Vodafone Head Office Development – Commercial – capital value $28m


Design and construction underway.
Ground improvement comprised 0.75m diameter stone columns at 1.7m spacing and 0.8m diameter columns at 1.9m spacing, with an ‘Area Replacement Ratio’ of 15%.
Ground improvement to between 3.5 to 5.5 m below ground level.
Ground improvement removed 90% vertical settlement for a 25-year earthquake event and 70% of vertical settlement for a 500-year event.


Ground improvement was adopted for this six-storey building over a surface area of 2000m². The initial proposal of soil mixing was deemed to be too expensive for a site which has minor liquefaction settlement predicted for a 500-year earthquake. The reasonable shallow and stable gravel layer made stone column ground treatment an economical ground treatment option for the proposed building and saved the client about $300,000 compared to the soil mixing treatment option recommended by the previous geotechnical consultant. The reasonably firm sandy silt soils overlying the gravel layer responded very well to an ‘ARR’ value of 15% compared to MBIE guidelines, recommending 18 to 20%. Good communication between the sub-contractor, McMillan Ltd, and Kirk Roberts Ltd concerning initial ground treatment results allowed the stone column spacing and diameter to be increased at the same ‘ARR’ value to speed up column installation, with a subsequent saving to the principal contractor.