Civil innovation

Brian Perry Civil is playing a key part in providing innovative solutions in constructing the Tauranga Harbour Link – Stage 2.

Tauranga.jpgTransit New Zealand’s Harbour Link project in Tauranga is the last remaining section of the central corridor of the SmartTransport network. The network will provide a continuous four lane expressway from Takitimu Drive to Hewletts Road – the main connection between Tauranga City and Mount Maunganui.

Fletcher Construction was awarded the $130 million design build contract for Harbour Link Stage 2 in March 2007. Brian Perry Civil, as a division of Fletcher Construction, is constructing the foundation piles, ground improvement works and temporary access staging for the project. For the first six months of the contract the effort was largely focussed on design, which is being undertaken on behalf of Fletchers by URS, Tonkin & Taylor and Peters and Cheung.

The scope of works involves construction of a new 525 metre viaduct with associated on and off ramps connecting with a 460 metre long new harbour bridge, which duplicates the existing bridge, doubling the capacity of the crossing. Works will continue on the Mount Maunganui side with the road along the causeway widened and the Aerodrome Bridge replicated.

To date, Brian Perry Civil has constructed 25 percent of the Chapel Street viaduct piles. The viaduct piles range in size from 1200 diameter to 2000 diameter and with depths up to 47 metres. BPC has also installed 250 metres of access staging into the harbour from the Tauranga side involving 105, 710 diameter, 24-36 metre long driven piles and associated beams and concrete decking. The staging gives access to drilling rigs and cranes to start construction of the 11, 2300 diameter, 56 metre deep piles for the new harbour bridge.

One of the interesting aspects of the job is the use of bentonite during piling. This allows deeper and larger piles to be installed. Bentonite is a mud-slurry used to support the pile bore during construction. The bentonite is injected into the bore to displace soil as it is drilled out and is later removed from the bore as concrete is introduced.

Important aspects of the bentonite system include the development of purpose built mixing, storage and “desanding” plants to supply and later clean the bentonite for reuse. The two plants built for Harbour Link can store 450 cubic metres of bentonite and are able to mix 15 cubic metres per hour of new mud each.

Specialised drilling buckets were designed and built to overcome issues involving flow of bentonite around the tools as they move up and down the bentonite column, and reducing contamination of the bentonite with sand and debris.

The new buckets are able to drill a 2000 diameter by 35 metres deep (110 cubic metres) pile in six hours, including base cleaning.

Before concreting takes place, the contaminated bentonite in the bore used during drilling is replaced by clean bentonite using an airlift. During the concreting process conventional tremmie equipment is used to introduce the concrete to the base of the pile and displace the bentonite from the top. The bentonite is then sent back to the plant for “desanding” which allows it to be reused in the next pile.

The bentonite piling method significantly reduces issues with bore stability that in the past were overcome by expensive full length permanent steel casings, which in turn create the noise and vibration that is usually associated with piling. Thus the bentonite method saves money and reduces potential issues with neighbouring stakeholders.

Piling using bentonite, while common overseas, is a first on this scale in New Zealand. Brian Perry Civil and Fletcher Construction plan to use this method on a range of projects in the future. 

Contractor Vol.32  No.2  March 2008
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