Study on mechanical properties of shotcrete lattice girder in a tunnel considering the excavation effect at early-age

Abstract

The lattice girder (LG) and shotcrete work together to provide support in tunnel engineering, being influenced by the hardening process of concrete and the excavation process. To elucidate the dynamic changes in the mechanical behavior of the LG during this process, the present study conducted indoor experiments using a self-developed loading system suitable for early age flow-plastic state shotcrete LG. Through the secondary development of the ABAQUS concrete hardening process constitutive model, an analysis of parameters such as curvature and load values was conducted to understand the evolution of the load patterns on the LG. Subsequently, relying on the background of the construction of an underground excavation station in Qingdao Metro, refined numerical simulations of the construction process were carried out. Combining the analysis with measured data, the following conclusions were drawn: 1. In the first three days after concrete casting, the LG specimen experiences relatively low stress, and after seven days, the LG specimen and concrete form a cohesive synergy; 2. The initial load caused the deterioration of the supporting performance of the unhardened grid concrete specimens, and the bearing capacity of the test group was reduced by 25% compared with the control group; 3. In the early stages of tunnel excavation and support, due to the low hardening process of shotcrete, the ability to transfer the surrounding rock load to the LG is limited, and the layout of the LG has no significant impact on the stability of the surrounding rock; 4. The collaborative support effect between the rebar bent frame (RBF) and concrete is greater than that of the LG, and it coordinates better with prestressed anchor bolts, presenting advantages in hard rock tunnels.