Evaluation of the effect of rock joints on the stability of underground tunnels

Abstract

To facilitate the water supply to the main cities in Malaysia such as Kuala Lumpur, Putrajaya and Cyberjaya, Pahang Selangor Raw Water Transfer Tunnel project is constructed in 2015. Facility tunnel of 44.6 km length with 5.2 m diameter are being constructed to transfer water from Karak to Hulu Langat. Granitic rock with dominant intrusive zones in various range of I until V were observed in the studied tunnel length. This tunnel has crossed sheared zone that categorized into fair, poor, and very poor rock classes with moderate to heavily jointed rock. At NATM-1, the discontinuities like joints that is close and far from the fault are one of the most important which caused to instability of the tunnel and reduced the performance of tunnel performance. In this paper, the joint orientation, overburden and distributions of discontinuities (joints and faults) into the tunnel were evaluated. These data were then simulated using discrete fracture networks with the direction of tunnelling excavation in the tunnel. Movement of blocks in the tunnel roof and wall is possible due to the creation of more intersection points in the critical zone. It also highlights how minor features, such as step-over joints in rock mass, can have a significant impact on instability.