Evaluation of Land Subsidence during Groundwater Recovery

Abstract

The Chao Phraya River basin is located in the central area of Thailand, which experiences many land subsidence issues due to groundwater pumping. The Department of Groundwater Resources (DGR) has been recording data on the changes in the groundwater level due to water pumping since 1960 until the present time. In 1997, the DGR issued a law regulating the use of groundwater due to its effect on the changes in the groundwater level. The changing of the groundwater level was separated into two periods. The first period is the high groundwater pumping ratio that led to a rapid decrease in the groundwater level of about 27 m from the ground surface. After the DGR issued the new law in the second period, the pumping ratio decreased and the groundwater level increased. The groundwater level tends to reach the ground surface. In the past, the groundwater level decrease was affected by the land subsidence. Therefore, this study focused on calculating and learning the behavior of the soil surface displacement during groundwater level recovery to the ground surface in Bangkok, Thailand. We obtained the 3D soil profiles adopted from eight boreholes from the Department of Public Works and Town & Country Planning. The soil profile data were verified by monitoring the data from the Department of Groundwater Resources (DGR) in the same area. The soil layers of the 3D soil profile were analyzed to calculate the soil surface displacement based on the consolidation theory of Terzaghi. We also examined the displacement behavior of the clay layers during the groundwater level recovery to the ground surface by assuming that the soil layers below the groundwater level do not settle or rebound. The surface displacement results showed that the surface ends to move upward or rebound, which is a similar trend to that reported in previous research. All the considered locations showed similar soil surface displacement trends. The soil displacement ratio is 0.21 to 0.53 cm/year during the groundwater recovery.