| Stratum resistance is one of the significant characteristics that distinguish substructure from suoerstructure,and its distribution is closely afffected by its shape and deformation.At present,the assumed elastic resistance method is mainly used in the design of metro shield tunnel,and the soft soil layer has the characteristics of low strength and high sensitivity.The magnitude of the resistance generated by the soft soil layer to the tunnel has always been controversial,resulting in the deformation of many tunnels during the service period.Therefore,this paper systematically studies the distribution form and calculation method of the stratum resistance of the special-shaped structural plane in the soft soil layer by combining the test and theoretical analysis.The main results are as follows:(1)Based on particle image velocimetry(PIV),a test device for the interaction between soil and abnormal structural plane is developed,Aiming at the problem that the texture characteristics of soft soil are weak and it is impossible to conduct PIV observation,a method for constructing the surface texture of soft soil is proposed,which uses black silicon carbide powder as tracer particle,lubricating oil and Vaseline as antifriction agent and adhesion agent.This method can improve the texture characteristics of soft soil at the observation surface,and solve the problem of obtaining soil displacement field changes by PIV system(2)Through the translation test of the rigid body special-shaped structural plane in sand and soft soil,it is found that the resistance distribution mode is mainly affected by the shape of the structural plane and the anisotropy of the resistance coefficient of the formation,and is little affected by the soil type.The resistance distribution of the convex and concave shapes is similar to a "oblique ellipse",and the resistance of the plate shape presents an "R" shape distribution.The resistance of the soil changes with the displacement in two stages and there is a limit displacement.Before the structural displacement reaches the limit displacement,the resistance increases rapidly with the increase of the displacement.After reaching the limit displacement,the resistance does not increase or slowly increases with the increase of the displacement.This extreme displacement is related to the nature of the soil.In this experiment,the limit displacement in the sand is about 1%D～2%D(D is the height of the structure),The ultimate displacement in soft soil is about 0.5%D～1%D.(3)Through the resistance distribution test of bending deformation of specialshaped structure in soft soil,it is found that the magnitude of resistance is affected by the deformation form of structure and the resistance coefficient of horizontal strata at different burial depths.The resistance distribution law of soil body of convex structure and concave structure is consistent,both of which are inclined ellipses.The resistance distribution range is in the area of convex deformation of structure,which is consistent with the definition of resistance;The resistance distribution of the flat plate structure is "water drop" shape,which is distributed along the structural plane as a whole.(4)Based on the interaction test of special-shaped structure and soft soil,the actual deformation curve of the structure under the action of load is obtained through the deflection differential equation of curved beam and straight beam.Considering the characteristic that the resistance coefficient of the stratum increases with the buried depth,the distribution form and calculation method of elastic resistance of the three types of structural planes under bending deformation are established.The accuracy of the theoretical model is verified by comparing with the model test results.The calculation results show that the distribution patterns of the formation resistance of the convex and concave structures are the same,and the action range is 45°～135°.The action range of the resistance of the flat structure is the whole structure surface.When the structure is subject to bending deformation,the deformation caused by the resistance of soft soil stratum is about 5%～10% of the deformation caused by the active load.(5)The deformation curve of circular tunnel under the action of water and soil load is analyzed.It is found that under the conditions of different buried depth and lateral earth pressure coefficient,the deformation of circular tunnel presents three modes: "horizontal duck egg","vertical duck egg" and "wave".The critical conditions of different forms are deduced theoretically.Through the analysis of the root of the deformation curve,the theoretical approximate solution of the resistance distribution range is obtained,and it is demonstrated that the resistance distribution range of the circular shield tunnel is 45°～135°.(6)The research on the limit resistance of soft soil layer shows that the deformation of circular shield tunnel may reach the limit resistance state,that is,the situation when the resistance reaches the limit state needs to be considered;Based on the ideal elastic-plastic p-y curve,the calculation model of elastic-plastic resistance distribution of circular tunnel is derived,and compared with the traditional triangular resistance distribution,the influence of tunnel diameter and surface overload on resistance is analyzed.It is found that with the increase of the surface overload,the bending moment calculated by the elastic-plastic resistance presents a nonlinear increase,and the bending moment calculated by the triangular distribution resistance presents a linear increase.When the overload reaches a certain degree,the bending moment calculated by the elastic-plastic resistance distribution is smaller than the bending moment calculated by the triangular distribution,and the difference between the two increases with the increase of the surface overload and the tunnel diameter.The calculation results show that the elastic resistance calculation may overestimate the influence of the stratum resistance for large-diameter tunnels and tunnels with overload on the ground,resulting in the low design rigidity of the tunnel. |
