Teretek® Used to Fill Voids During Stormwater Deck Replacement

On behalf of Melbourne Water, John Holland-KBR Joint Venture was engaged to upgrade the existing top deck covering an ageing bluestone underground stormwater drain. New precast slab sections were installed as part of a maintenance program in inner city Melbourne.

The stormwater drain features a bluestone culvert originally built in the 1850s that runs underneath the central median of Alexandra Parade, a major arterial road in Fitzroy. The culvert has a u-shaped base and allows stormwater to flow under the median towards the local waterway.

As part of the main deck replacement, new footings had been constructed on either side of the culvert to support the installation of a retrofit precast capping slab. When the concrete footings were placed beside the culvert, voids were identified in the soil between the existing bluestone drain wall and the new footing.

John Holland-KBR Joint Venture contracted Mainmark to deliver a remediation solution that would stabilise the soil around the culvert and fill the voids.

Mainmark’s proprietary Teretek® resin injection was identified as an ideal solution to stabilise and compact the substrata soil around the existing bluestone drain wall and the new foundation footings installed as part of the main deck replacement works.

Teretek® is a non-invasive and cost-effective method for ground improvement and void filling. It is suitable for most soil types, and can be applied quickly with pinpoint accuracy through small injection tubes that are inserted into the ground at regular intervals. The low viscosity resin then expands in the ground, penetrating planes of weakness within the soil profile and filling voids, without impacting existing structures.

The treatment zone was 300mm below the top of the drain wall. As the stormwater drain is situated under the central median of a busy main road, a tailored application technique was used to ensure the resin reached the section of soil that required stabilisation, an area estimated to be 500mm high and 500mm deep. However, the u-shape of the culvert combined with the ongoing risk of stormwater, restricted the ability for technicians to apply the solution vertically into the soil from inside the underground drain.

To overcome these access challenges, an injection sequence was marked out across the upper section of the drain. In a process similar to keyhole surgery, 12mm injection tubes were inserted at ground level through an existing 20mm gap between the drain wall and underside of the new deck roof. This allowed the Teretek® resin to be successfully injected to fill the voids and stabilise the soil and foundation footings.

To help avoid lateral heave and damage to the old bluestone drain wall during the compaction works, temporary mechanical props were installed inside the culvert against the drain wall to provide support as each section was treated.

Mainmark’s self-contained Rig, which is fitted with sealed hoses, was able to pump the resin solution quickly and efficiently to the below ground site from a distance of 60m. This meant deck replacement works could continue unobstructed throughout the soil remediation and void filling process, ensuring project timelines were not affected.

John Holland Senior Project Engineer, James Dekazos, stated that the asset owner, Melbourne Water, was happy with the deck replacement and soil stabilisation project.

“The Mainmark team was very helpful, informative and knowledgeable. When we encountered access challenges, they provided a solution within 24 hours and we were able to move forward, completing the project within the required timeline.”

Mainmark’s proprietary product, Terefil, is a technologically advanced, engineered lightweight cementitious-based filler that is well suited to projects where alternative granular fills or aggregate material are considered too heavy.

Terefil is significantly lighter than standard grout fill, minimising pressure to surrounding structures and substrate soils, while at the same time, able to withstand significant pressure and maintain structural integrity under heavy loads, such as road traffic.

Being highly-flowable, Terefil can be pumped over long distances while using lower pumping pressure, reducing the risk of damage to nearby pipes or other in-ground structures as the solution is pumped.

Mainmark adapted a delivery solution to suit the unique requirements and substantial size of the project, carefully selecting just 1 injection point for each pipe and culvert, pumping the Terefil at a slower than usual rate to fill the 990m3 of culvert and pipe treatment areas. Each segment of culvert and pipe was capped at one end and then filled with Terefil using “breathers” to reduce the risk of additional pressure on the damaged culverts. Integrity of the culverts and pipes was continually monitored at regular intervals throughout the fill process to ensure no leakage occurred during the application of Terefil.

Approximately 990m3 of Terefil was pumped into the affected culverts and pipes across the entire project area which was completed over a period of just 9 days, delivering an extremely cost-effective solution with minimal disruption to the local residential area, and without compromising project timelines or outcomes.

Senior project engineer for Diona, Gavin Bowen, had this to say: “Mainmark did a great job on the Terefil filling of the redundant stormwater culvert and pipes system in Mayfield. The crew were very efficient and very helpful to work with.”