Study On The Three Dimensional Land Subsidence Model And Its Application

Land subsidence is the lowering of the land-surface elevation.The ten-year plan for controlling land subsidence in China from 2011 to 2020 pointed out that land subsidence accompanying earth fissure has occurred and is increasingly more serious in many regions,for example Yangtse Delta,North China Plain,Fenwei Basin.In order to control land subsidence and protect the civil and industrial facilities,it is necessary to predict the future development of land subsidence based on present situation.An effective approach to predict the land subsidence due to overdrafting aquifers is numerical model for the interesting region.The numerical modelof land subsidence can be used to develop the strategies reducing water use and determining locations for pumping and artificial recharge that will minimize subsidence in some areas where surface water is not available.Therefore numerical model is important to predict and prevent land subsidence.In this thesis,the numerical model of land subsidence is investigated in both theory and practical application.The study on theory includes development of three dimensional(3D)uncoupling land subsidence model with stress-dependent parameters and comparsion with coupling 3D land subsidence model and contentional one-dimensional(1D)uncoupling land subsidence model.The study on practical application includes development of visualization software for convenience to develop land subsidence model and application of visualization software in Shanghai,numerical simulation of land subsidence for the whole Shanghai city,and numerical simulation of 3D deformation of aquifer system for the center area of Shanghai..The Conventional 1D uncoupling land subsidence model is the most popular to simulate regional land subsidence caused by groundwater pumping because it is easy to be solved.The conventional numerical model of large-scale land subsidence can only simulate vertical deformation,and cannot calculate horizontal deformation.However,real deformation is three-dimensional and horizonal deformation is important ingredient to true deformation,especially in some region with earth fissure.Therefore,horizonal deformation needs to be simulated.On the other hand,Biot model is not applicable to large-scale problem due to heavy compution requirements and complex algorithm though it can calculate horizonal deformation.Therefore,uncoupling 3D land subsidence model with stress-dependent parameters in this thesis was developed based on merit of conventional 1D land subsidence model and Biot model.Stress-dependent parameters mean that the model allows parameters like permeability coefficient,specific storage to be effective stress or groundwater level dependent.3D Uncoupling land subsidence model with stress-dependent parameters is appropriate to simulate 3D regional land deformation because it can calculate horizonal deformation with suitable computation requirements.3D Uncoupling land subsidence model is solved by the same method as conventional 1D uncoupling land subsidence model.It uses two-step method:the first step is that 3D-seepage is simulated by groundwater flow model and the second step is that deformation is simulated by deformation model.And deformation is simulated by 3D deformation model instead of 1D defomation model used by conventional model.Comparison of conventional model,Biot model and 3D Uncoupling land subsidece model by numerical experimentation,some conclusions are given:1)some PCG methods for solving Biot model are not robust,and many PCG methods for 3D uncoupling model are robust and rapid.2)the results from Biot model and 3D uncoupling model is similar,which implies 3D uncoupling model can be regarded as alternative model of Biot model to decrease computing complexity and time-consuming to a certain extent.3)in homogeneous large-scale areas,the results from conventional model and 3D uncoupling model is similar.However,in heterogeneous areas the results from the both model is obviously different,especially near the boundary and location of parameter variation.4)The biggest different of vertical deformation and extrusion force occur near pumping well.On the other hand the biggest tension occurs far away from pumping well.These imply earth fissure occurs not only near pumping well,but also far away from pumping well.Earth fissure near pumping well may be casued by difference subsidence;earth fissure far away from pumping well may be casued by tension in soil.The land subsidence model in this thesis is solved by finite element method.However,it is complexity that the finite element model for in some large-scale areas is used to develop land subsidence numerical model.Therefore,this thesis develops a visualization software to help reseacher analyze observation data and develop land subsidence numerical model.The visual software has many features for pre-processing and post-process of numerical model.The pre-process features include meshing,inversing parameters,managing and analyzing different types of observation data,for example,stratum deformation data from extensometer groups,groundwater level data from observation wells,and formation lithology data from boreholews.The post-process in the visual software includes drawing some diagrams to show the result from the numerical models.Meanwhile,global optimization algorithm in MATLAB software and UCODE softwar are embedded into the visual software in order to help users to get the parameters and perform sensitivity analyses or uncertain analyses.The visual software can reduce the threshold for developing numerical model because it is not necessary to learn a lot of the theory for numerical model and commercial software for pre-process and post-process.Using the visual software,this thesis develops two land subsidence models in Shanghai,china.Developing the model includes three steps:1.analyze observation data;2.Inverse parameters;3.develop 1D uncoupling land subsidence model with downscaling method for the whole Shanghai city and 3D uncouling land subsidence model for the centre region in Shanghai.The groundwater levels of the first,third,fourth,fifth confined aquifer rose by 2.1m,3.6m,12.4m,12.7m respectively from 1998 to 2011 due to reduction of pumpage and artificial recharge in Shanghai.Because the water levels recovered rapidly,the rate of land subsidence decreased to 1.83mm/a in 2011 from12.2mm/a in 1998,reducing by 85%.The main stratum contributing significantly to land subsidence is the first aquitard(includes the first and second soft clay)whose compaction rate is 2-4mm/a instead of the deep stratum(below the second confined aquifer)which is main stratum contributing significantly to land subsidence in 1990s.The deep stratum is upraiseing slowly on the condition of recovery of groundwater level.Meanwhile,in the process of groundwater level recovery,the author find that there are two patterns of deformation based on stress-stain relation.The first pattern is that the stratum expands with the recovery of groundwater level and irrecoverable deformation is small.The second pattern is that the rate gradually deceased without stratum swell and irrecoverable deformation is vast.The deformation of the second.third confined aquifer and the stratum below the fourth confined is the first pattern but stress-strain is not linear elasticity,rather than nonlinear elasticity.The recoverable specific storage is 40-60 percent of nonrecoverable specific storage.The deformation of the fourth aquifer and third aquitard includes both patterns.The deformation of the fourth aquitard is almost the second pattern.The parameters in land subsidence model are inversed using observation data The parameters include vertical hydraulic conductivity of the aquitards,aquitard irrecoverable skeletal specific storage,aquitard recoverable skeletal specific storage and skeletal specific storage of the aquifer.In aquitards,the irrecoverable skeletal is about 10-3m-1 and recoverable skeletal specific storage is about 10-5m-1.The huge gap between irrecoverable and recoverable skeletal specific storage explain why huge irrecoverable deformation occurs in aquitards.1D and 3D land subsidence model in Shanghai were developed.1D land subsidence model with downscaling method simulates land subsidence for the whole region in Shanghai.The model calibrated by observation data is used to predict future land subsidence in 2021.The result shows that surface will be able to uplift over the next decade if the state of boundary and pumpage is not changed.3D uncoupling land subsidence model simulated 3D land deformation in the centre urban area of Shanghai.The result shows that 3D uncoupling land subsidence model could be used as a tool to simulate 3D regional land deformation.