Sinkhole formation

This is an experiment of one type of sinkhole formation. The bottom layer (filled with water) is the confined aquifer which has a small opening in the middle (this is simulate the fracture and conduit). The confining layer is a thin clay layer overlaid the confined auifer. it is under the sand layer ( simulates unconfined aquifer). At first, the water head of confined aquifer is a little higher than that in unconfined aquifer. we lowered the water head of confined aquifer by pumping water out. Then the clay