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

Environmental impact assessments of the Three Gorges Project in China: issues and interventions

The paper takes China's authoritative Environmental Impact Statement for the Yangzi (Yangtze) Three Gorges Project (TGP) in 1992 as a benchmark against which to evaluate emerging major environmental outcomes since the initial impoundment of the Three Gorges reservoir in 2003. The paper particularly examines five crucial environmental aspects and associated causal factors. The five domains include human resettlement and the carrying capacity of local environments (especially land), water quality, reservoir sedimentation and downstream riverbed erosion, soil erosion, and seismic activity and geological hazards. Lessons from the environmental impact assessments of the TGP are: (1) hydro project planning needs to take place at a broader scale, and a strategic environmental assessment at a broader scale is necessary in advance of individual environmental impact assessments; (2) national policy and planning adjustments need to react quickly to the impact changes of large projects; (3) long-term environmental monitoring systems and joint operations with other large projects in the upstream areas of a river basin should be established, and the cross-impacts of climate change on projects and possible impacts of projects on regional or local climate considered. © 2013 Elsevier B.V.Xibao Xu, Yan Tan, Guishan Yan