Geotechnical modelling of station caverns for the Epping to Chatswood rail line project

The Epping to Chatswood Rail Line project comprises a twin rail tunnel, three new underground stations and the upgrading of one existing station. The station caverns intersect a sequence of horizontally bedded shale and sandstone. The major rock defects consist of bedding planes, bedding plane seams, low angled cross bed partings and sub-vertical joint sets. Local experiences in the Sydney Basin have indicated that the behaviour of the defects and their interaction with the roof support system are critical to the performance of underground excavations. Another key geological feature that can have significant performance impact is the relatively high locked-in in situ lateral stresses.

Each of the new stations comprises a large span platform cavern, an adjoining concourse cavern, associated escalator shafts and service buildings. The design roof support system is cable bolts with cement grouted end anchorage. The interaction between the bolts and the jointed rock mass within the influences of the various facilities is complex and thus rigorous modelling was employed. This modelling included 3D distinct element and boundary element analyses and 2D finite element approach. The complexity of the numerical models varied from homogeneous rock to layered rock with various discontinuities. Furthermore, both end-anchored and fully grouted bolts have been incorporated.

Various parametric studies were undertaken to assess the effects of various model components (rock mass, defects, in situ stresses, sequencing and roof support installations) on cavern performance. Based on the modelling results, a system of rock bolts and construction staging has been adopted to optimise the permanent support system.

This paper is confined to the geotechnical modelling aspects of the project and presents the geological setting and the various analytical procedures undertaken. The results of the parametric studies and their impact on the final selection of the support systems are also outlined.