Review of application of the Q-system for igneous rock tunnel intersection design in Hong Kong

Abstract

When Q-system is adopted as a design approach to provide support at tunnel intersections, a fixed adjustment is multiplied to the joint set number (Jn). The required support pressure is enlarged by a fixed extent relative to the support pressure out of the intersection area. Such factoring approach on the support pressure only provides a conservative support requirement in the intersection, while the project-based scenarios including stress state, strength of the rock mass and the extent of stress concentration in between two main tunnels were not included. The case of Tseung Kwan O-Lam Tin Tunnel was chosen to analyze the factors affecting the required support pressure and the determination on the extent of the intersection area. The current design approaches were also reviewed for their design mechanisms and limitations. By gathering the output from the review, an alternative framework to provide tunnel intersections design using Q-system is proposed to enhance the flexibility in design under different project settings.