Perry Infrastructure recently jacked a combined pedestrian/cycleway under a major road in Artarmon, a northern suburb of Sydney. The road was an on/off ramp for the Warringah Expressway and the subway construction was part of the roadworks for the new Lane Cove Tunnel. The inner dimensions of the culvert were 4.3 m wide x 3 m high.
There were many design problems to overcome being:
“¢ The roof of the tunnel was only 1.5 m below the roadway
“¢ There were five separate banks of 33 kVA cables that were to be crossed with only 200 mm clearance from the top of the culverts
“¢ There were setup limitations due to a restricted working area
“¢ The culverts were to be installed in half Hawkesbury Sandstone and the remaining roof formation in soft backfilled ground, necessitating roof support during excavation of the floor.
Perry Infrastructure manufactured a purpose built steerable front steel shield with all directional steering rams, a raked roof for ground support, Bisalloy cutting edges and a bond breaker system on the roof to minimise drag on the roadway above. Thrust was provided by Perry’s Herrenknecht jacking system capable of delivering 1600 tonnes of thrust to the jacked boxes.
Due to the restricted pit area the culvert segments were manufactured off site by Abergeldie Contractors in 1.5 m long segments. Each segment weighed 22 tonnes and had a keyed joint and multiple post tensioning conduits in each segment to allow the boxes to be tensioned together after placement. By the time the 30 m drive was completed Perry was pushing 440 tonnes of structure, 1.5 m under the roadway, with no disruption to the traffic. A concrete thrust block was manufactured by Abergeldie incorporating over 50 cubic metres of concrete and 3 tension piles into the rock, as a large part of the thrust block was above ground.
Excavation was initially carried out using a custom built mini excavator built in Perry’s manufacturing plant with a conveyor system for spoil removal to the tunnel entrance. Eventually it was found that a standard mini excavator with a rock breaker together with a skid steer loader for spoil removal increased production. Four all-directional 75 tonne steering rams were installed in each corner and two lasers provided line and grade information.
The 18 tonne steering shield manufactured in Perry’s facility by Robbie Ashton, Kevin McPherson and his team incorporated a purpose designed bond breaker system to reduce the drag on the roadway above the culverts and reduce skin friction. Ten rolls of steel were suspended within the shield and attached to the headwall at the tunnel portal. As the culverts progressed the steel rolled out onto the roof to support the material above. The rolls had individual brakes attached to ensure the rolls extended uniformly.
During construction the Perry crew had to deal with multiple unknown obstructions. At one time there were seven redundant services in the face including 600 mm stormwater pipes, 600 mm cast iron water mains, gas mains and high voltage electricity cables requiring excavation with jackhammers, quick cut saws and rock drills.
On completion of the jacking and demobilisation of the equipment Abergeldie installed post tensioning cables and tensioned the individual culverts together to form a rigid structure.