Experimental And Simulation Study On Groundwater Level Control In The Sandy Aquifer Along The Weihe River For Xi’an Metro

With the background of China’s rapid social and economic development and accelerated urbanisation,there is a serious shortage of urban land resources and the development of urban underground space is inevitable.Pit precipitation is a common method to reduce the groundwater level in the construction of underground space,especially for deep pits in waterrich sand layer areas,scientific and reasonable reduction of the groundwater level is an important means to ensure the construction safety of pit projects.Aiming at the large head difference,large flow rate,sand surge,near river and discontinuous permeable layer in the deep foundation pit precipitation project in the Weihe River section of Xi’an,this paper uses indoor physical simulation and numerical simulation to study the groundwater movement law and infiltration damage of the sand layer during the pipe well precipitation process,explore the influence of different filters on the stability of the sand layer.And then lay out different groundwater control schemes in the MODFLOW model to study the influence of precipitation in different pits under different schemes.The main findings of the thesis are as follows:(1)Before the infiltration damage occurred in the test sand layer,the flow rate of incoming water from the well wall increased proportionally with the hydraulic slope,and after the infiltration damage occurred the flow rate of incoming water from the well wall increased abruptly;the critical hydraulic slope for the infiltration damage of the sand layer was 0.6.The permeability coefficient of the sand layer before the infiltration damage was around 36.29 m/d,and the turbidity of the simulated well effluent was relatively small.After the permeability failure of sand layer,the turbidity of the effluent increased to more than 30 NTU due to the loss of fine sand particles with water flow,and the porosity of sand layer increased and the permeability coefficient increased.The smaller the filter screen size,the more sand particles are lost and the faster the permeability coefficient increased after permeation disruption occurs in the sand layer.(2)Under the tests of 120 mesh,100 mesh,80 mesh,60 mesh and 40 mesh filters,the turbidity of the effluent from the sand layer increased to about 30 NTU after the infiltration damage occurred,and only some of the fine particles followed the water flow into the well,and no large-scale particle loss occurred in the sand layer due to the blocking effect of the filter.However,under the 30 mesh,20 mesh and 10 mesh filters,with the increase of hydraulic gradient,the turbidity of water samples increases to more than 100 NTU,a large number of sand particles are lost,and the sand content in the water samples increases continuously,which eventually leads to the collapse of the sand layer.(3)When using tube well dewatering technology to dewater the foundation pit of Baimahe Station in Phase 3 of Xi’an Metro Line 1,the hydraulic slope around the tube well is less than0.6,which will not produce the risk of sand flow and pipe surge.The combined tube well precipitation technology inside and outside the pit is better than that outside the pit.After comparative analysis,the solution of 37 wells outside the pit and 8 wells inside the pit is the best tube well precipitation solution for the pit at Baimahe Station.The use of curtain for cutting off water combined with in-pit tube well precipitation solution can reduce the groundwater in the pit to the target level within a short period of time,and the total amount of water pumped after reaching the target level is small,causing less disturbance to the surrounding environment,but its cost is higher than that of the tube well precipitation method.The deeper the curtain for cutting off water is,the better the water stopping effect is and the higher the cost is.(4)The original precipitation plan for the Zhengyang Avenue-North passenger station contact channel foundation pit which is the north extension of the second phase of Xi ’an Metro Line 2 could only lower the groundwater level to about 1 m above the pit floor,which could not guarantee water-free construction of the pit.By adjusting the well spacing and the number of wells,the groundwater in the pit can be reduced to the target water level,but the hydraulic slope around the tube wells is between 1 and 1.3,which is greater than the critical hydraulic slope for infiltration damage to occur in the sand layer.Similarly,the use of out-of-pit tube well precipitation technology can control the groundwater in the pit of the liaison channel between Qinhuangnan Road Station and Baimahe Station of the Xi’an Metro Line 1 project to below 1m at the bottom of the pit,but the hydraulic slope around the tube well is greater than 1,which is greater than the critical hydraulic slope for infiltration damage in the sand layer,and is prone to infiltration damage such as sand flow and pipe surge.Therefore,for the foundation pit of the liaison channel between Zheng～North and Qin～Bai,the tube-well dewatering method is not applicable and the freezing method is recommended.