Seepage Failure Mechanism of the Gouhou Rockfill Dam During Reservoir Water Infiltration

The catastrophic failure of the 71-m high Gouhou concrete-faced rockfill dam in Qinghai Province, China, has drawn many studies. This paper aims at providing new understanding on the modes and process of this failure, which could be applicable to similar dam failures. The geometrical and hydraulic criteria for internal erosion in rockfill materials are assessed. Unsaturated-saturated seepage theory is used to analyze the Gouhou dam since the rockfill materials of the dam were unsaturated before reservoir water infiltration. The water infiltration is simulated by several cases of transient seepage analyses. According to the study, a perched-water table formed in the dam when the reservoir water infiltrated into the rockfill from the top of the concrete face. The perched water spread nearly horizontally along stratifications in the rockfill and exited from the downstream slope at a high elevation as observed before the breach of the dam. The hydraulic gradient of the perched water table was the highest near the wetting front, which might provide necessary hydraulic conditions to trigger internal erosion and piping failure. Based on the geometrical conditions of the rockfill materials and the hydraulic conditions in the dam, the susceptibility of internal erosion is evaluated and the possible modes and process of seepage failure of the dam are described.