Wagstaff Piling \ Innovative Foundations \ Yarra River Crossing

\ The vertical and horizontal alignment constraints meant that the tunnel needed to be relatively shallow and only just under the river bed in soft alluvial sediments. The 1200 mm diameter secant piles were installed to form the permanent walls for the majority of the river crossing.

\ The overall scheme involved a two stage crossing of the river, with the first stage involving the most challenging and complex deep foundation works.

\ The general sequence of the first stage was as follows:

• Install a sheet pile cofferdam 75 metres into the river (3000 m2 of sheets)

• Tie, brace and backfill the cofferdam to create a working platform in the river

• Install reinforced concrete secant guide walls

• Install 212 secant piles, 1200 mm diameter upto 28 metres depth at 1000 mm centres using the hard/hard method

• Excavate down to cut off level up to 10 metres below the river level

• The tunnel roof was built in the dry, within the sheet piled cofferdam

• Tunnel construction occurred under the roof beneath the river

\ Wagstaff Piling was responsible for the deep foundation component of the works. To create a working platform for the drilling rigs a steel sheet pile cofferdam was installed. The wall was braced and tied with walers and stress bars.

\ The sheet piles were up to 18 metres long. They were installed through temporary steel templates in the river using an ICE 44-50 vibrator and a 6 tonne hydraulic hammer. The cofferdam was backfilled with an engineered fill of crushed basalt.

\ next page...