The new Roma Street underground railway station in Brisbane is being constructed as part of Cross River Rail’s Tunnel, Stations and Development (TSD) package. The joint venture of CPB Contractors, BAM International Australia, Ghella and UGL (CBGU JV) is building the 5.9km long twin tunnels from the Southern Tunnel Portal near Dutton Park station, beneath the Brisbane River and CBD to the Northern Tunnel Portal in Spring Hill. The Cross River Rail project includes excavation and construction of four new underground stations.
Roma Street station comprises a 280m long cavern, five smaller connecting tunnels (adits) and three shafts. The station cavern has an excavated span of up to 24.4m with approximately 15m rock cover. It has been excavated within the Neranleigh-Fernvale Group (NFG) rock mass, which comprises weakly metamorphosed sandstone (meta-greywacke and arenite), phyllite and subordinate quartzite and meta-basalt. The station lies within the regional Normanby Fault Zone, characterised by a major fault up to 20m wide comprising a combination of intact rock, rock breccia and clay gouge. The fault zone encountered during the station cavern excavation required heavier primary support and localised foundation treatment.
The initial primary (temporary) support of the cavern and adits comprised rock bolts, cable bolts and a thin synthetic fibre-reinforced shotcrete lining. In some areas a passive shotcrete arch lining was required. Overlying piled footings from an existing busway overpass structure were within a metre of the adits’ excavated profile which necessitated a complex load transfer structure at the surface and verification of pile toe levels during tunnel construction.
The cavern permanent lining typically comprises steel fibre-reinforced concrete in the crown, bar reinforced concrete for the sidewalls, and bar and steel fibre-reinforced concrete invert slabs. Bar reinforcement is used in the cavern crown where it intersects the adits. Ground loads for the permanent structure had to consider the influence of future developments.
This paper presents some of the challenges of the primary support and permanent lining design of the station cavern and adits. It summarises the as-encountered ground conditions, aspects of the primary support and permanent lining design that were geotechnically challenging and the solutions developed to meet the project requirements.