Simulation of Complex Support Systems for Large Span Tunnels: Investigation on Support Interferences and Effects of Constitutive Models

Abstract

Ground settlement control is a critical aspect in underground projects with shallow overburden. In tunnels with large span, the use of common support elements such as shotcrete and lattice girder are not sufficient in order to provide tunnel stability with acceptable safety factor and additional supporting elements may be required. In this research, the effect of combination of multiple support elements, including shotcrete, fore-poling, nailing, and micro-pile to minimise ground settlements, have been investigated. This case study focuses on the Arash-Esfandiar tunnel, a shallow underground passage located in the northern part of Tehran, Iran with a total length of 1532 m. According to the geotechnical report, ground condition varies from silty sandy gravel to dense clay sand. Finite Element (FE) analyses were performed by assuming different constitutive models i.e., Mohr-Coulomb (MC), Hardening Soil (HS) and Hardening Soil with Small strain stiffness (HSS), to investigate the capability of linear and non-linear models on predicting the surface settlement in the study. The results indicate that axial and bending elements as tunnel support measures concurrently, affects more in ground settlement control. On-site measurements and the results of numerical modelling show a significant effect of removing temporary lattice girder on surface settlements. The research is novel in its application of various constitutive soil models – MC, HS and HSS – to predict surface settlement effects. Comparative analysis of FE results with on-site measurements reveals the significant influence of removing temporary lattice girders on surface settlements. It is found that while the MC model is unable to capture the full complexity of the conditions governing the project and the HS and HSS models demonstrate a higher fidelity in representing the soil behavior during the tunneling process. Whereas on-site measurements indicate a higher impact of excavation stages showing larger deformations. Considering the heights of the walls, during the final stage of excavation the invert didn’t have much effect on surface displacements.