Why use sheet piles?

Most areas on site are not particularly sensitive to lowering ground water levels. The excavation in these areas will be retained with horizontal timber poles lodged between steel posts(referred to as king posts).

The ground between the Carillon and Tasman Gardens Apartments is sensitive to settlement through loss of ground water pressure, so in this area sheet piles will be used. Interlocking steel sheet piling from a barrier to ground water loss. The sheet piles will be installed prior to excavation for construction of the underpass.

Installing sheet piles

In developing the plan for installing the sheet piles we have considered many issues including:
The presence of underground services such as water, electricity and sewers.
Ground water levels outside the excavation.
Noise and vibration effects upon people and buildings.
Vibration effects upon the old Tory St Sewer.
Disruption to pedestrian and vehicle traffic flow.

Method

First underground services are diverted from the path of the retaining wall.

Trials determine the best machinery for installing the steel sheet piling in the ground; noise levels and vibration levels are measured and assessed in order to establish the best methodology.

Holes will be drill to 12m depth for every alternate sheet piles installed .

An A-frame pile guide will be placed above the holes to guide the sheet piles into position and keep them vertical. The A-frame is 8m long, 4m high and spans 2m either side of the sheet pile wall.

The wall will be placed in 6m long sections. Each section will be partially driven into the ground - commencing at the Carillon end of the site. These steel sheet piling will be left sticking 5m above ground, just above the A-frame pile guide.

The pile guide and first crane will then be moved along to start the next 6m section of wall.

A second crane will drive the first section of steel sheet piling all the way into the ground - using a team of two cranes will reduce the overall time it takes to install the wall.

The completed length of the wall will be 90m.