Tunnelling at Forrestfield, the site that hosts the infrastructure vital to tunnel boring machine (TBM) operations, began in August (TBM Grace) and October (TBM Sandy) 2017. The site is operating 24 hours a day to support tunnelling activities, including processing excavated soil and treating water for re-use in the tunnelling process.
In a bid to unite the divided construction site at Forrestfield (dive structure, stowage area construction and station works to the west – TBM operations to the east), a section of Dundas Road has been realigned earlier this year. The road changes allow backfilling of the station retaining wall to continue.
The 300m long retaining wall for Forrestfield Station is almost completed, with 89 of the 93 panels in place. The remaining four panels will be installed once the multi-service vehicles (MSVs) no longer require access to the tunnels. More than 100 counterforts are being used to keep the 8.2m high wall securely in place. The counterforts support the 18 tonne wall panels, as well as the backfilling which is required to raise the ground level at the station by up to 6m.
Forrestfield Station is starting to take shape with the construction of the station’s platform foundations now in progress. The island platform will be 150m long, 11.5m wide and will help move an estimated 9800 passengers to and from the new station daily.
At the station concourse level installation of services is completed and construction of the concourse foundation slab has commenced. Works on the station roof will take place early-2019.
The stowage area, at the southern end of the site, is also coming along nicely with the platform well-advanced and the stowage building 85 per cent completed. The stowage area will be used to clean the trains and park them overnight, as well as for minor repairs and maintenance works.
The lift shaft for the service lift located at the dive portal further north on the alignment has been poured and formwork will be stripped before the end of the year.
On Saturday September 22, a leak developed at the first tunnel-to-tunnel cross passage, approximately 200m north of the Forrestfield Station site. Immediate efforts were made to stop the leak, however the significant water pressure caused the leak to grow to approximately 100mm in size. As a result, water and silt entered Tunnel One (TBM Grace’s tunnel) leading to the formation of a sinkhole at the surface alongside Dundas Road.
Immediately following the incident, a number of short-term measures were implemented to stabilise the tunnel and surrounding ground. These actions included installing temporary bracing within the tunnel, backfilling the sinkhole to avoid further ground movement and working to stem the leak. Compaction grouting to improve the ground conditions near Dundas Road was completed in several stages, with ground stability assessed after each stage.
A section of Dundas Road was closed following the incident, however two narrow temporary lanes have been opened to all motorists (including heavy vehicles) on December 21. Some traffic management, including reduced speed limits, will remain in place and further construction activity, including road repairs, will be ongoing in the New Year.