Putting plant on firm foundation

Thursday 21 January 2010, 11:58AM
By Gisborne District Council



Gisborne’s largest infrastructure project for more than 40 years is under way. The once-quiet Banks Street site is now bustling with people, machinery and materials as physical work begins for the city’s new wastewater treatment plant.

A massive 120 tonne, 80 metre crane dominates the skyline in the vicinity. The year-old crane owned by HEB Structures is fresh out of the Port of Napier where it was used in the construction of a new wharf. The crane is the site’s big brother to a 55 tonne, 30m crane being used by Hauraki Piling Ltd for ground improvement work, designed to put the new Banks Street buildings on a firm foundation.

Works to improve the predominantly sandy soil base began last week and are expected to take about 10 weeks. The work involves creating 1335 stone columns using more than 5000 cubic metres of stones. The 10 metre columns measure 60cm in diameter and are 1.5m apart, centre to centre. About half of the columns will support the pre-treatment and control building; the other half, the biological trickling filter tank.

It takes a crane and two diggers about 25 minutes to form each column, and so far about 20 are being created each day. The crane uses a metal drilling head, assisted by a water jet and vibration, to force the ground apart down to a depth of 10 metres. Once the hole is created and the silty sand displaced, about 4 cubic metres of gravel is progressively put in and compacted from the bottom up. The displaced water and sand is treated on site -- collected in a specially created pond from where it is gradually reabsorbed into the ground water.

CH2M Beca geotech engineer Dave Green says the process is being trialled and proven for the first 80 columns. “We want to make sure everything is going to plan. We tested the density of the sandy ground before the work and once a trial area of 80 columns has been created, we will repeat the tests to gauge the difference. We have a target density we are looking for and we expect to reach that.

“These columns will not only support the buildings, they will also – together with the compaction of the sand around them -- prevent liquefaction during earthquakes.”

About 15 people, including several local labourers, are now working on site, says HEB Structures project engineer Sheridan Peckett, who was also involved in the building of Port of Napier’s new port. HEB Structures is part of HEB Construction, which was awarded the contract to build the treatment plant and the main industrial separation scheme. Council and CH2M Beca site engineers are co-located with HEB at the Banks Street site to provide ongoing inspection and supervision.

She says the 120 tonne crane will be on site for the duration of the year-long project to assist with various lifting required including 18mm thick sheet metal pilings for the three pump stations and 11 tonne pre-cast concrete panels for the biological trickling filter tank.

HEB Construction has a tight but achievable timeframe to have the treatment plant built and commissioned by 31 December 2010.  Within the past month, the site has been cleared and fenced, old tanks removed, site offices established and water, sewerage, power and telephone services installed. ENDS