Client name: Thiess John Holland Joint Venture

Duration: 2009 - 2012

Location: Brisbane, Queensland, Australia


Situation 

This 6.7km multi-lane free-flow toll road is largely an underground roadway. It consists of dual 5.7km long tunnels which were constructed under some tightly constrained and difficult areas.

All works were carried out within the confines of an operational construction zone, adjacent to public infrastructure and facilities including major arterial roadways.

The first excavation was up to 18m deep and in a sensitive area - adjacent a church, and located over two proposed tunnels scheduled for construction post excavation.

The excavation design had to limit the amount of settlement at the church as well as maintain stability during the undermining of the excavation through the tunnel construction. 

The second major excavation had to cross an existing water course in two stages. As each stage was completed, a concrete roof slab was constructed to form the roof of the cut-and-cover tunnel which was to be excavated from the underside of the slab.

Solution

The original proposal was to use bored pile walls but our team nominated to replace these with soil nails, which while achieving all design requirements enabled quicker construction. It is believed that this is the deepest soil nail excavation undertaken in Australia to date.

Anchored sheet piles were adopted as the retaining system around the excavation. These were to be removed upon completion of the works, however a high level of rockhead existed and the sheet piles could not be taken to an adequate depth to achieve the required stability.

To overcome this, pre-drilling of alternate sheet piles was undertaken to enable the sheet pile wall to be adequately keyed in to the rock and enough stability achieved to install the ground anchors.

Result

All temporary works were constructed and performed to expectations and enabled delivery of the project ahead of schedule.