Ground Improvement at Poverty Bay Rugby Club

The presence of potentially liquefiable sand deposits also contributes to a greater need for ground treatment under large structures across the region.

The Poverty Bay Rugby Club serves as a key governing body for rugby in the Gisborne district and the surrounding area. Rugby Park, located at the end of Childers Road, is the home ground for the Men’s First team and serves a pivotal role for the Club.

Following fundraising from the community and sponsors, the club invested in redeveloping the grandstand overlooking Rugby Park. This included building an extension to the structure to better serve players, referees and other staff.

At the start of the redevelopment, the main contractor identified that there were potentially liquefiable soils below the Poverty Bay Rugby Club grandstand. This resulted in the structure not meeting the seismic NBS requirements.

The contractor also excavated the existing grandstand’s slab, removing the existing surcharge caused by the slab. This existing surcharge on the ground had to be accounted for in the final engineering design.

The following design targets were set for the project:

• Increase the bearing capacity of the soil beneath the foundation,

• Establish a non-liquefiable crust throughout the injection zone,

• Mitigate future liquefaction, and

• Achieve 67% NBS (New Building Standard) for the structure.

The project was further complicated by severe flooding events in the region between late 2022 and mid 2023. Special scaffolding with plastic wrap was constructed for the areas outside of the existing grandstand to ensure that works could continue without further inclement weather disruption.

Before work commenced, the team considered multiple options to provide necessary external surcharge to the ground. The chosen solution was a combination of a compacted hardfill on top of the excavated area, as well as a capping resin injection layer. The capping layer was installed at a depth of 1.25 m, or 0.25 m above the treatment zone. This provided added resistance for any vertical heave that the ground improvement resin injection works might cause.

The new grandstand had a proposed footprint of approximately 861m2, with work designed to be carried out in four stages. During trial works, Teretek resin was injected in 16 points across this area to verify that we could meet the set targets. For each stage of works, the team carried out two pre- and post-injection geotechnical investigations to ensure targets were met throughout.

Other construction considerations were also designed for and adhered to. This included the monitoring of heave to the structure (to not exceed the 25 mm threshold), as well as ensuring that all existing underground services were not damaged during works.

Both of these specifications were met, with the maximum of 16 mm heave recorded for the full-scale works. All underground services on-site were also successfully identified and avoided.

At the conclusion of the works, tests showed that key targets were met across the injection depth range. The works met the 67% NBS target with a non-liquefiable crust being established throughout the injection zone.

The Teretek resin injection method improved the ground, providing liquefaction mitigation.