MECHANIC RESPONSE OF A SHALLOW-EMBEDDED AND DOUBLE-ARCH TUNNEL UNDER PARTIAL PRESSURES DURING CONSTRUCTION

Taking an expressway in Jiangxi province as the engineering background, the behavior of a shallow-embedded and double-arch tunnel with partial-pressure during construction is simulated.Several aspects, such as stress concentration and transfer, plastic zone distribution, vault displacement and change of vertical stress, (σ_y)of the mid-leading wall with construction are studied systemtically. The study indicates that :1) asymmetrical excavation is the key cause of partial pressure in the mid-leading wall and the construction of the first and second lining can not ameliorate the conditions greatly; 2) under conditions of shallow embedment, vault displacement increases with embedment depth and displacement mainly happens during excavation of tunnel before lining construction; 3)after excavation of the up-bench tunnel, pulling stress concentrates around the top arch and lining should be constructed promptly or beforehand; 4)as a result of partial pressure, when plastic zones around the side-wall and mid-leading wall heel, which are near the mountain ridge, are better developed; and 5) when construction finished, compressive stresses at tunnel side-walls disappear and are transferred to the inverse arch and second lining, making them concentrated by shear stress and normal stress and improving the stability of the tunnel.