Sustainable infrastructure often discusses energy, water, materials and operational performance. But decisions that shape an asset’s long term environmental performance should be made when engineers are working out how the asset will physically stand up.
For example, Hobart’s Selfs Point Sewage Treatment Plant (STP) questioned what’s below ground. The project is expanding treatment capacity to receive wastewater flows redirected from the Macquarie Point catchment, but the new infrastructure is being built on former landfill and reclaimed land near New Town Rivulet and the Derwent River. Investigations identified around 5 to 10 metres of variable fill over soft, highly compressible alluvial soils.
For major tanks, process structures, buried services and other settlement-sensitive assets, those conditions created a long-term engineering risk. The foundation strategy had to manage settlement and uneven movement while providing confidence that critical wastewater infrastructure could perform reliably over 100 years.
The engineering constraint came first
Controlled modulus columns (CMCs) were selected as the principal ground improvement solution for settlement sensitive areas. CMCs are rigid concrete inclusions installed through weak ground so that structural loads can be transferred and distributed more effectively. At Selfs Point, the columns act together with an engineered load transfer platform above them to create a more stable foundation for the assets constructed on top.
Menard Oceania were the specialist contractors who designed and constructed the CMCs on the Selfs Point Project, and they were installed using a displacement auger method. Instead of removing large volumes of ground for each column, the installation process displaces much of the surrounding material laterally as the inclusion is formed. On a former landfill site, that approach reduced direct interaction with landfill affected material and provided low spoil and low vibration construction characteristics.
The sustainability question was not simply “did we use CMCs?”
By completion of the CMC installation, as-built records verified 5862 columns across the Selfs Point STP works. Together they represented approximately 71,702 linear metres of installed CMCs and 10,884 cubic metres of concrete.
That scale makes the material specification consequential. But it also illustrates an important point about sustainability claims: the engineering solution itself and the sustainability improvement need to be separated.
For the project’s Infrastructure Sustainability Council IS v2.2 assessment, the use of CMC ground improvement forms part of the agreed engineering baseline. In other words, the quantified sustainability benefit is not claimed simply because CMCs were selected. The measurable improvement comes from how the concrete used to construct those columns performed against the agreed business as usual benchmark.
Portland cement is a major contributor to concrete’s embodied carbon. Supplementary cementitious materials (SCMs) can replace a proportion of Portland cement while maintaining the required performance specification. For the Selfs Point CMCs, the agreed baseline assumed an average 10 per cent SCM replacement. The approved concrete mix delivered during construction achieved a verified 30 per cent replacement using fly ash.
When that improvement was assessed against the agreed baseline and applied to the final as-built concrete quantity, the project recorded an embodied carbon benefit of 936 tonnes of carbon dioxide equivalent.
The significance of that number is not just its size. It can be traced to an agreed benchmark, an approved concrete mix and completed construction quantities. That makes the outcome transparent, reviewable and bounded by evidence rather than by an assumed design intent.
Evidence needs to be designed into the work
The same principle applied to construction assurance. With thousands of CMCs installed across the site, the evidence system had to operate at a similar scale. Instrumented installation rigs captured digital information as the columns were constructed, including parameters relating to depth, drilling and concrete placement.
Complete telemetry records were retained across the CMC work areas, creating a traceable installation history that could be checked against design and construction requirements.
These records complemented engineering inspections, testing and acceptance processes, while noise and vibration monitoring was also maintained during the works.
The project’s measurement and verification framework then brought together final installation schedules, construction records, material information and quantity data. Importantly, it separated design stage estimates from final as-built results and distinguished between outcomes that could be quantified and those better reported qualitatively.
That distinction can seem administrative, but it is central to credible sustainability reporting. An impressive estimate is not the same as a verified outcome. Where a benefit is being reported to clients, rating bodies or the wider industry, the strength of the claim depends on whether someone else can follow the evidence trail and understand exactly what the data does – and does not – demonstrate.
Four lessons for infrastructure teams
First, bring specialist expertise into option development early. Difficult ground is not only a geotechnical problem, but it also influences constructability, material demand, environmental impacts and the evidence that will later be needed to demonstrate performance. Early collaboration between designers, constructors and ground-improvement specialists gives projects more room to make deliberate choices before the solution is locked in.
Second, match the foundation system to the asset rather than forcing a uniform approach. The Selfs Point strategy used CMCs where settlement control and ground improvement were appropriate and retained CFA piles where structural demands warranted them. Sustainability does not require a single green technology; it requires engineering decisions that respond to actual conditions and performance requirements.
Third, treat sustainability evidence as part of delivery rather than a retrospective reporting exercise. As-built schedules, mix records and digital telemetry are most useful when the required information is identified before construction begins. The same records can then support engineering assurance, handover, sustainability assessment and future project learning.
Finally, quantify only what can be verified. In this case, the embodied-carbon benefit could be calculated because the baseline, concrete mix and final quantities were all available. Where evidence is incomplete, a well-supported qualitative conclusion is more credible than a precise number built on uncertain assumptions.
Sustainability starts before the asset is visible
The broader lesson from this case study is that geotechnical engineering and sustainability should not be treated as separate workstreams. A difficult ground condition drove a foundation strategy that had to balance long-term performance, constructability and interaction with landfill-affected ground. The scale of that strategy then created an opportunity to reduce embodied carbon through a verified change in the concrete mix, while digital construction records provided the assurance needed to support the claim.
For future infrastructure projects on weak, reclaimed or variable ground, the useful question is not simply which foundation technology is more sustainable. It is how the engineering baseline is defined, where measurable improvements can genuinely be made, and whether the project is collecting enough evidence to prove those improvements once construction is complete.
At Selfs Point, sustainability was not added after the foundations were designed. It was embedded in the decisions, material specifications and verification process beneath the asset itself.
The author advises that AI was used to refine this article.
