Investigation On Environment Impact During Shield Tunneling In Weathered Granitic Gneiss Strata Covered By Soft Deposit

This study investigated the environment impact during shield tunneling in weathered granitic gneiss strata coverd by soft deposit.Literature reading,field investigation,numerical simulation,and theoretical analysis were conducted in this study based on the tunneling project of Guangzhou Metro Line 21.This paper did a systemic research on the granitic gneiss weathering profiles to study its geologic and engineering characteristics.A 3D finite elemnt model was eastablished to analyze tunneling in different surrounding conditions.Specially,shield tunneling in mixed strata were simulated and discussed under different strata loss and under different distance between two tracks.Major conclusions of this studay are as follow:(1)Investigated engineering characteristics and potential geo-hazards of weathered granitic gneiss and recommended solution measurement.The slightly and moderately weathered granitic gneiss are hard and compact enough to act as bearing layer for general construction.Cleavage and fissures are further developed in strongly weathered granitic gneiss so that it usually contains fissure water and acts as confined aquifer.Grouting is needed when shiled excavates in this layer.The completely weathered granitic gneiss and its residual soils are featured with polarized grain distribution and susceptibility to water alternation.This may induce potential geo-hazards in construction,such as quick sand,piping,surface collapse and instability of excavation face.In practice,deep hole blasting can efficiently explode huge boulders.At the same time,high-pressure fracuture grouting will reduce soil disturbance as much as possible.(2)Analysed the monitoring data of shield tunneling in real case.When tunneled in mixed strata,ground subsidence mainly derived from strata loss and consolidation of upper soft soils.Therefore,surface settlement along the tunnel is nearly in accordance with the upper surface of granitic weathering profiles.Transverse settlement groove of second tunnel are partially affected by the tunnel excavated first.Excavation of the first tunnel will cause soil disturbance and excess pore water pressure around the second-excavated one,which will aggravate ground subsidence around.(3)Investigated the impact of surrounding rocks on tunnel excavation.Land subsidence by tunneling is inversely proportional to the stiffness of soil layer it lies.The stiffer the soil layer is,the lower the surface settlement and soil plastic zone will be.However,harder surrounding rock will result in a bigger influencial scale of soil body by tunnel excavation.(4)Analysed environment impact of shield tunneling under different strata loss and under different separation distance.Surface settlement and horizontal displacement will increase when strata loss in tunneling increases.But it dose’t change the influencial scale and distribution of soil displacement;When separation distance is lower than i,internal forces of first-excavated tunnel are greater than the lower one.Internal forces of both two will decrease along with the increase of separation distance.Once the separation distance is longer than i,the surface settlement curves will separated from one normal distribution curve into a bimodal curve.In this stage,the horizontal displacement of soils between two tracks is nearly 0,and the interaction effect of two tracks on internal forces could be negligible.