A Recent Case Study of Portal Cavern Design

Abstract

A new dual-two lanes tunnel of about 3.8 kilometers long was constructed in Kowloon East in Hong Kong recently. It forms part of a major strategic road network to provide an express connectivity and improve the traffic condition between Kowloon East and Kowloon West. Two portal caverns, which are at the east end of the tunnels, are the first and largest of its kind with slender pillar constructed in highly fractured volcanic rock. A competent and optimised temporary cavern support design was required with the consideration of the pillar stability and construction logistics prior to the permanent support in place. A number of design reviews were carried out to suit the highly constrained construction sequence as the excavation works of the rock-cut slopes and the caverns were carried out concurrently. Some challenges that the project team had to deal with were installation of waterproof membrane and cast-in-situ reinforced concrete (RC) permanent lining for the crown that requires propping of steel shutter. Such challenges call for a cost saving design (CSD) with the use of sprayed waterproofing membrane and fibre reinforced sprayed concrete (FRSC) lining as the permanent support system for the portal caverns. This paper discusses the optimisation of the temporary support design, the CSD for the permanent cavern support faced by the construction works, and the design methodology of both the temporary and permanent cavern support with the details of the application of the sprayed waterproofing membrane.