Mechanical Behavior Of Metro Shield Tunnel Segment Under The Condition Of Upper-soft And Lower-hard Composite Stratum

Nowadays design and construction of the metro shield tunnel is more depond on the experience of the engineer when tunnelling under the condition of upper-soft and lower-hard composite stratum,and no theoretical design and construction system to rely on.The deformation and mechanical behavior of the segment lining ring in this stratum certainly be differ with in homogeneous soil layers.This paper attempts to explore some of the issues,focusing on some different aspects: design methods?construction phaseoperation phase and worked under earthquake on the following aspects:(1)3D simulation model was build,which included the segment and bolt,to simulation the deformation and stress during the shield tunneling under the condition of upper-soft and lower-hard composite stratum,and then analysis the influence of the ratio of soft soil in complex formation.The results show that there is almost no vertical displacement at the crown of the segment ring,and the arch segment convergences to the inner side under the condition of upper-soft and lower-hard composite stratum.The axial force calculated by spring-beam method can be used in shield tunnel segment structure design,the bending moment of segment is related to the ratio of soft soil,m=0.85 should be taken into consideration when constructional design.(2)The lifting jack force of the shield tunnel during the construction phase take the empirical value in simulation,while in this paper,the lifting jack force formula is obtained in the context of upper-soft and lower-hard composite stratum based on practical engineering,namely,the Zhuhai urban subway to airport.Three-dimensional finite element software is adopted to analyze the deformation and mechanical behavior of the segment through 10 segment rings of the tunnel when tunneling at the construction stage.Result from calculation of the influence of the parameters m,which is the indicator of the composite ratio of the upper-soft and lower-hard stratum,shows that: there exists only little difference between the result of theoretical calculations and actual measurements,which means that the theoretical calculation can be used to provide reference to the actual construction and numerical simulation.Longitudinal deformation and lateral deformation of the lining segment ring will change with the composite ratio m during the construction stage.The additional bending moments induced by the uniformly distributed loads Fs and Fl reached their maximum values when m=0.5,and accordingly,the longitudinal deformation difference will reach its maximum(1.4mm)at the tail pipe.(3)Based on stratum structure method,the 2D model of the operation stage of the subway and earthquake under the condition of upper-soft and lower-hard composite stratum were established to discuss the mechanical behavior of the segment lining ring under the train dynamic loading,and discuss the stability mechanism of the segment lining ring and the surrounding rock.Permanent settlement under long-term dynamic loading was predicted with the empirical formula,the theoretical analysis of when the shield tunnel from soft soil layers into a hard rock,the processing method of excessive differential settlement was given in this paper.