The project involves the replacement of section 6 of the Perth Main Sewer with over 2 km of 1,500 mm internal diameter CSR Humes Plastic-lined jacking pipe.
Depths range from 3 to 8 m on soft ground strata and in high groundwater table. Construction rates of over 20 m per 10 hour shift are being achieved, with program completion likely to be several months ahead of schedule.
Shafts were constructed using insitu caisson methodology, varying between rectangular and circular, dependent on design criteria and space access. Access chambers are being installed inside following the completion of tunnelling on each line. Access chambers mainly comprised of standard pipe with cutout sections to allow normal benching and flows through the pipes.
Benefits of the caisson method include the minimal dewatering required, as potential acid sulphate or contaminants are kept to bear minimum, therefore minimising water discharge quantities also. This is an important concern when working in close proximity to the community.
Proposals were received by the client for relining of the old sewer versus construction of a new pipeline with a 100 year life. The client saw the positive added value benefits of the new pipeline and given the need for bypassing of sewers was insignificant in comparison showing a large cost savings.
The new line was constructed in close proximity to existing sewer, but given the minimal vibration and the accuracy of the machine to meet critical tolerances, this was not a risk element on the project. A Herrenknecht AVN 1200 machine and tunnelling system was utilised by in-house staff with minimum of five years experience in this ground type. The machine is laser guided, steerable and the cutting wheels can be interchanged prior to each drive, dependent on ground conditions. If in sand, the soft cutting wheel is utilised, while in rock, the rock cutting wheel is chosen.
Traffic management was a high priority on the project, as much of the work was in the suburb of Northbridge – right next to Perth’s CBD. Due to the design fixed points either end, it was necessary for the pipeline to largely be constructed in roadways, however because of the reduced size of the shafts (as opposed to conventional methods) and the drive lengths of 250 m, traffic disruption was minimised.
Due to the large traffic volumes using the roads, careful consideration of the logistics of pipe delivery and TBM launching and receival was vital to ensuring that the project was a success.