TransGrid appointed GFWAust to clean the tunnel invert and install additional permanent support in the form of 900 GFR bolts and 200 sheets of stainless steel mesh
The MetroGrid project involves the installation of a 28-kilometre electricity cable between an existing substation at Picnic Point in southern Sydney and a new substation at Haymarket in the Sydney Central Business District (CBD). The majority of the cable is laid in a trench, however the last section of cable, between Alexandria and Haymarket, is laid in a specially constructed tunnel.
The cable tunnel is 3.5km in length and was constructed both by Tunnel Boring Machine (TBM) and roadheader. The tunnel commences from a shaft in Sydney Park at Alexandria and follows a route underneath road reserves through the suburbs of Waterloo, Redfern and Ultimo to a new substation in Haymarket. The TBM tunnel was constructed with a round profile diameter of 3.2m and a length of 3km. The roadheader section has a rectangular profile with and arched crown and is approximately 500m in length.
The contract to excavate the tunnel was awarded to Walter Construction in February 2002 and completed in 2004. On completion of the tunnelling works a permanent monitoring and ventilation system was installed prior to the 330kV and 132kV cables being made operational.
Water ingress into the tunnel brings iron rich deposits which block the drainage system and discharge into the local waterways. GFWAust were appointed to clean these deposits from the invert of the tunnel prior to any further work.
The overall ground support for the entire tunnel is rock bolts and mesh with 40% of the tunnel lined with 75mm thick steel fibre reinforced shotcrete. 330kV cables are installed throughout the entire length of the tunnel and are encased in sand/cement bedding with a retaining wall. 132kV cables for Energy Australia are installed in the roadheader section on wall brackets on either side of the tunnel.
Due to the nature of the ground water in the tunnel all steel roof support installed during construction were considered only to be of a temporary nature. TransGrid appointed GFWAust to install glass fibre reinforced bolts as permanent support in the tunnel and stainless steel mesh.