When Offset Developments, a residential building company began construction on a four-unit complex in Christchurch, it encountered a significant challenge when it came to laying the foundations.

After receiving a geotechnical report, the company was advised to place a raft under the foundation slab to protect the site from possible subsidence, soil liquefaction and other issues that may arise due to the weakness of the ground. The project was designed with a 2m deep gravel raft. However, as the water table was only 1.2m below the surface, the ground was too soft for gravel to be compacted.

Mainmark developed a unique solution using Terefil®, a proprietary, advanced, structural lightweight cementitious-based filler that is commonly used for void filling.


Mainmark was required to develop an alternative raft solution that could accommodate the shallow water table which was contributing to the site’s very soft ground conditions. Additionally, the solution needed to deliver ground improvement and liquefaction mitigation benefits for the proposed residential complex.


Mainmark worked in consultation with KGA Geotechnical Group Ltd, specialists in geotechnical engineering, to design a 900mm deep Terefil raft to be placed underneath the foundation slab. As the top of the raft was 200mm below the ground, the total excavation depth was 1100mm.

Terefil creates a non-liquefiable cementitious layer with a uniform mat that provides structural support. It requires no compaction, causes minimal site disruption during application and is self-levelling, lightweight and extremely cost effective. This fast and easy to place solution also has no detrimental effects on the environment.

The speed at which Mainmark was able to place the rafts, following excavation of the site, resulted in a considerably reduced project timeline. While traditional compacted gravel rafts would take a minimum of seven days each in favourable conditions, Mainmark was able to place four Terefil rafts in just five days.

According to George Nicol, Director, Offset Developments, “This is the first time that Terefil has been used for raft slabs. Given the predicament we were in, we were very happy to trial this solution as the alternatives were extremely expensive.”

“As it turns out, we couldn’t have been happier with the outcome. Not only is it a fantastic result, but we’ve discovered a new solution to address a common problem when it comes to improving the weak swampy soil that is prevalent in many areas of Christchurch.”

“Mainmark was also a pleasure to deal with and we’ll definitely be using Mainmark again in the future.”