Stability Analysis Of EPB Shield Tunnel Face Under Strata With Different Water Contents

Earth pressure balace(EPB)shield has been used more widely than slurry pressure balance(SPB)shield.Maintaining the stability of the tunnel face has been the most prominent problem for EPB shield.Therefore,many scholars have studied the failure process,and many calculation models were obtained by extracting and optimizing the results.However,the practicality needs to be further improved and developed.What’s more,in present researches,there are many studies about face stabiltity in dry sand or saturated sand stratum by carrying out model test,but there is less dry sand or saturated sand stratum in practical engineering.Therefore,it’s meaningful to study the stability of tunnel face under strata with different moisture content.And it is necessary to further study the dynamic failure process of the soil when tunneling and clarify the mechanical failure mechanism,so the safety of surface and underground construction works can be ensured and a real-time and effective warning system can be established.Based on the scale model test,the simulation of three groups of tunneling process under different moisture content was carried out.Meantime,the results gained by FLAC3D proved that it is reasonable to apply FLAC3D to the actual project.Then,the stability during the excavation process in Tai-Jing section of Harbin Metro Line 3 under saturated water condition was predicted and analyzed.Finally,based on the ground subsidence range and failure surface results,the calculation model was optimized.The conclusions are:(1)Based on the indoor large-size model test,the trend of the supporting force was investigated,and it was found that the supporting force showed a three-stage variation characteristic of stability,critical instability and failure.Furthermore,the ground settlement and earth pressure distribution with different moisture content were investigated.With the increase of moisture content,failure mechanism varied when C/D=1.0,and failure extended to the surface and forms a prominent failure mode when moisture content was small;with the increase of moisture content,due to increase of the apparent cohesion,the soil arching effect became obvious,and the height of the damaged area decreased,and the damage did not extend to the surface.(2)An equal-size numerical calculation model was established based on model tests,and by using the stress control method,the change law was obtained,as well as the change curve of the surface settlement value and the surrounding rock earth pressure.It is found that the numerical simulation curve lacks the stable stage,but the trend among them is alike.The surface subsidence value obtained by FLAC3D fit well with model test.Compared with change law of the earth pressure,the failure area obtained by FLAC3D is a little larger than that of the model test because of the small moisture content.By comparing the results,it shows the actual engineering can be simulated well by means of numerical simulation,which can achieve the prediction analysis at the same time.(3)Based on the engineering geological data of Tai-Jing section of Harbin Metro Line 3,a model was established to study shield excavation considering fluid-solid coupling.Statistical analysis of ground settlement caused by excavation showed that settlement value can meet the requirements well.Furthermore,the analysis result showed that the hydraulic funnel can be formed easily in front of the tunnel face and the stress there was significantly reduced.Finally,the variation law of plastic zone was determined by,and the ultimate supporting force of tunnel face was determined when the condition of seepage was taken into consideration(4)Considering model test results and the numerical simulation results,using FLAC3D to extract and optimize the failure area under the actual working condition,three kinds of assumed calculation models considering the damaged soil on both sides of the tunnel face were built.What’s more,the optimal solution was found by MATLAB;then a 3D wedge shaped failure model was proposed,and the results were more practical than some existing models.Finally,the limit support forces were compared with the established calculation model respectively,and the values of them were similar,which proved that 3D wedge shaped failure model was suitable for the actual engineering.