Study On Soil Consolidation And Three-dimensional Numerical Simulation Of Land Subsidence In Langfang Settlement Area

With the increase of water consumption for industry and agriculture,and the over-exploitation of groundwater for many years,the groundwater level in the North China Plain has gradually decreased,resulting in land subsidence and other secondary disasters.Langfang is located between Beijing and Tianjin,the geographical position is superior,geographically belongs to the Hebei plain and Beijing Plain intersectional area.In response to the national requirements for Comprehensive control of land subsidence in the Beijing-Tianjin-Hebei Plain,the paper relies on the project of National Natural Science Foundation of China "Soil Deformation Characteristics and Mechanism Driven by groundwater level change in Complex aquifer System"(41877294)and the project of China Geological Survey "Comprehensive Investigation and Monitoring of Geological Environment in Beijing-Tianjin-Hebei Region"(DD20190679),The consolidation characteristics of cohesive soil and the response of ground subsidence to pumping are analyzed.The main conclusions are as follows:(1)Through the analysis of high-pressure consolidation test data,By fitting the compression modulus and load of cohesive soil,the empirical formula of fitting the compression modulus and load is obtained,which provides a scientific basis for the settlement calculation of cohesive soil in Langfang settlement area.(2)Mohr-coulomb and Cam-clay model respectively are adopted to simulate the viscous soil layer,when the third water-bearing group is pumped water,it is found that under the same amount of water pumped,the calculated land subsidence of the cohesive soil layer simulated by Cam-clay model is relatively small due to the transformation of plastic strain energy,which is more consistent with the actual situation than that of the Mohr-coulomb model simulating the cohesive soil layer.(3)The Mohr-coulomb model is used to simulate the stress and deformation characteristics of the cohesive soil layer: a)The surface uplift occurs in the pumping water center area,which is inconsistent with the reality.Under the model of balance of groundwater exploitation and replenishment and gradually increasing pumping,the water flow on both sides of the model converges to the center,and the center of the first and second water-bearing groups is negative pore pressure,so the soil mass absorbs water and expands,resulting in uplift.b)Under the fluctuating pumping watr mode,the surface uplift no longer occurs,but the settlement in the center is smaller than that on both sides.When the pumping load decreases,the vertical compression decreases,the porosity of the third water-bearing group increases,and the vertical seepage effect of pumping increases,at this time,the uplift of the first and second water-bearing groups no longer occurs.c)Under different pumping modes,the central positive pore water pressure and compression volume of the third and fourth water-bearing groups are the largest,and then gradually decrease to both sides.(4)Cam-clay model is used to simulate the stress and deformation characteristics of the cohesive soil layer: a)There is no surface uplift in the pumping water center area,which is in good agreement with the reality.Although the water flow on both sides of the model converges to the center,due to the plastic deformation,the third water-cut group has a small compression amount,so the pore closure degree is small,and the vertical seepage intensity is large.Therefore,under different pumping modes,there is no uplift phenomenon in the center of the first and second water-cut groups.b)The stress and deformation characteristics of the third and fourth water-bearing groups under different pumping modes are basically consistent with the deformation characteristics of the moorcoulomb simulated viscous soil layer.(5)Fifteen kinds of groundwater exploitation schemes are adopted in the study area,and the influence of these schemes on the deformation of water-bearing group and the total surface settlement is analyzed by numerical model.Considering the combined groundwater exploitation of multiple water-cut groups,the land subsidence caused by 1+3+4,1+4 and3+4 combined schemes is small.Considering the amount of land subsidence,the first water-bearing group and the second water-bearing group should be minimized or even not exploited.If exploited,it should be combined with other water-bearing groups.