Model Test And Theoretical Studies On Face Instability Of Shallow Shield Tunnel

In recent years,the construction of shield tunnel is often carried out in more complex geological conditions.The corresponding risks and accidents are increasing,in which the failure of the excavation face caused by the lack of support pressure has occurred sometimes.Therefore,It has a great practical significance to study the instability of the excavation face of shield tunnel.In this paper,model test and theoretical analysis method were used to study the instability of the excavation surface in sand and clay strata.The three-stage regulation of the support pressure was found.According to the displacement and shear incremental fields,the failure zone was different in different stages of the support pressure.The settlement area of excavation surface front was much larger than that of excavation surface rear.The longitudinal trough of the clay is much larger than that of the sand in the same displacement of excavation surface.Based on the shape of failure zone in the ultimate support pressure,the modified wedge model of sand and clay was proposed to calculate the ultimate support pressure,respectively.The main research work was as follows:(1)The model test was carried out to study the development of failure of shallow shield tunnel in sand with the back rate of support panel.It was found that the three-stage regulation of the support pressure was not affected by the back-up rate of the support panel,and the corresponding displacement of the support panel is about 0.3%D.The greater the back-up rate of the support panel,the more sensitive the surface settlement and the taller and slimmer the transverse settlement trough.With the help of PIV,the law of displacement and shear incremental field in soil is obtained,and the failure zone and stage were divided corresponding to the support pressure.(2)Model test in clay was carried out to study the development law of support pressure and longitudinal settlement trough during the evolution of the face instability.The results were compared with the test in sand.Displacement of the support panel was found to be about 0.3%D at the end of the first stage of the support pressure.The difference is that in the second stage,support pressure slowly reduced in clay,while support pressure slowly increased in sand.With the help of PIV,combined with the change regulation of the support pressure,the failure zone and stage were divided.(3)Based on the analysis of the development of the instability zone in the the evolution of the face instability in the sand and clay strata,the failure zones in the sand and clay were analyzed.Meanwhile,the shape of instability sliding block in the strata was analyzed in the case of the ultimate support pressure.Depending on the different shapes of instability sliding block in the failure zone in the sand and clay,based on the wedge model,the wedge-four prism model and wedge-inverted four prism model were introduced to calculate the ultimate support pressure.It was found that the modified model was in good agreement with experimental and theoretical results.