Redevelopment of Wainuiomata High School using Terefil Cellular Concrete

Its complex geological history has resulted in the deposition of alternating bands of lake, swamp deposits and alluvium over the last 80,000 years. Near the location of the school, these soft sedimentary deposits are over 60m deep.

Multiple faults are present in the Wainuiomata area. The Whitemans Valley Fault is the closest active fault, approximately 2 km from the school, while the Wellington Fault Line is the closest major fault, approximately 5.5 km from the school.

Increasing student demand at Wainuiomata High School led to a Ministry of Education re-development project, with many existing buildings needing to be altered and new buildings constructed.

Detailed ground investigations across the site revealed a number of issues. Results showed damage due to soil liquefaction, as well as potential for further damage due to static settlement. Non-uniform ground conditions and a high water table made for further complications. Finally, proposed additions to existing buildings meant an increased risk of differential behaviour in both static and seismic conditions.

Options proposed by the geotechnical design team to address these issues included soil mixing, reinforced gravel raft, and cellular concrete soil replacement.

The geotechnical design team chose a Terefil cellular concrete soil replacement solution to meet the density requirement of 850 kg/m3 and strength requirement of minimum 1 MPa. This ensured a high-strength, engineered fill solution while also reducing the load on the underlying soft soils.

The Terefil raft extends 1.2 m below ground level, and at least 1.2 m out from the proposed building footprint. Mainmark carried out ground improvement works in 4 stages, before the new buildings were constructed:

• Learning Hub 1 – 3 (Stage 1 & 2)
• Learning Hub 4 to Block F (Stage 3)
• Learning Support Centre, Library, Canteen to Block D and Block L (Stage 4)

While the full re-development project is still ongoing, Mainmark has completed 4 stages of ground improvement works.

The lightweight nature of Terefil meant a reduced weight on the underlying soil, allowing for new school buildings to be added to the site. This additional structural load was possible without increasing the risk of static settlement. 

Using cellular concrete also meant that the installation time was much less than the alternative options, resulting in reduced build time and costs for the client.

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The new cultural precinct with wharekai, technology and arts hubs opened in March 2023. This project is ongoing — the finished result will see new classroom blocks to support increased student numbers, a support hub with essential student facilities like reading recovery, counselling services, career guidance, school deans, a school nurse, and a sickbay, and refurbishment of the school’s sports facilities, library and administration blocks (Ministry of Education, 2023).