Top-down build for Perth city bypass Jul 1998

A 1.6km long tunnel in Perth, Western Australia, is the first road tunnel to be construction in WA and is part of a US$197 million bypass to ease congestion in the centre of the state capital. The total 6.5km stretch of new highway, including a new bridge across the Swan River, began in 1996 and is scheduled to open in June 2000.

Fig 1. Route of the 1.6km tunnel around the Perth central business district

The need for the east-west highway around the city centre was identified in a traffic study in 1954. It was officially incorporated into the Metropolitan Planning scheme in 1963, when the state reserved the necessary rights-of-way and started buying property along the corridor. Construction of the project was announced in 1995, when it was included in the State Government’s ten-year state-wide additional road building programme which is financed largely by a levy on fuel.

Of the project’s US$197 million cost (increased to US$207.6 million in June 1997 with inflation and other adjustments), US$154.7 million is the tender value of the two design-build contracts awarded by the State’s Main Roads Department for construction of the new highway and US$20.5 million covers acquisition in 1995 of the final stretches of land needed to complete the corridor.

Originally, the route through the once run-down industrial Perth suburb of Northridge was at grade. Today, the area is the city’s rejuvenated cultural centre and property prices have soared. Following public objection to the surface alignment, and the number of heritage buildings that were likely to be demolished, the 23m wide highway was forced underground, adding an estimated US$58.8 million to the original US$60 million surface alignment plan for the 1.6km long section (Fig 1).

Of the two design-build packages, the SUS120 million west-end tunnel package was awarded to the JV of Baulderstone Hornibrook (owned by Bilfinger+Berger of Germany) and local company Clough Engineering in May 1996. Australian JV Transfield-Thiess was awarded the east-end bridge over the Swan River and road contract in March 1997.

Baulderstone-Clough JV (BCJV) started work on the tunnel in September 1996. Designed by The Maunsell Group, including local firm Halpern Glick Maunsell, and tunnel ventilation engineer Parsons Brinckerhoff (Asia), the JV is completing the underground works via a specified top­down cover-and-cut method.

The BCJV tunnel contract also includes laying the road deck, installing all M&E, ventilation, drainage and pumping services, construction of surface works, and extensive water, gas, power, telephone and sewer pipeline diversions. It also includes a 200m long cut-cover tunnel to relocate the Westrail rail tracks under the new highway alignment as it extends on the surface from the east end of the highway tunnel.

Top section excavation of the sandy core reveals the underneath of the installed roof slab, the outer diaphragm walls and the central column of temporary bored piles supports

When TunnelTalk visited the site in February 1998, some 50% of the diaphragm wall installed as the outer walls of the full width tunnel corridor was complete and the concrete roof slab was in place over about 40% of the route. Excavation of the core and integrated casting of the floor slab had advanced some 250m and casting of the two internal walls had also started as part of the rolling construction programme.

Time and cost saver

According to Project Director Ray Purdy, the use of an innovative conveyor system as part of the construction method has saved a significant amount of time and cost. The 100m long conveyor is suspended on a monorail attached to the top-deck ceiling and is installed to permits casting of the floor slabs to be carried out at the same time as core excavation.

Core excavation under the roof takes place in two stages. First, the top section is exposed and diagonal props are installed to support the outer walls until the floor slab is cast (Fig 2). Bulk excavation then continues, with a Caterpillar 966 front end-loader shovelling the sand into the 5m³ hopper at the front of the conveyor system. Running at a speed of about 1.5m/sec, the 1.2m wide x 800 tonne/hr capacity conveyor system takes about 3-4 minutes to discharge each 20m³ truck load.

When TunnelTalk visited the site, the JV was completing three concrete floor pours or sections every two weeks, working one 9hr shift/day, six days/week, each section comprising 320m³ of ready-mix concrete from local plants. It was aiming to improve this to two 14.75m long sections or 29.5 linear meters of floor/week to catch up on a two-month delay. Excavation work at the face continues during concreting but there is no loading via the conveyor.

Meanwhile, on the surface, construction of the 800mm thick x up to 30m deep diaphragm walls were progressing at a rate of seven 6m long panels/week using a 0.6m³ kelly bar grab. A second rig, a Bauer BG22 auger piler imported from Germany, has been installing a row of 30m deep x 1100mm diameter temporary piles at 4-5m centres down the middle of the tunnel to add support of the roof during excavation and until the floor and the central walls are constructed.

Fig 3. Breakdown of the US$120 million contract awarded to the Baulderstone-Clough JV in May 1996
Conversion rate at the time US$1.00 = Aust$1.70

Closer to the tunnel’s eastern end, where the construction corridor is wider, temporary Franki piles are used for the central supports and to speed up progress. Two rows are required under surface roads, which are re-opened to traffic once the diaphragm walls and the roof slabs are completed. Drainage installed in the floor slab and a central sump and pumping system will remove any stormwater entering the tunnel. The two 400mm thick internal walls, with evacuation doors at 60m intervals, create a 1.4m wide corridor to carry services and provide an emergency escape route.

Initially, the tunnel will have two traffic lanes and an emergency lane in each direction. As demand increases, the emergency lane will be used to provide a total of three lanes in each direction.

Ventilation system

The ventilation system is based largely on the piston effect of the traffic with large exhaust fans housed in two ventilation buildings, one at each portal, run continuously to extract fumes. In the event of high traffic loads, a series of jet fans attached to the ceiling of the tunnel will operate automatically to assist flow of air to the vent ducts. The ceiling will be coated with a four-hour fire resistant layer supplied by Promat and the walls will be fire treated and lined with ceramic tiles. Adequate lighting and other traffic signal and information services will be installed to provide a state-of-the­ art road tunnel facility.

As work progresses, an update from Perth indicated that the project remained on schedule for the June 2000 opening. By early June 1998, 620m of the tunnel core had been excavated, with 992m remaining; some 570m of the concrete floor slab was in place, with 1,060m or 65% to do; and casting of the internal walls was more than 400m into the 1,630m total. On the surface, diaphragm wall installation was 72% complete, with 452m remaining, and some 63% or 1,026m of the roof slab was in place, with 604m left to do.