Small Radius Curve Tunnel Shield Under Complex Conditions Mechanical Roperty Research

The shield tunneling of subway tunnels is constrained by the dense building complexes and underground pipelines,and cannot be carried out in straight sections only,but often requires small-radius curved sections to go through adjacent buildings and avoid dense pipelines.Compared with the straight section shield tunneling,the small radius curve tunnel shield is more complicated,especially in the complex stratum of soft top and hard bottom,which can easily cause engineering problems such as destabilization damage,sudden increase of deformation and soil seepage in the surface and neighboring buildings.In this paper,we take the shield project of Liuzhou Orbital Heike interval as the background,and study the various mechanical characteristics during the shield tunneling of small radius curved tunnels under complex conditions and the underpassing of neighboring buildings and pipelines.The main research points and results are as follows:(1)Based on Peck’s basic theory and Mindlin’s solution,the calculation equation of surface settlement deformation triggered by various factors such as frictional resistance of shield shell,synchronous grouting pressure and stratum loss during shield tunnel boring in small radius curve is derived and modified.(2)The influence of unbalanced force coefficients such as uneven frictional resistance of shield shell and uneven additional thrust of excavation surface on the mechanism and evolution of surface deformation is discussed.When the unbalanced force coefficient increases,the center offset and the surface deformation value also increase,with an overall positive correlation.However,the total surface deformation value is different from that of the straight section tunnel shield,which does not show a linear relationship.(3)The effects of factors such as grouting pressure,grouting thickness and boring speed on surface settlement were studied by using the control variable method and single factor analysis method and numerical simulation analysis software.The results show that as the synchronous grouting pressure and grouting thickness are varied within a certain reasonable range,the surface settlement deformation is linearly related to them,i.e.the surface settlement decreases when both increase;the tunnel shield boring speed in the small radius curve section should be controlled between 10～20mm/min.(4)The impact of tunnel shield on adjacent buildings and pipelines in small radius curve section is discussed,the mechanism of surface deformation on adjacent buildings is analyzed,and the actual project is simulated for shield underpassing adjacent buildings and compared with the measured data,and finally the corresponding safety control measures and optimized construction parameters are proposed.