STUDY ON OPTIMIZATION OF CRD CONSTRUCTION METHOD FOR METRO TUNNEL THROUGH GROUND FISSURE SITE

Based on the background of Xi'an Metro Line 6 section tunnel with shallow excavation construction through the ground fissure site, and considering the particularity of the ground fissure site, the construction optimization method based on the traditional CRD method was constructed. The three-dimensional finite element numerical calculation was carried out to simulate the excavation process of traditional CRD and optimized CRD method. Combined with in-situ monitoring test data, the surface settlement deformation of the ground fissure site and the stress deformation characteristics of the metro tunnel were analyzed. The results show that the surface settlement deformation is anti-s-shaped, which can be roughly divided into three stages: small deformation before excavation, rapid settlement deformation during excavation and stable deformation after excavation. The closer the monitoring points are to the tunnel face, the higher the ground settlement. Compared with the traditional CRD method, the optimized CRD method can increase appropriately the number of locking feet and waist bolts, improve the concrete strength of primary support properly, and so as to simplify the temporary support, expand the construction surface of the lower stage, facilitate the removal of temporary support, and improve the speed of the closure of the initial support and the removal of the temporary middle diaphragm. The construction speed of the optimized CRD method is 1.37 times faster than that of the traditional CRD method. The maximum surface settlement is reduced by 52.96%, the influence range is reduced by 22.17%, and the settlement of the tunnel roof is reduced by 54.53%. The optimized CRD method has the advantages of rapid construction speed, lower influence range, easily controlled surface and structure settlement, which can not only improve engineering benefit, but also guarantee the construction safety and the reliability of the tunnel construction. The research results can provide a scientific reference for the underground excavation construction of metro tunnel through ground fissure site in Xi'an city and other areas.