Simulation Of Groundwater Movement Characteristics And Water Environment Effect During Construction Of Water-rich Tunnel

The water-rich tunnel will cause great threat and influence to the safety of tunnel construction,groundwater and ecological environment,while the current tunnel engineering gradually considering the importance of groundwater and ecological environment protection,but for the sake of engineering economy,maneuverability,the destruction of the tunnel construction of underground water system and environment is still very big,It may even cause irreversible and permanent damage to groundwater environment and mountain ecological vegetation.Due to the complicated geological structure and hydrogeological conditions of water-rich tunnels,it is an important topic for scholars to study and discuss the distribution of groundwater flow field,the change of lining water pressure,the tracking and influence of leakage pollutants and the evaluation of the overall negative effect of water environment under the influence of tunnel construction.This paper takes a typical water-rich tunnel--Muzhailing Tunnel of Wei-Wu Expressway under construction as an example to carry out research,using the finite difference principle,combined with mainstream 3D seepage field simulation software such as Visual Modflow and Flac3 D,numerical simulation models of groundwater seepage field,water pressure outside lining and typical pollutants in tunnel construction were built,the weight of evaluation index of negative effect of water environment was determined by AHP,the comprehensive fuzzy evaluation method is used to evaluate the negative effect of water environment during tunnel construction.The main research results are as follows:1.The theoretical calculation formulas of tunnel water inflow and lining external water pressure considering seepage velocity are derived,including several conditions of grouting ring and lining,only lining and no grouting ring and lining,and the reliability is verified by traditional formulas.The above formulas provide a reference for the accurate prediction and calculation of tunnel inflow.2.The distribution and migration of seepage field in Muzhailing tunnel site under natural state before excavation,complete drainage state after excavation,and sealed state after excavation are simulated.Three months after tunnel excavation,the groundwater level in the tunnel site dropped rapidly,resulting in a "funnel-shaped" descent.From three months to 24 months,the groundwater level declined slowly,but continued to flow out in the way of water gushing,causing damage to the groundwater environment.After the drainage system of the tunnel is completely blocked,it will take 1-2 years for the groundwater level to gradually restore to stability,and the fall funnel will disappear and approach the original flow field.The research on the generation and recovery of the falling funnel in the seepage field of underground water in tunnel provides an argument for the reasonable "discharge limitation" in tunnel construction.3.Flac3 D software was used to simulate the external water pressure distribution of the lining of Muzhailing Tunnel under different construction periods and different discharge modes.With the increase of the thickness of the inner and outer wall of the grouting ring,the water pressure value of the outer wall of the grouting ring decreases,but the lining does not change significantly.With the increase of the thickness of the inner and outer wall of the grouting ring,the water pressure value of the outer wall of the grouting ring decreases,but the lining does not change significantly.With the decrease of the hydraulic conductivity coefficient of the grouting ring,the water pressure value of the outer wall of the grouting ring increases obviously,but the change of the lining is not significant.With the decrease of the lining hydraulic conductivity coefficient,the water pressure values in the grouting circle and outside the lining decrease,and the water pressure values at each monitoring point outside the tunnel lining are distributed as follows: lower arch bottom >left arch foot > left arch waist >left arch shoulder > upper arch.The above research results on pressure distribution of water plugging and drainage in tunnel construction can provide theoretical basis for the reinforcement of tunnel lining and the control of hydraulic conductivity coefficient of limited discharge.4.The simulation results of the migration characteristics of petroleum pollutants in the construction waste water of Muzhailing tunnel by using Visual Modflow software show that the petroleum pollutants will migrate in depth along the direction of hydraulic gradient after leaking into the groundwater,and will migrate laterally under the action of hydrodynamic dispersion of the groundwater,and finally form a pollution halo close to "elliptical shape".The concentration of petroleum pollutants in the pervious layer under the catchment tank increased from 100mg/L in 2 years to500mg/L in 7 years.In the 23 years after the leakage stopped,the outer boundary of pollutant halo spread almost to the whole simulated area,and the central extreme concentration decreased from 500 mg/L to 180 mg/L,but it still migrated slowly in the direction of the hydraulic gradient of water flow.After 20 years of migration,a small amount of pollutants migrated from the permeable layer downward to the aquifer,but no pollutants were found in the impervious layer during 30 years of simulation.It shows that once pollutants penetrate into groundwater during tunnel construction,they will migrate and diffuse in permeable layer and aquifer for a long time,which will pose a threat to groundwater environment and related organisms.5.Analytic Hierarchy Process(AHP)and comprehensive fuzzy evaluation method were used to evaluate the negative effects of groundwater environment during the construction of Muzhailing Tunnel.The evaluation index system structure was constructed with four factors of physical geography,hydrochemistry,geo-hydrogeology,tunnel engineering,and 23 specific influencing factors as the base layer and index layer,the evaluation grade standard of influence factors,fuzzy comprehensive evaluation model,membership function and quantitative index were established.The result of comprehensive evaluation on the negative effect of groundwater environment during the construction period of Muzhailing tunnel was medium,and the damage of water environment caused by the tunnel construction was obvious.The research results can be used as a reference for the evaluation of the negative effect of water environment in the construction of water-rich tunnels in China.