| At present,subways are under large-scale construction.The shield tunneling method is the best choice for construction of interval tunnels,and the stratum condition plays a decisive role in the selection of tunneling parameters.The sand layer at low moisture content is different from the dry or the saturated sand layer,and it has apparent cohesion between particles and soil arching effect which has a positive engineering effect for shield tunnels.In order to study the characteristics of the soil arching effect during the shield tunnel construction and reveal the face stability mechanism under the condition of the sand layer at low moisture content,Shijiazhuang’s subway project is taken as the background in the dissertation,and the physical and mechanical experiments on the sand at low moisture content and the model experiments on tunnel instability and face failure are carried out,and the corresponding discrete element method(DEM)simulation is performed.The main research results obtained are as follows:(1)The soil water characteristic curve(SWCC)for sand samples in the Shijiazhuang subway construction was obtained by the filter paper method,and the relationship between matric suction,cohesion and degree of compaction was acquired and analyzed.By the direct shear test and the slump test for the sand samples at low moisture content,the relationship between shear strength and matric suction was determined,and the calculation model of apparent cohesion affected by matric suction was put forward.(2)A device of trapdoor model test applying to the low moisture content sand was designed and a particle image velocimetry technology(PIV)system was equipped to realize the visualization for the instability process of the sand layer.In view of five water contents and three depths,fifteen groups of trapdoor model tests were carried out.The soil arching effect and the soil pressure law for the sand at low moisture content were obtained.Moreover,the corresponding numerical simulation was performed,and the influence of water content on the soil arching effect was revealed from the micro level,such as porosity of sand layer,coordination number and principal stress rotation.(3)According to the instability modes of tunnel under different water contents,loose earth pressure formulas under the ellipsoid failure mode for the sand layer at low moisture content were deduced,and it was also confirmed by the DEM simulation.(4)A device of model test for stability of shield tunnel in the sand layer was designed and a particle image velocimetry(PIV)system was equipped to visualize the instability process of sand particles on the tunnel face.In view of three water contents and three depths,nine groups of trapdoor model tests were carried out,and the instability evolution of tunnel face and the change of supporting forces were revealed.Furthermore,the corresponding DEM simulation was performed,and the distribution of force chains,the change law of porosity and coordination number indicated the arching characteristics of the sand at low moisture content.(5)Based on the model test results of the stability of the shield tunnel face,the formula of the ultimate supporting force for the face was deduced and it also was confirmed by DEM simulation.(6)A discrete-continuous coupling numerical simulation method for the dynamic construction process of shield tunnels was put forward.Based on the engineering background of Shijiazhuang’s interval tunnels in the sand layer at low moisture content,the shield tunneling process in the dry sand layer and the sand layer at low moisture content were simulated,respectively.The results from the mesoscopic level,including the displacement of particles,motion states and contact states,revealed the face stability and stratum deformation law under different tunneling parameters.The experience of the lower pressure tunneling mode was adopted in the Shijiazhuang’s shield tunnel construction in the sand layer at low moisture content,and it demonstrated the results of characteristics of the arching effect of the low moisture content sand. |
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