Numerical analysis of groundwater flow behaviour at a dam site in Karst area during its reservoir impoundment

Abstract

The Jiangpinghe concrete-faced rockfill dam, with a maximum height of 219 m, is located in the karst area in western Hubei Province, China. The bedrocks at the dam site are highly dissolved, in which karst conduits, faults, fractures and weak interlayers are developed. Given the complex geological conditions, a large-scale grouting curtain covering an area of 376,000 m2 was built to control the seepage in the dam foundation. However, the piezometers in the middle section of the lowest grouting tunnel at the right bank manifest an abnormal rise of hydraulic head after reservoir impounding. The hydraulic head varies with a good correlation with the reservoir water level. This abnormal rise of hydraulic head can be attributed to higher permeability of strata where small-scale karst pipes and conduits are developed. In this study, an inverse modelling procedure combined orthogonal design, finite element forward modelling of transient groundwater flow, artificial neural network, and genetic algorithm was adopted to evaluate the permeability of the strata. The inverse results show that the permeability of shallow-buried rocks is one order of magnitude larger than that of deep-buried rocks in the right bank. The numerical simulations show good agreement with field measurements. Based on the inversed results, the groundwater flow behaviour at the dam site was analysed and the performance of the seepage control system was evaluated. It is demonstrated that the abnormal rise of hydraulic head is induced by the permeable structures developed along karst strata, and the seepage control system performs well in lowering the hydraulic head and limiting the amount of leakage through the dam foundation.