Motuoapa Wastewater Treatment Plant

The Motuoapa Wastewater Treatment Plant upgrade required construction of new infrastructure on soft ground close to the shores of Lake Taupo. The site consisted mainly of low-density pumice sands and gravels from Taupō eruption deposits — presenting a bearing capacity and settlement risk that ruled out conventional shallow foundation approaches without prior treatment. Pumice soils present specific challenges for conventional foundation design — low density, high abrasivity, and variable bearing capacity — making ground improvement a more appropriate and cost-effective solution than deep piling in this context.

GSI was engaged by Marshall Projects to deliver In-Situ Mass Stabilisation (IMS) — a technique that mechanically mixes a cementitious binder into the existing soft soils in-place. GSI adapted its methodology to address the highly abrasive pumice conditions, adding steel facings to the mixing equipment. Laboratory binder trialling was conducted on site-specific soil samples before field works commenced, with results exceeding 2.1 MPa UCS, confirming the mix design parameters.

The completed IMS programme delivered a treated ground mass meeting the specified strength criteria, with full as-built documentation including GPS coordinates for every treated zone delivered to Marshall Projects. Works were completed within programme and to client satisfaction, providing the bearing capacity and settlement performance required for the new wastewater infrastructure.

The Motuoapa project demonstrates GSI's capability to deliver specialist ground improvement in sensitive environmental contexts — bringing IMS technology to a New Zealand client where it provided a more cost-effective and less disruptive alternative to conventional piling or excavation and replacement, while meeting the strict environmental requirements of the Taupō freshwater catchment. For clients across New Zealand working with soft, low-strength, or highly abrasive soils — including pumice — IMS ground improvement is a proven, cost-effective alternative to conventional piling, and Motuoapa is one of the most technically rigorous examples of the technique applied in this country.