Analysis Of Groundwater Seepage Characteristics And Study On Environmental Impact In Tunnel

With the rapid development of China's transportation industry,more and longer tunnels are being constructing.As these tunnels go through complex strata environment,underground structure like tunnel engineering will inevitably cause disturbances and damages to groundwater,which could bring many difficulties for construction.The gushing water in the tunnel is becoming more and more obvious.A large number of underground water transportation and discharge will cause a series of geological disasters,such as surface water depletion,ground subsidence and surface subsidence.Simple drainage cannot meet the requirements of tunnel construction and environmental protection.Therefore,it is necessary to study the seepage field of groundwater either from environmental protection or construction safety.This paper takes the Zhongliangshan tunnel,part of the second stage of line six,Chongqing railway transportation as background.Based on the investigation and analysis of tunnel spot geological basis,it takes numerical simulation software of Visual Modflow,genetic algoritms optimizing neural network and other means to carry out numerical simulation and theoretical analysis of groundwater seepage field of tunnel spot.Main contents and conclusions of this paper include the following aspects:(1)Generalize the geological model according to the survey data.The geological condition of the tunnel site is complex.On the condition that tunnel passes through anticline and fault.The geological model can be generalized as inhomogeneous equivalent continuum model;the fault belongs to compression-wrench faults,the water conductivity is weak,generalized to wall of water insulation boundary conditions;try to select the natural boundary,avoid artificial boundary randomness.The generalized geological model accords with the actual terrain.(2)the genetic algorithm is used to inverse the permeability coefficient of aquifer,and the Modflow simulation data is used as training sample to observe the water head as the prediction value.The inversion results show that the computed head and observat ion head have less error.Within the allowable range,the generalized model is more accurate and can be used to simulate the seepage field of groundwater.(3)The simulation of groundwater seepage field for tunnel excavation for 2 years shows that drawdown funnel of groundwater has been formed in the early stage of construction.When the tunnel is excavated,the amount of drainage is large,while,the recharge of ground water and leaky cannot keep up.The hydraulic gradient of the earlier stage is larger.As time goes by,the change of the drawdown funnel becomes slower because of recharge.After the completion of the tunnel,the drawdown depth reduced to 63 m,and the influence area expanded along both sides of the tunnel axis to 2.5km.(4)The construction of different elevation of tunnel has obvious influence on groundwater seepage.The higher the elevation,the shallower the buried depth,and the weaker the damage to the groundwater environment,and the lost ground water can recover in a short time.On the contrary,the lower the elevation,the greater the buried depth,the greater the pressure of head water and the pore water surrounding rock,the wider the groundwater fall and discharge range.Therefore,the proposed tunnel should be in weak stratum water,when the problems are unavoidable,reducing the depth of tunnel can be taken into consideration.(5)During the tunnel operation period,the groundwater seepage field was calculated and analyzed in 0.5,1,2,3,5,10 and 20 years.The results showed that the groundwater level recovered rapidly within six months,and reached steady state after 2 years,and the drawdown depth decreased from 63 m to 38 m.In the early stage of operation,the hydraulic gradient around the funnel and the aquifer is larger,and the leaky recharge intensity of the groundwater is recovered quickly by the ambassador.The water on both sides of the tunnel moves to the center of the funnel,which expands the scope of influence.The completion scope of the tunnel is 2.5km,and extends to 4.1km after stabilization.The groundwater aquifer and aquifuge were conducting during tunnel excavation,seepage failure system of tunnel area,the seepage field changes,the formation of drainage channel in the tunnel and the surrounding surface.Even after the seepage field is stable,groundwater is difficult to recover to its initial level.(6)Predict groundwater recovery time with different water discharge,it is found that it recovers faster when the displacement is 500m~3/d.After 1 year of operation,the groundwater level has been restored to 90% of the initial level.After 2 years,the water level stays stable and basically consistent with the initial seepage field,has little impact on the ecological environment.The more the displacement is,the greater the drawdown depth after the seepage field is stable,and the environmental impact is difficult to recover.Therefore,it is suggested that the amount of drainage should be less than 500m~3/d during the construction process,so as to minimize the negative impact on the environment.