damage to the slope protection could occur if the uplift pressures exceed the dead weight of the soil-cement. A steady state seepage analysis cannot model the changing boundary conditions associated with reservoir drawdown. This paper describes transient seepage analyses that we performed to evaluate possible uplift pressures on the soil-cement. During the transient seepage analyses, we developed unsaturated hydraulic conductivity functions and non-linear transient boundary conditions, and investigated the model sensitivity to its inputs. Parametric analyses indicated that the expected uplift pressures are highly sensitive to the hydraulic conductivity of the upstream shell and drainage provided to it. The model results were used to design a drainage system within the upstream shell that is expected to prevent the development of excess uplift pressures beneath the soil- cement slope protection.