A Study Of The Fault-zone Control On Groundwater And Its Coupled Stress-seepage Modeling And Pumping Effect Analysis

The movement and distribution of groundwater in a bedrock aquifer are mainly controlled by faults or fractures and are quite complex due to its strong heterogeneity and anisotropy.Therefore,it is theoretically important and practically significant to study how groundwater distributes and flows in the complex bedrock system along with their main controlling factors.More specifically,it is quite important to understand the relationship between the capacity of water-supplying wells and the distribution of faults and fractures,the interaction of the stress-seepage field near a fault,and the pumping effect on the groundwater flow pattern in a fault-zone aquifer.In this thesis the relationship between the specific capacity and the faults was investigated based on the pumping well data at the bedrock area of southwest,China,the impact of the nonuniform crust movement on the stress field-seepage field near a fault was modeled,and an analytical solution was derived for the groundwater flow to a pumping in a fault aquifer.These results not only improve the theories for groundwater movement in bedrock aquifers but also are useful for the field practice of groundwater exploration and pumping data analyses in bedrock areas.Firstly,based on the active faults map at a scale of 1:4000,000(Qidong Deng,2002)and 635 wells distributed in Yunnan province of China in 2010,a statistical analysis of the relationship between the active faults and the wells was performed with the geologic statistics tools in ArcGIS software.Results indicate that these wells distributed mainly along the faults whose strikes are in the directions of NNE-SSW,NEE-SWW,NWW-SEE,and NNW-SSE.The shorter the distance of a well from a fault,the more fractures found in the well,and the larger specific yield of the well.Secondly,the impact of the non-uniform crust movement on the stress fieldseepage field near a fault was modeled for four models:Model ? is a homogeneous and isotropic formation in which there is no existing fault;Model ? is the same with Model I except there is an existing fault in the formation;Model ? is a heterogeneous intact formation of different lithologic strata and Model ? is the same with Model ? except there is an existing fault.Results show that:(1)The maximum principal stress(Smax)in Model I highly concentrates in the middle of the model,and a plastic deformation area or a fault will appears here.Corresponding to the stress,the pore pressure also highly concentrates in the middle of the model,groundwater flows from the middle areas with high pore pressure to the surrounding rocks with low pore pressure in the early time,and then flows from the surrounding rocks with positive pore pressure to the middle areas with negative pore pressure at later period;(2)The Smax in Model ?,as ?=90°,mostly concentrates around the fault zone and increases with time.Corresponding to the stress,the pore pressure highly concentrated around the fault zone,especially near the interface of the fault zone and formation,groundwater flows from the fault zone with high pore pressure to the surrounding rocks with low pore pressure and then flows from the surrounding rocks with positive pore pressure to the fault zone with negative pore pressure at later period,suggesting that the hydrological and mechanical properties of the fault zone play a significant role in the stress and pore pressure distributions as well as the groundwater flows.And Model ?,as ?=75°,60°,30°,the smaller the dip,the larger area of the compressive stress concentrates as well as the tensile stress,and the fault zone highly concentrates tensile stress due to the boundary conditions and the weak mechanical properties.Corresponding to the stress,the negative pore pressure highly concentrates in the fault zone and the positive pore pressure concentrates outside the fault zone,groundwater flows from the surrounding rocks with positive pore pressure to the fault zone with negative pore pressure,and the dip of the fault zone on the distribution of the pore pressure is not obvious;(3)The compressive stress in model ? concentrates in the middle of the model,while the tensile stress concentrates near the middle of the top layer.Corresponding to the stress,the pore pressure concentrates firstly in the middle of the model,and then decreases,the groundwater flows from the middle area with high pore pressure to the surrounding rocks with low pore pressure,then flows from the positive pore pressure to the negative pore pressure at later period;(4)The Smax in model ? concentrates in the fault zone and increases with time.Corresponding to the stress,the pore pressure concentrated near the fault zone,especially at the interface of the fault zone and formation,and increases to the largest at the interface of two layers,groundwater flows from the middle area with high pore pressure to the surrounding rocks with low pore pressure and then flows from the surrounding rocks with positive pore pressure to the interface of the fault zone and formation at later period.Finally,a semi-analytical solution is derived for the three-dimensional groundwater flow towards a pumping well in a sloping fault aquifer surrounded by permeable matrices.The flow in the aquifer is described by a three-dimensional flow equation,and that in the upper and lower matrices is described by a one-dimensional flow equation.A first-order free-water surface equation at the outcrop of the fault aquifer is used to describe the water table condition.The Laplace domain solution is derived using finite Laplace-Fourier transform techniques and the solution in the real time domain is obtained by the Stehfest method.The solution is excellent agreement with Theis solution combined with superposition principle.It find that in early time,i.e.,before the cone of depression reaches the aquifer boundaries,the upper and the lower matrices have little effect on the drawdown and the influence of the free-water surface can be neglected,and the larger the dip of the fault-zone aquifer,the larger the drawdown in the aquifer.In later time,i.e.,when the cone of depression reaches the aquifer boundaries,the larger the ratio of Sy/Ss,the larger the effect of the free-water surface boundary.The smaller the ratio of the specific storage and the ratio of the hydraulic conductivity of the matrices to those of the fault aquifer,the smaller effects on drawdown,and this effect can be neglected when the ratio is less 0.001.