Experimental study on the vertical deformation of sand caused by cyclic withdrawal and recharging of groundwater

Fast urban growth and an ever-increasing frequency of extreme weather-related events demand a better understanding of land subsidence and the potential measures to control it. This paper presents an experimental investigation on the deformation behavior of sand undergoing cyclic withdrawal and recharging of groundwater. A sand-box model with dimensions 1000 mm × 600 mm × 892 mm was used to investigate soil deformation following lowering and rising of the water table. The sand used was Pingtan sand (Fujian, China) a quartzitic formation composed of elongated to slightly spherical grains. Three sand samples with different initial densities, i.e. loose, medium dense and dense, were tested on repeated cycles. The vertical deformation of the sand was measured and the corresponding strain and effective stress were calculated. The observations show that initially the sand behaves as an elastic material with the subsidence caused by water withdrawal being recovered as the soil rebounds during recharging. With further withdrawal–recharging cycles, subsidence becomes larger as plastic deformations as well as time-dependent deformation occur in the soil. The distinct patterns of deformation were identified to be dependent on both the initial density and the number of withdrawal–recharging cycle the sand has been subjected to. The different behavior of the medium dense sample compared to the loose and dense samples, in particular for the first two cycles, indicates that the soil response cannot be explained in respect of the initial void ratio alone and further microstructure considerations were discussed. It is suggested that the orientation of the grains resulting from the sample preparation technique and the consequent induced anisotropy can be used to explain the results herein presented. Finally this paper provides the rationale for a micromechanical interpretation of soil deformation when subjected to changes in stress due to rising or lowering of the water table, which will help in establishing measures to control land subsidence, in particular to assess the efficiency of water recharging following subsidence by withdrawal.