Model test and discrete element method simulation of shield tunneling face stability in transparent clay

The stability of the shield tunneling face is an extremely important factor affecting the safety of tunnel construction.In this study,a transparent clay with properties similar to those of Tianjin clay is prepared and a new transparent clay model test apparatus is developed to overcome the"black box"problem in the traditional model test.The stability of the shield tunneling face(failure mode,influence range,support force,and surface settlement)is investigated in transparent clay under active failure.A series of transparent clay model tests is performed to investigate the active failure mode,influence range,and support force of the shield tunneling face under different burial depth conditions,whereas particle flow code three-dimensional numerical simulations are conducted to verify the failure mode of the shield tunneling face and surface settlement along the transverse section under different burial depth conditions.The results show that the engineering characteristics of transparent clay are similar to those of soft clay in Binhai,Tianjin and satisfy visibility requirements.Two types of failure modes are obtained:the overall failure mode(cover/diameter:C/D≤1.0)and local failure mode(C/D≥2.0).The influence range of the transverse section is wider than that of the longitudinal section when C/D≥2.0.Additionally,the normalized thresholds of the relative displacement and support force ratio are 3％-6％and 0.2-0.4,respectively.Owing to the cushioning effect of the clay layer,the surface settlement is significantly reduced as the tunnel burial depth increases.