Teretek® | Voids and erosion beneath road and around culverts


Causeway Culverts | Mountain Creek Causeway, Tidbinbilla Nature Reserve, ACT | 2022

In early 2021, a causeway located across Mountain Creek in ACT’s idyllic Tidbinbilla Nature Reserve suffered serious damage during a series of torrential downpours. The causeway features three 12m long concrete reinforced culverts, each measuring 750mm in diameter, situated beneath the road and within an embankment.

The embankment consisted of compacted bedding material and backfill, with an ageing bluestone protection wall supporting the culvert pipes. The bluestone wall was significantly compromised in multiple places due to the force of the creek’s water flow, which became a raging torrent during rain events. Prior to the heavy rainfall, the reserve was exposed to bushfires which had destroyed vital areas of protective vegetation. This allowed the creek’s fast flowing water to breach the bluestone wall in a number of places and washed away large amounts of soil fines from the embankment, creating several voids in the protective wall and erosion beneath the causeway.

Worried about the ongoing stability of the road and culverts, the ACT Parks and Conservation Service commissioned a ground penetrating radar (GPR) test to assess the damage and severity of the voiding beneath the roadway. While the road was determined to be structurally adequate for general vehicle use, the load limit had been compromised, preventing trucks and tourist buses from accessing the road.

To arrest the damage, ground improvement works were required to stabilise the embankment and re-support the ground beneath the road, while also protecting the long-term performance and durability of the culverts.

As the nature reserve was located in an environmentally sensitive area, replacing the existing culverts was quickly ruled out. Instead, Mainmark was commissioned to provide an environmentally safe, non-intrusive and cost-effective remediation solution.


Mainmark was required to deliver a ground improvement solution by filling voids, strengthening the weakened backfill within the embankment and re-supporting the surrounding culverts and concrete roadway so the bluestone protection wall could be repaired. The solution also needed to provide long-term protection against further erosion and prevent water infiltration into the bedding and embankment to enable the road to reopen for tourist buses and trucks.


Mainmark’s Teretek® engineered resin was used as an ideal remediation solution due to the product’s ability to increase ground-bearing capacity without damaging the already compromised culverts. Teretek is environmentally inert and therefore has no detrimental effects on groundwater, soil quality or local wildlife. It is also backed by a 50-year product warranty.

Prior to commencing the project, a leak barrier was constructed between the work zone and the creek to ensure the safety and protection of the surrounding area.

Once the area was fully contained, Mainmark commenced the Teretek resin injection treatment. Approximately 500kg of Teretek engineered resin was injected into the ground through very small tubes, in a process likened to keyhole surgery. Technicians targeted areas where weak soil and voids had been identified in the ground penetrating radar (GPR) report. The project immediately re-established ground support to the embankment and concrete roadway.

Teretek proved to be an extremely cost-effective and environmentally safe alternative to replacing the culverts, which would have required road closure and expensive excavation works.

The entire project was successfully completed within two days, after which the road was immediately re-opened to traffic including heavy vehicles.


“the project ran smoothly; Mainmark provided an effective solution to a challenging project site. Based on the success of this project we would be happy to consider Mainmark again if a similar situation arises.”

Martyn Ellis, project officer for the ACT Parks and Conservation Service, was happy with Mainmark’s approach and said.