COMPARATIVE STUDY OF ROCK LOAD THEORIES FOR TUNNEL DESIGN OPTIMIZATION

Abstract

Because of increasing world population, urbanization and industrialization, the construction of underground tunnel structures is preferred, as they limit interferences with existing surface uses of the land and water bodies. Although underground tunnels are a common flexible construction alternative, they are high hazard risk structures. The risks are mostly related to ground conditions. Tunnels buried at depth disturb in-situ conditions, cause ground instability and ultimately failure. Permanency of underground structures is ensured by provision of adequate resistance in the form of primary and secondary supports to any impeding failure of the ground surrounding deep underground excavations. The effectiveness of the ground support interaction depends on geology, material properties, geotechnical parameters, loads of the surrounding ground mass and mechanism of the interaction. Using actual project information of Swat Twin Tube Tunnel, a 1.2 km long, transportation tunnel project in Khyber Pakhtunkhwa, Pakistan, various methods to select the required supports are explored in this dissertation. Geo mechanical classifications (RMR, Q and RMi) have been used for designing the primary supports (shotcrete and rockbolts), whereas a comparative study of various rock load theories was done to design the secondary support (reinforced concrete lining). The Finite Element Method (FEM) used the Rocscience PHASE2 software package for evaluating the primary support systems and the Computers and Structures, Inc. (CSI) SAP 2000 software package for designing the secondary support system. The study demonstrated that all explored methods were independent and dissimilar for the same geotechnical engineering challenge of the underground structure. Also, the outcomes of the different rock load theories in this study were unique functions of their underlying scientific philosophies and each required different input parameters. The study proposes that in designing adequate support systems (primary and secondary) to resist forces causing failure of underground tunnels, several methods should be considered rather than opting a single method for design, along with detailed laboratory and field testing. Also, a cost comparison analysis shows that the most conservative design should not always be chosen to ensure permanency, because it proves to be uneconomical. Rather a proper vigorous design of both the primary and secondary supports should be carried out to save huge amounts of money.