Study on Soil Conditioning Experiments during Earth Pressure Balance Shield Tunneling in Sand Gravel Strata

During the tunneling of shield machine in sand gravel strata containing high content and large diameter gravels,the soil mass cannot be timely discharged from earth chamber due to poor plasticity and fluidity,which often results in such phenomena as the abnormal increase of shield thrust and cutterhead torque as well as extremely slow advancement speed.Since the force transfer among soil particles in earth chamber is point-to-point,the support pressure in earth chamber cannot be effectively applied to excavation face,which may easily result in the overrun of ground settlement and collapse,etc.Therefore,with the shield tunneling engineering of Beijing Metro Line 9as background,soil conditioning tests are carried out in the process of EPB shield tunneling in sand gravel strata.Results show that it is feasible to adopt foam plus bentonite as soil conditioner for sand gravel strata.During soil conditioning,such parameters as the injection time and volume of soil conditioner should be adjusted in time with the varying of these parameters,such as the volume of excavated earth,earth pressure,shield thrust and cutterhead torque,etc.Under the premise that the volume of excavated earth is controllable,the advancing mode in lower pressure can be used for improving the efficiency of shield advancing in sand gravel strata when earth chamber is full.