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Sydney orbital heads east

The M5 East motorway project in Sydney, Australia is the latest link in the 2000 Olympic city's growing orbital road network and is throwing up some interesting tunnelling and geotechnical challenges.

While the focus on construction in Sydney has been firmly on the state of the art facilities for this year's Olympic Games, equally important projects have been under way to improve the city's infrastructure.

The new eastern distributor road, linking Sydney Harbour Bridge to the airport and forming part of the city's orbital road network, was recently completed, along with a new rail link to the airport.And after all the medals have been won and records broken, work will continue on another important part of the orbital link, the M5 East motorway.

The 10km long four-lane highway will close the gap between the M5 at Beverly Hills to the west and the airport's General Holmes Drive.The project has a 100-year design life and includes upgrading the M5 from two lanes to four, a 3.8km long twin-tube rock tunnel beneath the city, an underpass tunnelled in soft ground beneath a railway and a 200m long tunnel beneath the Cooks River.

More than 4km of the new route will run at grade with bunds to cut traffic noise as it passes through the city centre. Fill will be a mixture of spoil from tunnel and cutting excavations.

Project client, State of New South Wales Road & Traffic Authority (RTA), awarded the $A750M (£300M) design and build project with a 10-year operation and maintenance period to a joint venture of local firm Baulderstone Hornibrook and its German parent, contractor Bilfinger + Berger (BHBB M5 East JV) in August 1998.

BHBB began site works six months later, in February 1999, giving a very short lead-in time for design of such a major project. Tunnellers broke ground within a month on the twin-tube tunnel's western, Bexley Road portal, with the eastern, Marsh Street portal, getting under way in August. The medium to high strength (20MPa-80MPa) Hawkesbury Sandstone contains some coarse-grained lenses and bands with a high quartz content of between 50% and 80% causing high abrasion on cutting tools.

Shale breccia and lenses of shale are also present.

The tunnel has a cross-section of between 52m 2.Mitsui and Voest Alpine road headers excavate the rock, while spoil is carted away by Caterpillar dump trucks. Tunnel progress has averaged more than 35m per week, and rates are steadily improving as the teams gain experience.

Emergency and passenger crossings run between the two tubes every 120m, excavated by drill and blast or road headers depending on ground conditions.

During tendering, B+B's Munich-based tunnel division expressed concerns over the design of the tunnels, advising a flat roof profile.

Discussions continued after the contract was awarded but after only a few metres of excavation it became apparent that tunnels in Sydney's Hawkesbury Sandstone have previously been excavated with flat roofs for good reason.

The mainly fine to medium grained Triassic sandstone was deposited in horizontal beds of between 1m and 2.5m thick, and is cemented and partially separated by fissures, with tight jointing and rare faulting. An arch roof means cutting the horizontal layers of sandstone twice, which with a wide cross-section would make the roof unstable.

Furthermore, the ratio of horizontal to vertical stresses in the sandstone varies from 1.5:1 up to 2.5:1 for deeper layers, giving a good bearing for horizontal roofs.

During construction, rock bolts and anchors installed by semiautomatic drilling rigs provide immediate roof support. Steel fibre reinforced shotcrete is used for the final roof lining, with secondary rock bolting where needed. This is the standard approach for tunnels in the Hawkesbury Sandstone, says BBHB.

A major aspect of the project is the excavation work for the tunnel ventilation system.

This comprises two vertical shafts, an exhaust tunnel and an underground 'ventilation station'.

The 6.5m diameter and 60m deep Duff Street intake shaft, housing fans, ventilation equipment and control rooms, is located above the running tunnels. However, the Turella exhaust shaft is 700m from the alignment, linked by a 52m 2cross-section exhaust tunnel.

The 12m diameter and 45m deep Turella shaft carries exhaust air to the 70m long underground ventilation station which is built in open cut, using up to 24m long permanent sheet piles from Luxembourg manufacturer Arbed.

The shaft is hexagonal in plan and was formed using diaphragm walling installed by Bachy, working as subcontractor to Frankipile Australia.Walls are made up of 7m long panels, 0.8m thick, installed through 25m of soft ground down to rock head. The final 20m is being excavated using hydraulic impact hammers.

Underground work is due to finish in March 2001 but by March this year 40% was already complete.

At the eastern end of the tunnel, the road will rise up and run on an elevated section before plunging back underground to cross the Cooks River. The 700m long crossing includes access ramps on the river banks built in cut and cover with 1m to 1.2m thick diaphragm walls up to 40m deep. Similar sized diaphragm walls form the permanent walls of the 200m long tunnel section.

The crossing is being built in three stages so that flood control in the event of a 100-year flood is assured at all times. Central roof support for the approaches and the tunnel is provided by a row of 1m diameter bored piles.

Additional site investigations carried out after the contract was awarded revealed that the sandstone beneath the river bed was at a higher level than expected and the original plan to use circular cofferdams and open cut bottom-up construction was abandoned.

Instead, a temporary rectangular sheet pile cofferdam, propped with heavy trusses and steel tubes, is being used. This is filled with sand to 1m above mean riverbed level to provide a working platform for the diaphragm wall equipment.

The cofferdam was designed, with input from Baulderstone Hornibrook, to be dismantled and reused for all three sections of the tunnel.

Diaphragm walling is being carried out by IP Foundations. Heavy grabs excavate down to bedrock which is then dug out with freefall chisels. Panel joints are sealed with stop ends with water stops.Once complete, the roof slab is cast, the cofferdam moved and top-down excavation carried out beneath.

Other elements of the contract have called for CFA, bored and driven piling.There are several viaducts, bridges and underpasses, plus retaining walls, sewers and mains crossings, and an open flood channel for Wolli Creek.

The M5 East motorway is due to open in mid-2002.

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