Numerical Simulation Of Land Subsidence Due To Groundwater Withdraw In Xi'an And Its Prevention Schemes

Xi'an is one of the four major ancient civilization capitals, locating in the center of Guanzhong Plain. From the 70s to late 90s of last century, the overdraft of groundwater especially the confined water led to rapid decline of groundwater table, which consequently triggered and aggravated serious land subsidence. Land subsidence is one of the most prominent geological disasters which are troubling the city construction of Xi'an. So how to prevent the occurrence and development of the land subsidence is a pressing problem needs to resolve. Land subsidence is a complex problem relating to the coupling of liquid and solid. A main method of studying land subsidence is to build a groundwater- subsidence model for quantificational analysis and calculation. Former research on the land subsidence of Xi'an, though have got some achievements, was still on the qualitative analysis, some quantitative analysis was based on the relative simple gray models or biology growth model. Till now, there is no real coupling model of groundwater and land subsidence for Xi'an.Orienting directed from the theory of "interbed" put forward by Leake, based on the Terzaghi's effective stress principle, the principle of coupling groundwater flow equation and soil deformation equation is analyzed in this paper. And the land subsidence model coupled by three dimensional groundwater flow model and one dimensional soil deformation model was built. In this model, the skeletal storage coefficient was used for coupling, different storage coefficient were used for the calculation of storage change of aquifer and interbeds, the elastic and inelastic skeletal coefficients were used to account for the elastic and inelastic deformation, a one-dimensional diffusion equation was used for describing the delayed dissipation of unequilibrated heads within the interbeds, these make the model not only be able to simulate the elastic and inelastic deformation of soil, but also the delay phenomenon. In addition, the three dimensional geological model built according to a lot drill data with the method of slicing can represent the actual structure of the geology body much better than the traditional geological model, and is much more befitting for the analysis and calculation of land subsidence in this paper.With the land subsidence model built in this paper, the change of land subsidence and groundwater level in the next 20 years was forecasted under present groundwater exploitation. On this base, five schemes of preventing the land subsidence were put forward. For each strategy, a corresponding exploitation (or recharge) plan of groundwater was designed. Then each designed exploitation plan (except scheme 3) was put into the model for calculation and simulation. The analysis on the forecasting results every plan indicates that:(1) Shuting down the self-provided wells is the most direct and effective method, playing an important role in the land subsidence prevention.(2) The resumption of compressed soil by groundwater artificial recharge is limited. So the artificial recharge can't provide essential effect for the resumption of land subsidence, unless the artificial recharge is much more than the groundwater withdrawal.(3) The method of "adjusting the exploitation stratum of groundwater to the shallow" has an obvious effect on the land subsidence prevention.(4) "Sand wells" can increase the leakage recharge for confined aquifer, and thus can slow and prevent the development of the land subsidence.(5) For the groundwater exploitation in Xi'an, emergency self-provided wells should be managed according to the well spacing of 1600m, 2000m and 1600m, respectively, in the river terrace area, alluvial-proluvial fan area and loess tableland area to exploit the first confined groundwater. Corresponding individual well yield can be set as 2500m~3/d, 1500 m~3/d and 200 m~3/d. Make emergency self-provided wells pump in the dry year and stop pumping groundwater to restore and conserve underground water sources in average years and wet years. In this way, groundwater withdrawal of 162000m~3/d can be obtained to provide for the supplement of water supply in Xi'an. As a result, the water supply stress can be released in dry years. The groundwater depression formed in the exploiting year can resume in 3-4 years. At the same time, this exploitation manner will not lead to obvious land subsidence.