| With the rapid development of China's transportation infrastructure,some tunnels will inevitably pass near or even below the landfill.The impact of tunnel construction on the landfill leachate migration law has become a growing concern of scientific question by scholars.The impact of tunnels on hydrological environment mainly involves groundwater level,solution transport path and solute concentration distribution.Generally speaking,pollutants are accompanied by groundwater flow.The landfill leachate migration law is based on groundwater flow law.Therefore,to study the impact of tunnels on landfill leachate migration law,we must first study the impact of tunnel excavation on groundwater level,and also consider the stratigraphic disturbance caused by tunnel excavation when analyzing groundwater level changes.The sulfate ion has a certain stability,and the migration of sulfate ion in the study represents the migration of the landfill leachate.Tunnel excavation changes the permeability of geological medium.The change of geological medium permeability will lead to the change of landfill leachate migration law.At the same time,landfill leachate is a high concentration and multicomponent pollutant,which is inevitable when transporting in the stratum.It will react with geological medium,and the complex reactions will inevitably affect the various properties of geological media.For example,if it affects the permeability,it will change the migration law of leachate.If it affects the strength parameters,it will change the surrounding rock mechanical properties after excavation.In this paper,GMS(Groundwater Modeling System)software is used to simulate groundwater movement and leachate migration from the perspective of stress field-seepage field coupling and chemical field-seepage field coupling.At the same time,combined with the rock strength test,the GTS software was used to simulate the excavation of the surrounding rock of the tunnel considering the chemical action and the seepage action.The main work and results are as follows:(1)The simulated water level head before the tunnel construction is successively descended from the northeast to the southwest and merges with the pumping well PW-1.The groundwater solute transport model before the tunnel was constructed.For the dispersion of the aquifer(first layer)in December 2030,the longitudinal dispersion of sulfate ion reached the PW-1 monitoring well as far as 500 meters from the southern end of the Landfill 1.(2)The stress field-seepage field coupling experiment is used to illustrate the change of rock permeability after excavation,and then the migration law of leachate is revealed from the change of permeability.The permeation experiment found that the confining pressure had a great influence on the overall trend of the permeability curve.The difference of the permeability of the rock specimens that were not soaked and soaked under the same confining pressure and osmotic pressure was statistically analyzed,and it was found that the permeability of the soaked rock specimens increased by an average of 21%.(3)The groundwater flow model after tunnel excavation forms a precipitation funnel near the middle section of the tunnel and the pumping well PW-1.The change of water level before and after excavation also shows that the tunnel has an impact on the direction of groundwater flow.In the model of solute transport after tunnel excavation,the concentration of sulfate ion in the south of the tunnel decreases and the migration range decreases.The concentration of sulfate ion in the north of the tunnel increases and the migration range increases.(4)The porosity change after rock corrosion is explored through experiments,and the solute transport law is explained from the change of porosity.In general,the higher the concentration of the immersion environment,the higher the porosity,and the increase rate of the porosity of the immersed rock in the low concentration and high concentration leachate will gradually slow down with the increase of the immersion time.The porosity of the rock specimens immersed in the high concentration and low concentration leachate for 88 days under the action of the chemical field were 0.0245 and 0.0225,respectively.(5)The simulated water level under the action of the chemical field after tunnel excavation is lower than the simulated water level after tunnel excavation.This is because the chemical field acts after excavation,which leads to an increase in porosity.It has also accelerated the downward penetration of groundwater.(6)The elastic modulus of rock specimens soaked by landfill leachate under same confining pressure is smaller than that of unsoaked rock specimens.The average value of the Poisson's ratio of the unsoaked rock specimens is 0.21;the average value of the Poisson's ratio of the soaked rock specimens is 0.2;The cohesive force of the unsoaked rock specimen was 5.32 MPa,the internal friction angle was 37.23 °;the cohesive force of the soaked rock specimen was 4.69 MPa,and the internal friction angle was 33.61 °.(7)In the simulation of tunnel excavation under the action of chemical field-seepage field,the maximum settlement of the vault is 2.37 mm and the maximum floating bottom of the arch is 2.50 mm at the end of construction step 1.The maximum settlement of the vault is 3.05 mm and the maximum height of the arch is 2.52 mm at the end of construction step 2.Comparing to the model of normal tunnel excavation of surrounding rock,the maximum settlement of the vault increased by 0.22 mm and the maximum floating of the arch bottom increased by 0.07 mm at the end of construction step 1;the maximum settlement of the vault increased by 0.36 mm and the maximum floating height of the arch bottom increased by 0.06 mm at the end of construction step 2. |
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