Analysis of seepage effect on the formation of sinkhole in unconfined aquifer-aquitard system caused by groundwater changes

Cover-collapse sinkholes are an extremely complex process that often cause unpredictable geological disaster. They occur abruptly and can lead to catastrophic damages such as human death and injury, property damage, losses of soil and water and environmental problems, as well as may cause secondary disaster. Groundwater is one of the crucial triggering factors for sinkhole development and collapse. To have a better understanding of the mechanical effect of groundwater on sinkhole formation，the force exerted by the groundwater on the soil particles in saturated zone with porous medium was analysed. In this case, the entire hydraulic force acting on the soil particle by groundwater depends on the seepage force or the value of hydraulic gradient which causes the velocity of groundwater movement. A 1D groundwater flow model was simply established, where the unconfined aquifer-aquitard system overlies the confined aquifer, in order to study the hydraulic gradient distribution in the confining unit. The differences between the effects of the phreatic level arising and piezometric level lowering were compared. Moreover, the effects of drawdown rate and drawdown of piezometric level in confined aquifer on the confining unit were mainly discussed. The results show that groundwater movement with variation of the water table or piezometric level in karst area will erode and disintegrate the soil, resulting in the formation of soil cavity and upward propagation of the erosion. The unsteady seepage due to groundwater level fluctuations goes against the stability of the confining layer above the karst opening; the impact of seepage force resulting from piezometric level lowering is greater than that of the phreatic level arising; the size of the soil cavity formed is related to the degree of the decline of piezometric level which is also influenced by the decline rate; under the conditions with the same piezometric level declines in the confined aquifer, the higher the drawdown rate is, the larger the maximum hydraulic gradient will be formed at the bottom. It was demonstrated that under the same drawdown condition, a critical drawdown rate should be made to prevent the soil failure; meanwhile, when a huge drawdown rate is needed, we should shorten the time of continuous drawdown. In other word, a critical drawdown should be proposed to protect the stability. Thus, controlling maximum operating head and maximum drawdown rate is critical in managing the groundwater exploitation in confined aquifer, in order to reduce or avoid the occurrence of cover-collapse sinkhole.