DJ Mac Cormick Contractors, with its head office in Perth Western Australia, has recently completed another large diameter microtunnel project on the Larrakeyah Defence Base – Darwin NT Stormwater Drainage Concrete Jacking Pipeline of 1,950 mm diameter with an internal diameter of 1,650 mm.
The project was undertaken utilising its Herrenknecht AVN 1600 Control Container and closed face Tunnel Boring Machine with steerable laser guidance, utilising a slurry spoil removal system.
Ground conditions encountered at 6 to 8 m depth of drainage line included rock of up to 150 MPa so a rock cutting wheel was utilised to cope with the high strength required. Groundwater was also encountered on the first drive.
The tunnelling involved two drives of 330 m and 354 m with an 850 t jacking system in the launch shaft and drives both ways from that location. Due to the length of drive, several intermediate jacking stations were necessary and a lubrication system was used to minimise friction on the outside of the pipe being installed.
Given the pipe diameter, man-entry was possible from inside the TBM should cutter changes be required along the drive. A ventilation system was necessary not only at the shaft locations, but inside the tunnel and was designed to suit the drive lengths and high humidity in Darwin.
The TBM weight was approximately 24 t and required a crane pad, hence a lift analysis was necessary to ensure safe lift loads. A 200 t crane was utilised to ensure all risk contingencies were covered. The system included the TBM and Trailing Pipe housing the Power Pack for running the machine.
In addition, there was an intermediate jacking unit which could power four sets of intermediate jacking stations.
This enabled capability of jacking in excess of 1 km drives given four intermediate jacking stations from the control container and four from the intermediate jacking unit which could be housed inside the tunnel line. Pipe lift/lowering was undertaken, utilising a 48 t excavator with pre-start lift analyses being undertaken.
The detailed temporary design was undertaken by DJ Mac Cormick Contractors including the Thrust Block, Entrance wall, Base Pad, entrance and exit rings, plates etc., which were independently checked and signed off.
Structural checks and verification of the shaft designs for launch and reception shafts were signed off as works were undertaken. Shafts shoring requirements were supplied by Ground Support Systems (Aust) and all were certified to KPA requirements for the ground conditions at depth.
Several management plans were developed and implemented by DJ Mac Cormick Contractors during the works, including detailed ITP’s for all works and QA packs.
Initially there were three drainage pits to be constructed in-situ to suit the pipe diameters and future open cut take offs that were required; however, DJ Mac Cormick Contractors proposed a precast solution with pit components supplied by Humes Darwin. The structural S40 design and construction sign off was undertaken by Cardno Consultants Darwin
