Motuoapa Water Treatment Plant

The new build Motuoapa water treatment plant upgrade required construction of new infrastructure on liquefiable soils close to the shores of Lake Taupo. The site consisted mainly of low-density pumice sands and gravels from Taupo eruption deposits — presenting a liquefaction risk that ruled out conventional shallow foundation approaches without prior treatment. Pumice soils present specific challenges for conventional foundation design — low density, highly abrasive, and high ground — 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 in-situ soils. GSI adapted its methodology to address the highly abrasive pumice conditions, adding steel facings to the mixing equipment. Laboratory binder trials were conducted on site-specific soil samples before field works commenced, with results exceeding an unconfined compressive strength of 2.0 MPa, 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. Works were completed ahead of programme and to client satisfaction, providing the bearing capacity and liquefaction performance required for the new water 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 Taupo freshwater catchment. For clients across New Zealand working with soft, low-strength, and liquefiable soils — including pumice — IMS ground improvement is a proven, cost-effective alternative to conventional ground improvement, and Motuoapa is one of the most technically rigorous examples of the technique applied in this country.