Foundation works progressing well

Friday 12 February 2010, 10:08AM
By Gisborne District Council


Ground works for the city’s new wastewater treatment plant in Banks Street shifted up a gear this week with a third crane joining the other two on site.

Two cranes are now being used for ground improvement work that includes the creation of more than 1300 stone columns to ensure the new buildings are set on a firm foundation. The third crane, a massive 120 tonne, 80 metre machine that dominates the skyline, is involved in sheet piling for the three pump stations being built on site.

More than 300 stone columns – each measuring 10m long and 60cm in diameter -- have been built since the ground improvement work started in mid-January. About half the columns will support the pre-treatment and control building; the other half, the biological trickling filter tank. With about 25 on average being constructed a day with the use of one crane, the second crane will soon see up to 56 being built daily. The results from the first trial area are just in and currently being analysed.

HEB Structures project engineer Sheridan Peckett says work is also progressing well on preparation for the three pump stations being built on site – one for the biological trickling filter tank itself, one for the wastewater coming into the site, and the third for the treated wastewater going to the outfall, then to the existing outfall pipe 1.8km to sea.

Each below-ground pump station is being lined with 18mm thick sheet metal piles, which interlock to create an impenetrable barrier. These piles, which range in weight between 1T and 1.37T are slowly driven into the ground using a combination of mainly vibration and weight. The largest crane and a 5 -8 tonne vibro hammer are employed to do the job. The outfall pump station, the largest of the three, has 13m piles driven 11m into the ground. The biological trickling filter piles are 10m long and driven into the ground to a depth of 8m.

Council projects engineer Michael Yukich says the alignment of the first sheet pile is critical as this becomes the ‘cornerstone’ that the other sheet piles will be set to.

“Driving in the piles is time-consuming with each pile being stopped and checked for vertical alignment several times on its way down. As the pile gets closer to its final resting place, greater friction slows the speed of entry,” he says.

Once the piles are in place, a massive 1700 cubic metres of soil needs to be excavated –1000 cubic metres for the outfall pump station alone – before a concrete lining can be poured. The excavated material will be stockpiled on site, potentially for future use.

HEB Structures is part of HEB Construction, which was awarded the contract to build the treatment plant and the main industrial separation scheme, initial works for which are due to start within the next two weeks. Council and CH2M Beca site engineers are also located on site to provide ongoing inspection and supervision.