Majestic Centre Piles

Geotechnical Drilling, Micropiling

2014

Wellington, New Zealand

Fletcher Construction

744m - length of micropiles installed
26.5m – Deepest 300mm micropile
$83.5m – Total cost of restrengthening

A property assessment following the Canterbury earthquakes in 2011 found that the Majestic Centre was between 34 per cent and 66 per cent of the new building code.

The project was initially expected to cost $35 million and bring the building up to 70 per cent of code when it began in 2012. A year later, Kiwi Property decided to increase the strengthening to 100 per cent. The budget was subsequently increased to $83.5 million in 2014.

The strengthening required a massive new foundation to be constructed within the existing multi-level car park below the Majestic Centre.

Webster Drilling also undertook extensive low-headroom geotechnical investigations on behalf of the designers in the lead-up to the works.

Fletcher Construction tasked Webster Drilling & Exploration with installing 38 large-diameter vertical micropiles required for the seismic restraint of the new foundation.

The building was underlain by a mixture of extremely weathered sandstone, hard, unweathered siltstone and highly fractured argillite containing clays and silts (Wellington Greywacke sequence), making the selection of a suitable drilling system challenging.

To complicate matters further, the drilling system needed to be capable of installing the micropiles within a limited headroom of less than 2.2 m.

A large-diameter pneumatic Down-The-Hole (DTH) hammer was purchased as the only practical solution for penetrating the hard rock in a low-headroom environment. Our Engineering Workshop then fabricated a 7 5/8-inch drill pipe string in 1.0 m sections, along with a full suite of pipe-handling tools and racks to allow the heavy-duty equipment to be handled in the confined space.

An air/foam drilling medium was used for the majority of the micropile drilling to provide borehole stability and remove cuttings at a lower velocity when drilling through the highly fractured formation.

Webster Drilling’s in-house engineering capability, hard-working staff, extensive experience in low-headroom micropiling, and innate understanding of the behaviour of Wellington’s notorious geology provided us with a unique ability to select a drilling system that met the challenge and delivered a cost-effective solution for the client.