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Save time and money with a gravel raft alternative

New Zealand’s complex geology presents unique challenges for construction, particularly when it comes to foundation performance.

Traditional soil replacement involves removing poor soil and replacing it with gravel fill, or piling — but there’s a more efficient way to do this. Over the last 30 years, developments in cellular concrete technology have revolutionised construction.

Theo Hnat is the General Manager at Mainmark New Zealand. He has a decade of engineering experience, including as Technical Manager of Mainmark during the years following the 2011 Christchurch earthquake. As well as his considerable work in testing these technologies, he has managed many commercial projects over the years.

What is a Terefil raft?

Terefil is a cellular concrete soil replacement solution for building sites situated on soft soils, or challenging sites that would be difficult to build on without piling or other forms of ground improvement.

“It is an engineered, fast solution. This is better than a gravel raft and is a potential alternative when you would otherwise need to pile for geotechnical reasons” says Theo Hnat, General Manager at Mainmark New Zealand. 

Terefil is created when a foaming agent is added to a cementitious slurry, forming a large number of air bubbles within the mixture. These air bubbles give cellular concrete its characteristic low density, with significant advantages over traditional materials like gravel or regular concrete. 

In a raft application, Terefil cellular concrete is around three times lighter than the soil it is replacing. It’s designed to reduce static settlement and cut down on raw material use, with superior thermal insulation properties. This means better temperature regulation in both hot and cold climates, leading to lower heating costs and a consistently warmer home.


Saving time and money

The main traditional methods of ground improvement (piling or a gravel raft) can cost developers in time and money. 

Piling has inherent uncertainties, such as depth. “You have to have really good geotechnical investigation and there’s always the risk you have to go deeper, increasing cost and time,” says Hnat. “It’s a good solution in some cases but you might have to go 10 metres deep or more to find competent ground.”

Gravel raft is a simpler method than piling. However, it may not meet all ground improvement requirements, particularly on soft soils, and can take several weeks to deliver onsite. This method also requires more raw material to achieve the same results in ground strength and stiffness, with little thermal insulation benefit, as well as insulation.

The recent modelling shows that we can reduce the thickness of Terefil by up to 60% compared to a gravel raft and still get the same, if not better, performance,” says Hnat. (source)

In addition to reducing the thickness and thus the amount of material needed, there are other benefits to using Terefil over a gravel raft: “Reduced excavation depth means time saved on excavation and reduced disposal costs. Those are direct savings before we even turn up,” says Hnat.

Gravel raft is more labour intensive as it has to be installed in layers, with compaction and external validation for each layer. If it rains, it could take longer as light erosion requires re-testing and re-compacting the gravel. By comparison, a Terefil raft can be poured in a day.

What does this mean for property developers? 

“You would be able to build more houses in a year.” Hnat elaborates: “With your current team and resourcing, you might build 20 houses a year, and maybe 10 of those need a gravel raft. By switching to a cellular concrete solution, you could save 2 weeks per project on average. So the question is: what could you be doing with that extra 20 weeks?”



Reduce risk and minimise impact

There are also long term benefits to using Terefil over traditional methods. The lighter and more robust material means reduced risk of damage to the property from any earthquakes or aftershocks.

A gravel raft on soft soils can also increase subsidence risk because it adds weight to the ground — risk that falls back on the builder or developer.

With piling, the risks are different. “We’ve had engineers reach out to us, who were expert witnesses in cases where a building on piles subsided,” Hnat says. “The building is now out of level and also connected to these piles which are locking in the whole thing. So it becomes really difficult to re-level and could be hundreds of thousands of dollars to replace.”

In more densely populated areas, developers may also face requirements from councils and residents to minimise disruptions. With Terefil there is no loud machinery and heavy vibrations — equipment can be set up 30-60 metres away from the site and done non-invasively.

Fewer truck and site movements also mean a reduction of environmental impact, up to six times less than a gravel raft.

Engineering expertise built in

Terefil is an engineered product, and complies with New Zealand guidelines. External engineering validation on the product isn’t needed as Mainmark provides this with site specific testing.

The Mainmark team can provide all inclusive engineering consulting, or work alongside an existing team, says Hnat. “If a property developer already has an engineer on their team, we can work with them. They might have done the site design, or be able to review and sign off on the Terefil soil replacement work. If they prefer our team handles everything, that works too.”

Mainmark can also provide Producer Statements and documentation for all stages of a project, which are required for building consent submissions.



A proven solution

Cellular concrete has extensive use cases both globally and in New Zealand.

Mainmark has used Terefil in a range of applications across many sectors, including: residential multi unit townhouses, data centres and warehouses, a high school, and a substation.

“For example, Wainuiomata High School had several stages of up to 2000 m2 to fill,” recalls Hnat, who was involved with the design of the project.

“Even aside from the technical benefits of the 1.2 metre thick raft, up to 2 metres in some places, it would have taken months to deliver a gravel raft. For the main contractor on the project, the fact that they could deliver a section that size in a few weeks rather than a few months was a big saving.”

Many international design standards specify cellular concrete for a range of construction applications. There is growing demand for cellular concrete worldwide (source), with widespread use in Australia, Brazil, China, France, Germany, Japan, South Korea, the UK, and USA.


Shannon Franks
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