Evaluation of groundwater flow through a high rockfill dam foundation in karst area in response to reservoir impoundment

Constructing high dams in karst valleys remains a challenging issue as karst conduits provide preferential channels for flow through the foundations. The uncertainty of the karst system drastically increases the risk of leakage and hence the difficulty in seepage control at the foundations. This study examined the transient flow behaviors through a karst foundation on which a rockfill dam with a height of 219 m had been constructed. During reservoir filling, an abnormal pressure rise occurred in the foundation downstream the grout curtains, typically with a magnitude of 53 similar to 69 m higher than the pore pressure head at the neighboring positions. This phenomenon was comprehensively investigated by site characterization, inverse modelling, numerical simulation and cross-correlation analysis, demonstrating that the abnormal pressure rise is locally induced by eroded fault (left bank) or contact zones between karst formations (right bank). These conduits have an effective permeability value one order of magnitude higher than that of the karst formations, which quickly transmit the reservoir pressure pulse across the grout curtains, as evidenced by a higher cross-correlation and a smaller lag between pool level and piezometer measurements. The grout curtains have sufficiently low permeability in limiting the amount of leakage through the foundation, and hence ensure the safety of the reservoir operation in spite of the pressure rise. This study confirms the applicability of the equivalent continuum model in addressing groundwater flow problems in karst formations, as long as the site conditions are properly characterized.