Tunnel design for fractured rocks and suitable support selection

Abstract

This research focused on design of a tunnel in highly fractured rocks, and to propose suitable support classes, based on the study carried out at the Upper Kotmale Hydro Power project location. The necessity arises in order to cope up the unpredictable changes in geological patterns which highly influence the continuity of tunnel works. Present tunnelling method in fractured rocks need to be further evaluated to suit the engineering geological patterns and rock types of the area. The rock strength and the fracture intensity are assessed through field and laboratory works, including tunnel mapping and joint intensity surveys. The suitable support class selected based on Q-value system formulated by Nick Barton (Norway) in mid 70's.The data and the borehole samples obtained from site location facilitated to propose a proper support design, suitable to Sri Lankan geotechnical conditions. The variation in the rock mass from unweathered to highly weathered, sound to heavily fractured along a tunnel was accounted in selecting a suitable supporting method to avoid disastrous consequences and to provide a secure working environment. The results of the study were used to develop a model for tunnel design, and to propose modifications to the existing support classes in fractured rocks at Upper Kotmale Hydro Power Project. And it was understood Q-value evaluation could be more suitable than RMR, for Sri Lankan terrain.