A comparative modeling on the failure of shotcrete for the calculation of safety factors of tunnels

Abstract

In a tunnel, failure of its supports can cause collapse of the whole tunnel. Thus, in estimating the safety factor of the tunnel, it is important to take the failure of its supports into account. In previous studies, shotcrete, which is one of the main supports for tunnels, was usually modelled by the elastic beam element which never fails in numerical calculations. According to the Working Stress Design (WSD) concept, the failure of the beam element can be indirectly checked by comparing the allowable stress and the calculated bending stresses inside the beam element. This approach has been also applied in estimating the safety factor of a supported tunnel. However, the calculated safety factors are usually underestimated due to the excessive stress concentration on the shotcrete. In this study, therefore, it is suggested how to properly model shotcrete and consider its failure. To this end, shotcrete was modelled by both the beam (elastic) element and the continuum (elasto-plastic) element in two dimensional numerical analyses. In addition, how the rock mass class, coefficient of lateral pressure, thickness of shotcrete, and excavation method effect the safety factor of a supported tunnel were analyzed. (A) This paper was presented at Safety in the underground space - Proceedings of the ITA-AITES 2006 World Tunnel Congress and the 32nd ITA General Assembly, Seoul, Korea, 22-27 April 2006. For the covering abstract see ITRD E129148. Reprinted with permission from Elsevier.