A rational method for assessing the occurrence of brittle failure in deep tunnels

If not properly predicted and accounted for during the tunnel design process, brittle failure can lead to time, cost and safety issues. To support designers in addressing these dangerous and critical phenomena, this paper presents a suggested design process for deep tunnels with suspected brittle failure potential. The strength of this approach is that it enables understanding whether brittle failure is likely during excavation and, if so, predicting its magnitude. The first step is to apply the recently developed and presented Tunnel Brittleness Index (TBI). TBI quantifies the potential of ductile and brittle failure around deep tunnels by analyzing the competition between the two failure mechanisms. The collection of the input parameters needed to calculate TBI will be described, and its operating principles will be recapped. When brittle failure mechanism results predominant over the ductile one, based on the TBI calculation, numerical analyses by the hybrid Finite-Discrete Element Method (FDEM) represent a powerful tool to address its occurrence and intensity at the site. Hence, the execution of such analyses to study the sitescale problem constitutes the second step of the suggested design process. The main aspects to be careful to when performing such analyses will be outlined. After describing the steps of the methodological approach, the case study of the Turin-Lyon Base tunnel will be addressed to demonstrate its applicability.