Ground Engineering
Rotorua Lakefront
In-situ mass stabilisation of 6,000 m³ of sensitive geothermal soils to create a bearing foundation for Rotorua Lakes Council's lakefront redevelopment.
- Location
- Rotorua, Central North Island
- Main Contractor
- HEB Construction
- Designer
- GSI NZ / Tonkin & Taylor
- Timeline
- February — June 2020
- Scope
- 6,000 m³ in-situ mass stabilisation to 4 m depth
Design
Geothermal Soils Requiring Specialist Treatment
HEB Construction engaged GeoStabilization NZ at the project's initial stages to provide site sampling, laboratory testing, and physical trial panels ahead of the main works programme. The Rotorua region's soils are heavily influenced by the Central North Island geothermal field — very soft, sensitive silt materials with natural bulk densities of 1,400 kg/m³ and moisture contents exceeding 100%. Standard stabilisation approaches were not appropriate for these conditions.
Design requirements specified a 2 MPa unconfined compressive strength and 500 kPa shear strength from the stabilised block — challenging targets given the material characteristics. Working alongside geotechnical designer Tonkin & Taylor, GSI NZ developed an optimum mix design through an exhaustive iterative laboratory testing programme using sulphate-resistant cement (NZS 3101) and hydrated lime to reduce moisture content. Three trial panel variations of the preferred mix were constructed in late 2019, then cored and UCS tested after curing to verify strength development.
Build
6,000 M³ Stabilised in Four Months
Working within environmental controls of sheet-piled cofferdams and protection screens to isolate works from the adjacent lakefront, 6,000 m³ of sensitive soils were blended with cement-lime binders using Allu mixing arms to a depth of 4 metres below design level. The programme ran from February to June 2020, including a one-month stand-down during COVID-19 lockdown.
Binders were prepared as wet grout and blended into the ground under strict quality controls, including 3D working and as-built models to ensure 100% soil stabilisation coverage. Daily wet grab samples were UCS tested at 7 and 28 days throughout the programme. On completion, core drilling and 46 UCS tests on borehole samples verified soil strengths — all samples achieved greater than 2 MPa. Triaxial testing of wet grab and core samples confirmed shear strengths exceeding 500 kPa across the stabilised block.
Deliver
Strength Verified. Walkway Delivered.
The completed stabilised block provided the engineered bearing foundation required for Rotorua Lakes Council's new lakefront walkway — a key component of the Vision to Action lakefront development programme that has transformed public access to Lake Rotorua's foreshore.
All specified strength targets were achieved and verified through a comprehensive QA programme. Core drilling and 46 UCS tests on borehole samples confirmed strengths exceeding 2 MPa across the full 6,000 m³ treatment zone. Triaxial testing verified shear strengths above 500 kPa. Zero structural incidents were recorded during or following construction.
The Rotorua Lakefront project demonstrates GSI NZ's capability in specialist ground improvement for geothermally influenced soils — a soil type found across the Central North Island that requires specific binder selection, iterative mix design, and rigorous QA to achieve reliable strength outcomes. It is one of the largest in-situ mass stabilisation projects completed in the Rotorua region.
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