Excavation of the 20 km northside storage tunnel in Sydney, Australia is complete. The fourth TBM breakthrough for the last 1.2km of drive finally took place on 16 July 2000.
The troubled project has made up ground, and completion of all excavation means that the four Alliance partners – project owner Sydney Water; project consultants Connell Wagner and Montgomery Watson; and contractor Transfield – should at least have achieved the start of commissioning before the Sydney Olympics. The tunnel may be able to operate at some point during the games; a race is now on to complete the final tunnel support structures, clean the tunnel out and complete the mechanical and electrical installations.
The tunnel has been built to capture the majority of the largest wet weather sewer overflow events at four locations on Sydney’s North Shore.
The breakthrough took place after two years almost to the day, when ground was first broken at the North Head sewage treatment plant – which treats all of Sydney’s North Shore "output".
Four TBMs varying in size from 3.8m to 6.6m in diameter were used, with five up to 300kW roadheaders excavating 240,000m3 of Hawkesbury sandstone for access declines, pump and screening chambers.
A total of 1.8Mt of excavated sandstone (spoil) has been barged across Sydney Harbour to a central off-loading point and onwards on a 50km long haulage route by train.
The major challenge on the scheme was at Middle Harbour, where geotechnical investigations indicated a paleochannel that had experienced severe "valley bulging" (T&TI May). Open horizontal bedding planes and sub-vertical stress fractures, allowed ground water to reach the tunnel level.
"For this zone of extremely high permeability, we had to adopt a specialist pre-excavation grouting technique," said project manager Russell Cuttler.
The 6.6m Wirth TBM fitted with twin Tamrock probe drills pre-grouted a 250 m length of tunnel, underneath the base of the paleochannel. Up to 42 (38 long) probe holes in advance of the face were repeatedly drilled for more than 13 km and 180 dry tonnes of cement grout were injected – thus reducing the potential inflow of an estimated 200lt/sec to less than 2 lt/sec. This equals a grouting efficiency of almost 99%.
The 6.6m Wirth TBM under Middle Harbour advanced at less than 100m per month, compared to the 200m achieved when excavation conditions were more favourable. Under the harbour there were 10-20m grouting cycles and it took three months to complete the crossing.
