Evaluation of Seepage from an Embankment Dam Retaining Fly Ash

Observations, data, and analyses used to investigate the cause of fly ash-laden seepage from the right abutment of an earthen dam are presented herein. The investigation shows that the sediment-laden seepage occurred through permeable/jointed bedrock in the right abutment that was exposed by a landslide prior to construction of the dam. When the level of the impounded fly ash reached the level of the prior landslide, the fly ash-laden seepage migrated through the jointed bedrock of the abutment and exited on the downstream right abutment. The joint bedrock was exposed to the fly ash reservoir because the landslide removed the clayey colluvium and/or residual soil overlying the jointed bedrock that formed a natural impervious barrier to seepage. This sediment-laden seepage initially was a great concern because of the potential for erosion and piping in earth dams. However, the rapid investigation into and subsequent monitoring of the seepage revealed that accumulation of fly ash and other coarser particles created a filter cake that reduced the seepage and eventually sealed the joints and fractures in the sandstone abutment. No fly ash-laden seepage has been observed on the downstream abutment since April 2004 after first appearing on February 16, 2004. This filter cake development and self-healing process averted additional seepage and illustrates the beneficial effects of fly ash-laden seepage in controlling reservoir leakage.