DESIGN OF A TUNNEL BY NEW AUSTRIAN TUNNELING METHOD (NATM)

Abstract

There are several methods for designing rock supports in tunnels. The Q-system is one of such methods, which is based on the rock mass classification. The Q-system was developed at Norwegian GeotechnicalInstitute (NGI) between 1971 and 1974 (Barton et al. (1974)). The rock supports recommendations fromQ-system are based on the Q-value of the rock mass, which is calculated from 6 rock parameters. The rock parameters for calculating Q-values are based on the orientation and number of discontinuity sets present in the rock mass, surface condition of discontinuities, groundwater conditions and insitu stress state in the rock mass. For a known Q-value and span/height of the tunnel, the Q-system recommends rock support in terms of thickness of fibre reinforced sprayed concrete layer, rock bolts, reinforced ribs of sprayed concrete (RRS) and in some cases cast concrete lining (CCA). There are several types of fibres available to reinforce the sprayed concrete, however it is chosen to use steel fibres for the present work. The reinforced ribs of sprayed concrete (RRS) are similar to a reinforced concrete beam and generally recommended for a very poor quality of rock mass. Based on the quality of rock mass, the Q-system recommends thickness and the reinforcement in the RRS. The installation of rock supports is a risky task, especially when the rock mass quality is very poor as the rock blocks may fall during installation. Moreover, the reinforcement for RRS need be placed duringinstallation, which makes the construction of tunnel difficult, time consuming and expensive. Several research papers are published to assess the validity of Q-system’s supports recommendations, one such study was conducted by Palmstrom and Broch (2006). The authors concluded that the Q-systemis a optimum method to design the supports for fair to very good rock masses, but it may provide impractical, unrealistic and conservative outcome for poor rock masses. The rock supports from Q-system are compared with the outcome from numerical analysis in this thesisaiming to obtain the optimized rock supports and subsequently improving the safety and pace of construction. The main objective set for this study is to check the possibility of reducing the requirement of reinforcement in the RRS focusing mainly on the poor quality of rock masses in general, having Q- values less than 1. The results from numerical analysis show that the rock supports recommended by Qsystem for the verypoor rock mass (Q <1) are conservative and can be optimised in terms of reducing the reinforcement inRRS, given that the detailed rock-lining interaction analysis is carried out.