The early works for Melbourne’s North-East Link project posed a technical challenge: decommissioning the old, recently bypassed sewer pipework below the new motorway. The sewer needed to be filled in a way that removed any risk of collapse once the road was operational.
The series of pipes were up to 2.74m2 in cross-sectional area, meaning each metre of length could contain up to 2,740L in volume.One way to do this was to excavate and remove the old sewers, however much of the pipes’ length lay 20m underground. The pipes also criss-crossed other subterranean infrastructure including gas mains, water mains and telecommunication cables. Plus, the pipework ran directly below a very busy road that could not be closed during the day. This meant excavation and removal was not a viable option.
Another method was to create plugs at various points and then pump concrete into the voids, but conventionally placed concrete is not very flowable over long distances and experiences shrinkage (decrease in volume) as it cures. This would potentially create a gap along the top of the fill, meaning the structural integrity and complete abandonment of the sewer pipework could not be guaranteed.
The project required a more effective solution, which is where Mainmark’s vast experience in below-ground utility abandonment came to the fore.
Mainmark is a renowned global leader in geotechnical and asset preservation solutions, specialising in foundation subsidence repairs. It’s proprietary cellular concrete material and technical expertise made it the preferred partner for this significant infrastructure project in Victoria.
Meeting the technical challenges
The scope of the project included filling the large sewer main below Bullen Road, as well as the filling of smaller reticulation sewer lines feeding into the mainline, including reticulation manholes.
Mainmark’s Regional Manager (Southern Region), said accessing the pipework was very technically challenging. The concept was straightforward – fill a sewer with grout – but the methodology to get it in there, and be certain that it was full, was the hard part.
“We rolled out every night, six nights a week for about three months, precision-drilling small, 75-millimetre diameter holes directly down from the surface. These drills broke into the top of the sewer. Through those holes, we inserted PVC injection sleeves,”
Mr Kudrenko said that on site the crew utilised a specialty batching process.
“Conventional concrete agitators come to site delivering a water and cement slurry, and we then introduce specialty treated engineered surfactant into the slurry to produce Terefil®. It develops properties that make the material very suitable for backfilling of voids or pumping very long distances, as well as producing a lower density material,” Mr Kudrenko said.
However, for this project, the concrete batch plants were unable to supply the site with cement slurry. Supplying such a project with the quantities required overnight would negatively affect their day-to-day capabilities.
“So, in what was another small, technical triumph, we ended up utilising a volumetric mixer. That meant every single night we essentially had a mobile concrete batch plant, rather than agitators, on site”.
“The Terefil® fills more volume with less cement, so it’s more cost-efficient and has a smaller carbon footprint. It’s aerated, so the final compressive strength is not as high as concrete, but the result is still much stronger than the surrounding soil, which the client required.”