The city of Toowoomba in Queensland is situated on a plateau on the edge of the Great Dividing Range, overlooking the Lockyer Valley. The Toowoomba Second Range Crossing (also known as Toowoomba Bypass) is a 41km long dual carriageway bypass and partial ring road constructed to the north and west of the city.

After years of planning, this major infrastructure construction project commenced in April 2016 and was completed in September 2019. It has been jointly funded by the Australian and Queensland Governments, with Nexus Infrastructure contracted to design, construct, operate and maintain the crossing.

Nexus Delivery, the division of Nexus Infrastructure that is responsible for the design and construction of the project, identified that 7 reinforced concrete pipe culverts located beneath the road had lost structural integrity due to cracking and would need to be repaired to ensure the ongoing compliance, of the bypass, with industry standards.

The 7 culverts, measuring between 1350mm and 2200mm in diameter, each contained at least 1 damaged barrel, with a number of culverts compromised in up to 5 separate locations along their length. Nexus Delivery needed to deliver an efficient remediation solution that would not hinder construction timelines.

Working closely with Nexus Delivery and Roundel Civil Projects Pty Ltd to develop a bespoke approach, Mainmark drew on its breadth of infrastructure and asset remediation experience, to propose a number of installation and gouting solutions to facilitate the relining of the redundant culverts and restore structural integrity quickly and efficiently.


A solution was required that would restore structural integrity to the compromised reinforced concrete pipe (RCP) culverts beneath the road with minimal impact on construction timelines, allowing the project to achieve the required completion date in June 2019.

With access to the interior of the RCP culverts limited by their size and location, the solution needed to allow for effective and efficient delivery, while also protecting the health and safety of all project operations personnel and subcontractors working in the area. The proposed solution needed to be carefully planned and executed to address the extreme isolation and site access constraints, and to eliminate the potential for any future degradation of the underground culverts, all whilst maintaining the overall project deadlines.


Working closely with Nexus Delivery and Roundel, Mainmark developed an engineered solution using a multi-faceted approach to relining the RCP culverts which involved the insertion of site based custommanufactured corrugated metal pipes (CMPs) inside the damaged RCP concrete culverts and then filling the void (or annulus) between the CMP and the original RCP concrete culverts with Terefil®. The CMP solution that was finally adopted was designed and developed by Roundel.

Mainmark’s Terefil is a highly specialised lightweight cementitious void fill solution for geotechnical applications. The proprietary, versatile solution was pumped from a single injection point, quickly and easily flowing to fill the annulus between the CMP and culverts. A custom grout mix, Terefil 6000, was developed to comply with specific project requirements determined by the Queensland Department of Transport and Main Roads (DTMR).

The 12m lengths of CMP were connected through custom abutment joining metal bands, created to be rigid enough to maintain the seal between each adjoining section, and flexible enough to yield if any float occurred during the Terefil injection.

Mainmark also developed a landing skid, reel and winching system to install the custom CMP inside the concrete culverts, providing reinforcement and stability to the compromised reinforced concrete pipes prior to application of the Terefil solution.

Engineers from Mainmark and Nexus Delivery collaborated closely on initial testing through to final implementation, to ensure the safety and suitability of the proposed solution. The engineered solution using Roundel’s CMP and Mainmark’s specially formulated Terefil mix successfully delivered the required project outcomes, while providing a 100-year warranty for the customer.

The Toowoomba Second Range Crossing Health and Safety Manager Leigh Mehmet acknowledged Mainmark’s standard of work and safe approach throughout the project. “The works undertaken by Mainmark for culvert relining have been excellent, and they are a contractor who is setting the standard and leading the way for safety.”

Good design award winner Mainmark

Mainmark is proud to win the Good Design Award in the Engineering Design category, in recognition of outstanding design and innovation for this project. Read the blog here.