New sewers with internal diameters of 1,500 and 1,800 mm were constructed using closed face earth pressure balance slurry machines.

D.J. & M.B. Mac Cormick selected AVN 1200 & AVN 1800 from their fleet for the project. Ground conditions were watercharged sand.

With two fixed points, the depth of pipeline could not be adjusted in design consideration. As a result, machines with 1,800 and 2,150 mm outside diameter only had cover of 1.5 m under roadways and bike paths in many locations (the ideal depth is 2.5 times the diameter of the machine).

This highlights the fact that minimal settlement is achievable with closed face tunnelling. The other consideration is ensuring the experience of the operator in similar ground strata.

Cassion shaft methology was undertaken for thrust and receival shafts. This reduces the dewatering requirement which would otherwise be required using other techniques.

Temporary lowering of the ground water table was a high risk for the project, given the proximity of buildings/bridge structures.

Treatment of acid sulphate soils and treatment of large quantities of water was also minimised through cassion shaft methology. Access chambers were located inside the cassion shafts following completion of tunnels, serving dual purpose and increasing cost efficiency.

Settlement monitoring of surface levels was undertaken at 10 m intervals and monitored on daily basis until two months after completion. The greatest settlement on the whole route was 8 mm.

As part of the construction process grouting of the outside of pipelines after completion of each drive was undertaken as a further assurance against possible settlement.

CSR Humes sewer jacking pipe was chosen for the project. Design detail and calculations and test results formed part of the tender submission. These included maximum jacking loads allowable on the pipe and pressure testing of the pipe and joint.

The design engineers for the project were GHD, who involved the contractor in order to ensure drive lengths, shaft locations and other tunnelling considerations were included in the scope and ensure successful delivery of the project to the client, Water Corporation.

The majority of the works were in Leederville Parade in Leederville, a busy arterial road which links up to the Graham Farmer Freeway and Mitchell Freeway. Prior to undertaking the tunnelling two drainage lines required conversion to siphon structures in order to avoid clashing with the tunnel alignment. One was in the Vincent St Roadway, requiring two 7 m deep access chambers with connecting pipework in watercharged ground to be completed over one weekend, including reinstatement of roadway and asphalting for Monday morning peak hour traffic.

The project construction scope included 123 m of 1,500 mm tunnel and two in situ access chambers which were plastic lined, as well as 707 m of 1,800 mm tunnel and seven in situ access chambers which were also plastic lined.

Construction of reticulation sewers of 150, 225 and 300 mm, totalling 1,080 m was also required. Reconnection of the 900 mm Havelock St Branch Sewer, reconnection of a 300 mm branch sewer crossing Mitchell Fwy, reconnection of the plumbing for properties to new reticulation sewers, decommissioning of 369 m of 1370 RC pipe including grout filling, decommissioning of 1,366 m of 1525 RC pipe including grout filling, removing redundant access chambers, and construction of a 1,200 mm main drain through a carpark.

Planning and management for the project was a critical factor with numerous stakeholders involved who needed to be consulted and approvals that needed to be gained prior to construction. Project specific safety, quality and environmental plans and procedures were developed for the works.

The traffic management plan developed enabled the contractor to reduce traffic on Leederville Parade to one lane, however, the contractor saw the trenchless methology as a good way to demonstrate to the client and local councils the benefits of a trenchless solution – which allowed two way traffic to be maintained during the project. As a result community and business impact costs were minimised and compensation claims from businesses and restaurants were nil.

Internationally D.J. & M.B. Mac Cormick has been working in the Middle East for several years and provides an Australia-wide tunnelling service for infrastructure pipelines.

For further information on the company’s capabilities visit their website at www.maccormick.com.au