Influence of spalling failure and post-peak strength degradation on a deep tunnel construction in Taiwan
Journal
ISRM International Symposium - EUROCK 2020
Date Issued
2020
Author(s)
Abstract
Tunnelling in hard rock is normally stable at shallow depth except for wedge failure. However, the excavation depth is increasing in Taiwan. A road tunnel with a deep overburden of 1288 m is being constructed. At depths where high in-situ stresses and stress concentration near the excavation boundary might result brittle failure occurrence due to hard rock overstressed. The brittle feature assessment and the tunnel deformation simulation considered the post-peak strength degradation on marble were performed. The results show that the post-peak strength beyond failure will rapidly drops down to a residual strength at low confining stress condition. The impact of post-peak strength degradation on tunnel stability should not be disregarded. During the tunnel construction, slightly weak schist encountered in the deepest area. Violent rock burst, therefore, does not occur. Unfortunately, the tunnel design is based on traditional elastic-plastic behaviour, that is, no post-peak strength degradation. Rock bolt system was only installed in the tunnel crown. Spalling type failure was found on the tunnel sidewall as the rock overburden over 800 m. Meanwhile, the actual tunnel deformation exceeds the design value. A re-mining work was executed due to the insufficient tunnel space for inner section. The instability phenomenon for the case tunnel construction illustrate that the effect of post-peak strength degradation would affect tunnelling behaviour. Cautious design and systematic monitoring should be fulfilled to overcome the possible problems caused by brittle failure in deep tunnelling. ? International Society for Rock Mechanics and Rock Engineering Norwegian Group for Rock Mechanics
Subjects
Deformation; Elastoplasticity; Rock bursts; Rocks; Spalling; Tunneling (excavation); Deep overburdens; Elastic-plastic behaviour; High in situ stress; Post-peak strength; Residual strength; Rock overburdens; Tunnel construction; Tunnel deformation; Tunnels
Type
conference paper