From the magazine

Tackling critical relines

Stretching more than 12 km and carrying 60 per cent of the city’s sewage, the S1 Main Sewer is more than 100 years old, spans 1.5 m in diameter and lies up to 20 m below Kingsford Smith Drive, one of the busiest roads in Brisbane.

Due to the size, depth and location of the pipe, the multi-million dollar rehabilitation of the pipe is one of the most unique and challenging sewer rehabilitation projects ever undertaken in Australia.

The project is also a powerful demonstration of the benefits of Trenchless Technology – especially when the utility and contractor work together effectively.

The first phase of the project involved relining a 1.3 km length of sewer with a nominal diameter of 1,350 mm.

During the works, both the sewer and road needed to remain operational, as flow bypass and traffic detours were not possible.

A robust community engagement strategy, which included cooperation from both contractor and utility, resulted in minimal complaints from residents.

Lining up a challenge

Before work had even begun, the project presented major design challenges.

The close proximity to the Brisbane River means the water table in many sections of the sewer is very high and the hydrostatic pressure on the pipe can be as high as 20 m.

The design team also needed to develop an innovative liner capable of withstanding the extreme conditions, including soil load.

Queensland Urban Utilities then had the challenge of finding lining techniques that would satisfy both the necessary structural strength and construction requirements.

Several safety issues also needed to be addressed, including working inside a deteriorated pipe prone to sudden and frequent changes in flow and gas levels.

The utility carried out extensive investigations and improvements to the sewer network, which included repairing sewer gates, installing new flow gauges, rehabilitating maintenance holes and carrying out flow trials for sewer diversions.

Geological studies were also undertaken and finite element modelling analysis was completed to obtain a realistic assessment of likely loads onto the sewer in alluvial areas and the interaction of the pipe-soil system under various scenarios.

Analysis from these studies showed that, typically, the appropriate loads for liner design were considerably less than the default value of the weight of the prism of soil above the pipeline.

This allowed the design of a liner that could be practically installed with minimum risk and meet required design parameters.

Sticking to Ribs

Two sewer lining technologies were trialled: steel reinforced polyethylene spiral wound Ribline from Interflow and a cured-in-place pipe lining method.

Following the trials, Queensland Urban Utilities awarded the contract for the first phase of the S1 Main Sewer Upgrade to Interflow.

A major factor in the award was that a sewer flow bypass and traffic detour around the 1.3 km section of pipe were not feasible.

Ribline liners are made by passing a continuous strip of steel reinforced polyethylene through a winding machine placed in the invert of the sewer manhole.

The winding machine simultaneously winds the liner and welds the edges together to form a continuous liner which “÷corkscrews’ up inside the sewer.

During this process, the liner floats on the sewage flow inside the pipe.

This reduces friction, allowing long continuous lengths of liner to be inserted in a single session.

Though spiral winding is traditionally composed of black polyethylene, Queensland Urban Utilities requested a light-coloured liner for the S1 so that future CCTV inspection would be clearer.

To accommodate this request, Interflow, together with supplier Sekisui Rib Loc, developed a light grey polyethylene that could be used instead of the black material.

They only come out at night

All on-site works were carried out at night during off-peak traffic and low sewer flow conditions.

At the end of each night shift, the road and the sewer were restored to their full operating capacity.

Work was carried out five nights per week from Sunday to Thursday.

The small site footprint meant that if the manhole was located in the lane closest to the kerb, only one of the four lanes of Kingsford Smith Drive needed to be closed.

Where the manhole location didn’t allow this, two lanes were closed and traffic was diverted under a contra-flow arrangement to the other side of the road.

This meant removing sections of the median strip prior to the start of the project to allow contra-flow, as required, then reinstating the lanes at the end of the night shift, using traffic calming barriers during the day.

Work was carried out from existing manholes – some of these had their top sections enlarged under a previous contract, while others had standard 600 mm openings.

Work continued in the sewer each shift until about 4 am. All equipment and material was then progressively removed from site and traffic management dismantled ready for full re-opening of the road each morning at 5 am.

While flow bypass was impractical, diversion of night-time flow was possible.

Queensland Urban Utilities managed the required flow control with full-time supervision on-site to maintain a safe working environment during the lining process.

While flows at night are typically low, they can rapidly increase, together with the concentration of sewer gas.

For safety reasons, the sewer gate upstream of the site was closed at night and flow diverted to other parts of the network.

Following completion of site works each night, it was re-opened to handle full daytime flow.

No action was required to control local incoming flows along the section being lined.

The work procedure took advantage of Ribline’s capacity to be installed with some flow in the line.

Flow height in the 1,350 mm diameter sewer during installation was always at least 300 mm.

Going with the flow

Flows were monitored continuously on-site using a live supervisory control and data acquisition (SCADA) system to prevent overflows.

Predictions were made after studying the upstream and downstream flow information, including the performance of pump stations.

Further helping to eliminate the risk of overflows was the flexibility of the Ribline technology, which can be stopped at short notice.

With distances between manholes of up to 197 m, it was not possible to complete liner winding in a single shift.

When work needed to be stopped for the night, the installed section of liner was secured in position in the sewer and the manhole lid replaced.

The full flow capacity of the sewer was therefore restored without compromising the structural integrity of the partially installed liner.

This feature of the process also meant that full flow capacity could be rapidly restored at any time in the event of a sudden flow increase, such as a storm event.

As Ribline is a fixed diameter liner, there is always an annulus which needs to be filled with cement grout. Interflow has developed a grout specifically for such applications.

It is classified as “÷fluid’, meaning it can flow long distances in small annular spaces; however, it does not shrink or separate and, due to its density, can displace any water in the space.

Interflow used its own portable batching plant, allowing the grout to be mixed on-site from components delivered to site in bags.

Up to 20 cubic metres of grout could be mixed in a night. Like all of the equipment used on this project, the batching plant was established on-site each night and then removed at the end of the shift.

Success in stages

A highly challenging project, one of the main reasons for its success was the advanced identification of design and construction issues, as well as the collaboration between utility and contractor.

Together, these stakeholders worked to innovatively eliminate or mitigate all challenges to acceptable levels.

The successful completion of these works has demonstrated the suitability of the design, products and work methodologies for the renewal of the remaining sections of the S1 Main Sewer.

The rehabilitation project is expected to be completed in stages, finishing in 2019.

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