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  Design of Large Span and Shallow Ramp Tunnels

  The St. Peters Interchange ramp tunnels are situated at the northern end of the New M5 tunnels, part of the larger WestConnex project in Sydney, Australia. This paper presents an overview of the design for two large span and shallow cover ramp tunnels as designed for the New M5 tunnels. These ramps were excavated through the Ashfield Shale Formation, crossing dykes and faulted ground with a live and heavy traffic dual carriage roadway and a heritage building directly above the portal openings. The shallow ramp tunnels were designed to be arched profile tunnels up to 23 m in span and with a ground cover to span ratio as low as C/D = 0.3 near the portal. The design considered a sequential excavation method with up to 3 headings plus bench targeting to sustain long-term ground loads and control tunnel deformation. The tunnel permanent primary support system comprised canopy tubes, rock bolts, lattice girder, fibre reinforced shotcrete lining and allowance for micropiles if required. A secondary sprayed shotcrete lining was also designed to address fire safety requirements and potential long-term deterioration of the lattice girders in case of poor encapsulation. Both two and three-dimensional finite element numerical analysis were carried out to estimate the ground movements and stress redistribution associated with proposed tunnel excavation sequence and short-term and long-term performance of the proposed tunnel support system design. Seismic and fire design were assessed for secondary sprayed linings to take into consideration the long-term durability requirement for 100 years design life. The full span excavation is now complete and comparison with as-built performance is also discussed. 1.

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