Investigation and evaluation of the leakage caused by seepage failure in karst reservoir

The karst leakage that occurred after the impoundment on the right bank of Xiaojiagou Reservoir in Nanchuan district of Chongqing has been taken as an example in the investigation and evaluation of karst leakage caused by high-pressure infiltration of the anti-seepage system during the long-term operation of karst reservoir. By means of geological analysis, historical investigation of leakage, drilling, hydrogeological test, geophysical prospecting and connectivity test, the range and causes of karst leakage on the right bank of the reservoir have been identified. According to the geological analysis, groundwater level observation and water permeability characteristics, the dam area is composed of karst rock, where dissolution occurs and fracture develops, and the local water permeability rate is large, hence there exist basic conditions for leakage. Besides, the limited groundwater recharge near the dam site and the low and flat groundwater level under the influence of limestone karst contribute to the hydrodynamic conditions for leakage. Combined with the analysis of temperature field, conductivity field and chemical field of reservoir and leakage point, the areas of abnormal groundwater temperature and conductivity on the right bank are mainly concentrated near the groundwater level of ZK2 and ZK4, and the water of the seepage point does not go through the deep groundwater circulation. The leakage path is short, which may penetrate through the dissolution fracture and small karst hole at the inlet on the right bank of the dam site to the PD5 flat cave section between the dam and anti-seepage line on the right bank. The connectivity test shows that the karst cave (dissolution fracture) on the right bank upstream of the dam have good connectivity with the leakage outlet. Geophysical exploration results show that CT-2, CT-3 and CT-5, the main channels of fracture-type leakage distributed above the elevation of 630 m on the right bank, mainly control the leakage of dam foundation and around dam.

Study results are shown as follows. (1) The leakage boundary of the reservoir is clear, and the leakage range is mainly above the elevation of 630 m in the soluble rock area on the right bank. The seepage failure of the anti-seepage body is the main reason for the leakage of reservoir. (2) The first leakage path is through the weak area of the original plugging plug or the surrounding of the karst plugging body, resulting in a new channel for leakage to converge into the karst cave outlet downstream. The second bypasses the dam shoulder on the right bank through anti-seepage line before leakage converges into the karst cave downstream of the right bank slope along the erosion fissures or pipelines of rock mass, and then it seeps out on the surface. In this study, the method of investigating and evaluating the leakage caused by seepage failure of anti-seepage body in karst reservoir has been proposed as illustrated by the project. To preliminarily determine the cause and range of seepage, it is advisable for us to analyze historical data and use such methods as exploration, testing and geophysical prospecting. The study results may provide a reliable basis for anti-seepage treatment.