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Concrete In Australia : June 2013
4 Concrete in Australia Vol 39 No 2 NEWS Top-down tunnel completed in Perth Major tunnelling works on the new Fremantle Line tunnel in Perth have been completed, marking a signi cant engineering feat that has involved detailed planning, an Australian rst innovation and around-the-clock monitoring of existing structures to ensure any movement was within the allowed parameters. e project is being delivered by the Perth City Link Rail Alliance, consisting of the Western Australian Public Transport Authority, John Holland and GHD. e rail project is the rst stage of the overall Perth City Link project and involves sinking the Fremantle Line from William Street to Lake/ Kings Streets, building a new pedestrian underpass and special events platform inside Perth Station. e rail project is on schedule for completion in 2014. e new Fremantle Line tunnel has been constructed directly above the existing Joondalup Line tunnels with a gap of only 1.2m. Work was carried out adjacent to the live railway, and throughout construction the Joondalup Line tunnels remained operational with no impact to train services. e new tunnel is 600m long and built using top-down construction which involved four main stages: building the walls, installing the roof panels, excavating the soil and constructing the base slab. One hundred and y diaphragm walls make up the Fremantle Line tunnel, eight of which sit directly above the existing Joondalup tunnels. e diaphragm walls vary in depth from 8m (the section directly above the Joondalup Line tunnels) to 40m. While the diaphragm walls were up to 170m3 in volume and were poured via tremie pipe, Perth City Link alliance manager John Anderson said the largest single pour on the project was a tunnel base oor slab with a volume of 375m3. e oor slab thickness varied between 550mm to 1200mm throughout the tunnel, largely depending on the future load requirements of above ground structures set out in the master plan. Anderson said all pours required continuous concrete supply and the start of each concrete pour was highly variable, based on excavation progress, cage installation and cleaning of the support uid (Bentonite). e roof consisted of precast tensioned planks with a topping slab poured once they were in position. A er a thorough tender process, Anderson said Holcim was selected as the preferred supplier, based on provision of technical support, commercial and batch plant locality factors. Grace additives were used to increase ow, reduce water usage and increase the workability life of the mix. He said the major challenge with building the Fremantle Line tunnel above the Joondalup Line tunnels was the potential for movement, as the pressure keeping the Joondalup Line tunnels in place was altered by excavation and dewatering. "A dedicated monitoring team worked The new tunnel is 600m long and built using top-down construction which involved four main stages.