On 4 November 2020, McRobert Contracting Services, in association with Power and Water Corporation, completed the structural relining of a box culvert inlet channel at the Palmerston Waste Stabilisation Ponds in the Northern Territory.
The channel is 125m long and 1.2m wide, and is a concrete precast box culvert, which includes a flume section and 90 degree bend. The inlet channel was installed in the early eighties and has been in service ever since, connecting to a DN 1200mm sewer trunk main servicing the suburbs of Palmerston.
This channel was open to the atmosphere until just over two years ago, but due to odour complaints from residents in the area, it was decided that aluminium covers would be installed over the length of the channel to reduce odour from the sewer.
When the covers were first installed, there was a small amount of corrosion just above the water line and a small amount of cement loss from the culvert walls. Approximately two years after the lids were installed, operators noticed during a regular maintenance inspection that the channel walls had a significant increase in corrosion, with exposed aggregate visible.
After further inspection, corrosion was detected along the entire length of the channel. By enclosing the top of the inlet channel with the aluminium covers, the sewer gases were retained within the system, accelerating the corrosion process in a short period of time.
Due to the location of the ponds in a residential area and the potential for a sewer spill as a result of a rupture of the channel walls into the lagoon, McRobert Contracting Services was engaged to design a relining system to save the channel.
McRobert Contracting Services carried out research and design at its workshop in WA to find a suitable lining material and methodology to line the precast culverts. After purchasing two precast box culverts with the same dimensions as the culvert in Palmerston Ponds, McRobert Contracting Services was able to set up a channel simulation at the workshop to design a suitable bracing system and moulds.
Three sets of moulds were fabricated from aluminium and form ply which could be used continuously. Bluey’s AKS 2.5mm HDPE in black was used as the lining material because of its UV qualities and ability to accommodate Darwin weather conditions.
To reduce the amount of on-site welding, the channel was lined using one sheet per section (2.6m long) to eliminate any horizontal joints along the base of the channel, which included bending the sheet hard against the curved sections of the base.
The project was undertaken in the Northern Territory wet season, which meant it was crucial to keep the HDPE lining out of direct sunlight and stored in containers to minimise sheet expansion resulting from high temperatures and humidity. Once the lining was grouted in position it could be exposed to these conditions.
A U-shaped capping was fabricated using 3mm HDPE sheeting which was welded on the inside of the channel and then fixed to the outside using a 3mm aluminium strip along the channel length. This ensured a gas-tight seal when the lids were reinstated.
The lining was completed in four weeks with a total of 375m2 installed. The base of the channel lining was secured using Bluey’s products, and the walls were secured using a high rapid-set grout that allowed McRobert Contracting Services to install three sections per day, reducing the time on site.
On completion of all lining works, high-voltage spark testing was conducted for quality control and to ensure any faults were detected and rectified – all part of the McRobert Contracting Services guarantee for all materials and workmanship for a minimum of 50 years.
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