Abstract: Controlled by karst development and groundwater fluctuation,soil cave usually forms indefinitely and develop upward progressively until a sinkhole happens.It is important to find an efficient way to monitor the soil cave formation in good season for decision maker to take treatments.TDR has been used as a cost-effective method to monitor soil voids and sinkholes.The principle of soil void monitored with TDR is introduced in this paper.Cable tester lines need to be plane installed.Coaxial cables ...