Murray Irrigation delivers water, sourced from the Hume Dam, to over 2,000 properties across the NSW Riverina region. The Mulwala Canal forms the backbone of the Murray Irrigation delivery system, controlling the flow of local river and creek systems. The Mulwala Canal is 2,880 kilometres long and is the largest private irrigation canal in the southern hemisphere, spreading across the southern Riverina plain to Deniliquin and suppling water to 700,000 hectares.
Numerous gates along the Mulwala Canal (known as regulators) maintain the water depth and control water flow along the channel. The regulator gates are supported by reinforced concrete footings, designed to support the regulator structures and their applied loads, the roadways that pass on top and direct the canal water through the regulators.
During a program of maintenance works, 65 regulators were scheduled to be upgraded, with plans to custom-fit concrete upstream sections onto existing structures and establish sill heights for new regulator gates.
Visual inspections of three existing regulators, Sprys, Mundiwa and Mulwala 20, revealed significant cracking and evidence of ground settlement and wash-out affecting the foundations of the structures. Work had been undertaken at these sites some years earlier to facilitate the installation of lay-flat gates, at which time there was no indication of any structural issues or voids.
Further investigation using drills and probes, revealed a number of structural problems including extensive voids, washout of the subgrade, as well as the saturation and weakness of the residual materials beneath the footings supporting the three regulators.
The concrete foundation slabs had also subsequently cracked and exhibited signs of settlement, impacting the floor and apron of the regulator at each site. The Sprys site was the worst affected, with a void measuring approximately 23m3.
To prevent destabilisation of the regulator’s structural footings and the road bridges running across the top of them, Murray Irrigation Limited (MIL) approached Mainmark to deliver an efficient and cost-effective remediation solution that was suitable for the canal conditions and capable of improving the ground bearing capacity beneath the regulators.
The primary objective was to mitigate severe subsidence affecting the regulator foundations, filling extensive voids and compacting the weak soil to improve bearing capacity of the substrate surrounding the footings. The solution needed to rectify the structural issues and overcome the need to replace the affected regulators.
Only when the 500km channel was drained of water was it possible to determine the full scope of works, which was more extensive than forecasted, encompassing a total of 15 different sites. Due to the location, project timelines were strictly limited to a short window when the canal would be completely drained of water.
Mainmark worked closely with MIL to adapt the solution to complete the full scope of works, treating 15 sites, before the channel was refilled for spring irrigation.
Mainmark’s proprietary Teretek® engineered resin injection provided a cost effective and efficient remediation solution that was perfectly suited to the project’s site conditions. Being extremely durable and environmentally inert, Teretek was also considered a safe solution that would have no determinantal impact on the canal.
Teretek is a unique two-in-one solution that increases both ground bearing capacity and fills voids. Technicians apply the solution via a number of very small injection points under the impacted structure in a process similar to keyhole surgery; injecting the expanding polyurethane resin mix through 16mm tubes into the ground beneath the footings. On entering the ground, the resin rapidly expands, filling any voids encountered, compacting weak soils and adding volume to the ground, thereby increasing its bearing capacity.
Teretek also assists in reducing water movement below the impacted slabs, mitigating the risk of water leakage or further washout of subgrade materials. The solution helps to mitigate soil subsidence and further structural deterioration including cracking and movement.
The Teretek injection points at each site were laid out in an approximately 1.2m centre to centre grid allowing the resin solution to be injected under the affected slabs, including under the reinforced mass concrete flooring beneath the lay-flat gates. The slabs ranged in thickness from 250mm at the outlet apron slab, to 1000mm thick at the inlet slab under the layflat gates.
Three self-contained Mainmark Rigs were used to access the 15 sites which spanned a distance of 250km. The Rigs were used to transport all required Mainmark technicians, equipment and materials necessary to deliver the project to each site. Additional Mainmark personnel were on standby to cover shifts allowing the agreed project timeline to be met.
To help ensure strict safety protocols were adhered to throughout project delivery, a bespoke mobile access platform with stairs was constructed to allow technicians to access the sites safely and efficiently. Close collaboration between MIL and Mainmark was required throughout the project to ensure technicians were mobilised quickly and efficiently.
This complex project was completed over a period of 26 days using a total of 14.5 tonnes of Teretek across the three regulator sites. The project delivered a successful outcome that avoided the need to replace the regulators, and the mobile access platform constructed for the project will be a useful asset for future MIL works.
Murray Irrigation Limited Project Manager, Matthew Close, spoke of the success of the works:
“Mainmark’s void filling program for our undermined bridge regulators was thoroughly successful. The works were completed well within the limited time constraints and under budget. The crew was also a pleasure to have onsite with all challenges met with optimism. We look forward to the opportunity to work with Mainmark again in the near future.”