Effect of Closely Spaced, Non-Persistent Ubiquitous Joint on Tunnel Boundary Deformation: A Case Study from Himachal Himalaya

Abstract

The stability of an underground tunnel excavated in a jointed rock mass is studied using the field investigation and numerical modelling. This research aims to numerically analyze the rockmass behavior as a function of closely spaced non-persistent joints. For this purpose, the Kainchi-mod Nerchowck twin tunnels (Himachal Pradesh, India) is chosen for the in-depth analysis. The host rock encountered is mainly gray sandstone and maroon sandstone with many closely spaced, non-persistent joints, dipping critically into the tunnel. The detailed rockmass properties were collected from the field and intact rock properties were tested in the laboratory. A series of finite element numerical simulations were conducted based on the filed/laboratory data with different values of joint spacing, including the actual values of field joint spacing. It was found that the extent of deformation above the excavation was predominantly controlled by the joint spacing. The results of this study provide an explicit correlation between geometrical features of the rock mass with the total displacement values around the excavation. The study will help the engineers to design an appropriate support system for heavily jointed rocmkass.