With the introduction of a new four-year degree program at The University of Hong Kong, a Centennial Campus is being developed at the western side of the existing Main Campus. The scope of this infrastructure project comprises the design, construction and commissioning of two new salt water and two new fresh water service reservoirs. Cavern was excavated in sandstone and tuff to accommodate the new salt water reservoir in a twin-cell tunnel system. The cavern was constructed inside a sloping ground due to the need to find adequate rock cover. Starting at the portal, an about 30 m long, 7.2 m span access tunnel was constructed and then separated into two reservoir tunnels. Two 10 m long transition zones were constructed and then the tunnels were enlarged from 7.2 m span to 17 m span to create the cavern for the new salt water reservoir. With the requirements of minimal damage and disturbance to the rock mass during the excavation, it provided an ample opportunity to study the convergence of the cavern as the excavation approached an undisturbed zone. This paper presents an evaluation of the magnitude of stresses acting on the crown of the large span tunnel at different stages of ground movement.

Back analysis was carried out based on the observed stresses and deformation resulted from approximately 6 m high top heading tunnel excavation. Two-dimensional finite element analysis program was utilized for this back analysis. Different empirical equations such as Bieniawski, Serafim and Pereira, Barton and Hoek et al. have been used in the numerical analysis to simulate the rock mass behaviors. It revealed that the Serafim and Pereira estimation was generally applicable for this particular HKU cavern project with volcanic tuff bedrock of as-mapped Q-value higher than 2.3 or RMR value higher than 50.

The observed monitoring records also demonstrated that approximately 2 mm of vertical deformation was mobilized to provide an efficient temporary support to the tunnel crown. Little stress relief and deformation was observed after the temporary rock support has been substantially mobilized.

This paper presents a case-study of the rock behavior due to the underground opening. It also demonstrated the performance of immediate support to the tunnel crown in controlling the ground settlement and stress relaxation.

1.1 Project Background

In April 2007, Gammon Construction Limited was commissioned for the design and construction of re-provisioning of the Water Services Department (WSD) utilities and infrastructure works for the proposed Centennial Campus at The University of Hong Kong. The re-provisioning service reservoirs included the fresh water and salt water service reservoirs with a capacity of 26,500 m3 and 12,000 m3 respectively, in which the proposed salt water service reservoirs were located inside the 17 m span rock cavern in a complex geology (see Figure 1 for the site layout plan). In this project, drill and mechanical breaking was adopted rather than drill and blasting to excavate the caverns.

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