The new Gundagai Sewerage TreatmentPlant is located within the Murrumbidgee floodplain in southern NSW. The upgrades to the plant represent a long term investment in regional infrastructure, replacing facilities that were almost 100 years old. As a result, maintenance costs and flood related repair costs will be substantially reduced.
SKCE was engaged to develop the structural design, documentation, and certification for the retaining structures forming part of the upgraded sewage treatment facility. This included geotechnical review, wall system analysis, hydraulic loading assessment, and full engineering certification. The design was required to withstand flood loads and groundwater pressure during submergence, while maintaining stability under long-term exposure and minimal maintenance requirements.
The project requirements included
- Design of retaining walls using Dincel 275 mm permanent formwork system with integral reinforcement cages.
- Analysis of wall behaviour under flood conditions, including combined earth and hydrostatic pressures from both retained fill and external inundation.
- Footing design to address variable soil conditions and differential settlement along the 210 m alignment.
- Engineering of wall buttresses and base anchorage for uplift and overturning resistance during full submergence.
- Pour sequencing and construction methodology review, including staged pour design (initial 2 m pour, then full-height completion).
- Integration with hydraulic design and access structures, ensuring drainage and scouring protection were maintained.
- Site inspection and engineering certification during construction, verifying reinforcement placement and pour methodology.
- Close collaboration with Dincel Civil Solutions technical team to adapt the modular system for non-standard wall heights and flood performance requirements.
The SKCE design team assessed the retaining wall structure under extreme flood conditions. The design was developed to withstand challenging conditions, including a 1-in-100-year flood event, high hydrostatic loads, and reactive foundation soils.
The project included approximately 210 metres of retaining walls, with heights ranging from 2.7 to 5.7 metres. The Dincel 275 mm system was selected for its composite strength and reduced temporary works requirements, minimal joints and reduction in formwork.
To simplify site operations and minimise labour, the wall design allowed for full-height continuous pours up to 5.7 m, supported by limited external bracing. The abutment design allowed for the efficient construction of a tall wall, minimising propping and thickness of the RC concrete walls.
The approach adopted significantly reduced construction time, eliminated the need for conventional formwork, and achieved robust flood-resilient structures.
The solution delivered significant value for the client:
- Cost savings when compared with traditional formed concrete retaining walls,
- A shorter construction program due to simplified installation and single‑lift wall pours,
- Long-term durability and reduced maintenance due to the polymer‑encased Dincel system,
- Successful collaboration between Orion, DET Constructions and Dincel Civil Solutions that ensured seamless execution and the successful implementation of innovative construction methods.
