WORK STARTED this week on rebuilding the exit ramp of an Australian motorway tunnel which collapsed during construction last year.
The collapse happened last November at the junction of the Pacifi Highway exit ramp tunnel and the Marden Street ventilation tunnel, 17m below ground level in north Sydney (NCE 10 Nov 2005).
The collapse zone was stabilised within a day using pumped concrete.
No one was hurt, but the incident caused part of a block of flats to partially collapse.
Contractor Theiss John Holland this week started boring through the concrete plug to create a retaining wall made up of 900mm diameter piles.
The retaining wall will isolate the collapse area from the realigned tunnel route.
It will penetrate at least 2m below the base of the collapsed section of tunnel.
Debris between the retaining wall and the new tunnel alignment will be removed via the completed ventilation tunnel which lies at right angles to the running tunnel (see diagram).
Pressure grouting behind the wall should ensure no further settlement.
The new tunnel will then be constructed about 16m south of the original alignment, beginning about 100m west of the collapse zone and meeting the existing exit portal.
The tunnel construction is expected to use the New Austrian Tunnelling Method with additional probing. This involves excavating using road headers, installing rock bolts and applying two passes of shotcrete.