Analysis Of Ultimate Support Force Of Shield Excavation Face In Composite Ground

The stability excavation face of water diversion tunnel is related to the construction quality,safety and efficiency in the whole water diversion line.Composite stratum is a common type of stratum structure in underground tunnel engineering.The stability of tunnel excavation face has become a hot and difficult issue for researchers at home and abroad in composite strata.The determination of the limit support pressure in the excavation face is one of the key issues to reveal the catastrophic instability process and stability analysis of the tunnel excavation face.Further study on limit support pressure of tunnel face in composite strata is urgently needed.Based on a large number of domestic and foreign literature research.the applicability of the existing commonly used tunnel excavation face instability mode and the limit support force calculation model of the excavation face is summarized and analyzed.The material point method is used to simulate and analyze the deformation and failure characteristics of the stratum in front of the tunnel excavation face and the evolution law of the support pressure in the excavation face under the influence of different parameters.Based on the upper bound theory of limit analysis,the calculation model of instability sliding mechanism and limit support pressure of excavation face in composite strata under deep and shallow buried conditions is constructed,and the influence of parameters is analyzed.The main conclusions as follows:(1)By changing the geometrical ratio of composite strata,the strength parameter ratio of strata and the buried depth-diameter ratio of tunnel,the material point simulation analysis of the deformation and failure characteristics in front of the excavation face,the evolution law of supporting force and the law of surface settlement are carried out.The results showed: The depth coefficient of composite stratum,cohesion and friction of stratum and the ratio of buried depth to diameter of tunnel have obvious influence on the deformation and failure of excavation face and supporting force.With the increase of stratum cohesion and friction,the maximum surface settlement point gradually shifts to the excavation surface.(2)Upper bound theory based on limit analysis,the rotation failure mechanism of soil in front of the excavation face under two buried depth modes is proposed,and the evaluation model of limit support force of composite strata suitable for shallow and deep buried conditions is established.Compared with the numerical simulation results and the calculation results of other methods,the results show that the method has good reliability and applicability.(3)The influence of depth coefficient,buried depth diameter ratio,soil cohesion and friction is analyzed respectively.The results show that the buried depth diameter ratio is the most sensitive to the limit support force.The friction is logarithmically negatively correlated with the limit supporting force,and the cohesion is linearly negatively correlated with the limit supporting force.The cohesion and friction of the lower soil are more sensitive to the limit supporting force.