Abstract: Under the disturbance of shield tunnel construction,the dense sand layer is prone to exhibit strain softening Behavior,which leads to the deterioration of strata stability.In this context,greater support pressure is needed to maintain the tunnel face stability.In order to clarify the instability characteristics of the shield tunnel face in dense sand layer,based on the critical state constitutive model of sand material,the nonlocal theory is introduced to regularize the ill-posed numerical problem associated with sand softening.The progressive failure process of shield tunnel face under the condition of insufficient support pressure in dense sand layer is simulated and analyzed.The results show that with the accumulation of the tunnel face deformation,the support pressure decreases to the minimum and then increases gradually,indicating that the softening of dense sand leads to the increase of the critical support pressure needed to maintain the stability of the excavation face.Compared with the ideal elastoplastic model,the sand softening model leads to the expansion of the failure region determined by the displacement field and plastic shear strain distribution.The distribution of soil pressure and state parameters in the stratum determines that the softening of sand occurs at the sliding surface,and the anti-sliding force at the sliding surface and the soil arch effect in the stratum are reduced,which leads to the degradation of the tunnel face stability.The increase of sand critical friction angle reduces the critical support pressure and restrict the instability range of tunnel face,while the decrease of critical friction angle magnifies the negative effect of sand softening on the tunnel face stability.