Study On Failure Modes And Influencing Factors Of Slurry Shield Excavation Face In Gravel Stratum

With the acceleration of urbanization in China,the development of underground engineering in China has entered the "fast lane".Shield tunneling method has many advantages,such as fast construction speed,small environmental disturbance and so on.It is often used in urban underground engineering.At present,many shield tunneling projects have been put into use using slurry balance shield construction method.How to ensure the safety and stability of slurry shield excavation surface and reduce the impact of construction disturbance on urban construction is a difficult problem in the field of shield construction.Therefore,it is of great significance to study the stability of excavation face of slurry balance shield.Based on the mud shield excavation project in Qing-Shi District of the first phase of Nanning Metro Line 3,the active and passive failure modes of mud shield excavation face in gravel stratum are systematically studied by means of model test,numerical simulation and theoretical analysis,and the parameters affecting the stability of excavation face are analyzed.The main research and work results are as follows:(1)Based on the results of field investigation,the adaptability of slurry balance shield is evaluated according to the stratum permeability coefficient,rock and soil particle composition,construction characteristics of slurry shield and the actual conditions of the project.The construction risk analysis of the strata mainly crossed by the excavation project is carried out.The results show that it is appropriate to use slurry balance shield tunneling in Qing-Shi District of Nanning Metro Line 3.Among the three types of strata that the project mainly passes through,the construction risk in the whole section gravel stratum is the greatest.Six specific control measures are put forward to ensure the safety and stability of the construction.(2)According to engineering conditions and similarity theory,the model tests of active failure of excavation face in gravel stratum under different coverage-span ratio were designed and completed,and the stability of excavation face of slurry shield under different coverage-span ratio was studied.In the test,the influence of the ratio of overburden to span on the limit support force of active failure and the value of ground settlement is analyzed.Through the analysis of the stress variation law of the soil in front of the excavation,it is found that the active failure of the excavation face of the slurry shield in the gravel stratum follows the progressive failure mode.The front soil body gradually develops from partial loosening to overall failure.In the process of the progressive development of the damage area,the soil arching is constantly formed,developed and destroyed inside the soil body.(3)On the basis of active failure model test of excavation face,the passive failure model test of excavation face under different coverage-span ratio was designed and completed,and the passive failure mode of excavation face caused by excessive pressure of mud chamber of mud shield was studied.The influence of coverage ratio on passive failure limit support force and splitting failure mode of excavation face is analyzed in the test.Combining with the variation law of soil pore pressure in front of excavation face,it is found that mud shield will undergo three stages of dynamic equilibrium,mud film splitting and splitting through in the process of passive failure of excavation face.Its essence is that mud water splits mud film and then splits stratum and develops upward.(4)Using FLAC3 D finite difference software to simulate the failure of excavation surface,and compared with the experimental results.The failure modes of excavation face are verified by analyzing the development law of soil plastic zone,displacement law of excavation surface and surface and stress field of model.(5)Based on the typical working conditions of the project,the influence of tunnel burial depth,water head height,soil properties and mud properties on the stability of excavation surface is analyzed by numerical simulation.Taking the displacement of excavation surface as reference variable,the sensitivity analysis of factors is carried out based on grey correlation method.The results show that the main factors affecting the stability of excavation face are tunnel depth,water head height and friction angle in soil,while the secondary factors are the gradient of mud support pressure and soil cohesion.