Massive concrete pour at wastewater site

More than 60 truck loads of concrete were carted to Banks Street yesterday [subs:22 April] in what was the biggest concrete pour on the wastewater treatment plant site to date.

The pour, comprising 270 cubic metres of concrete from 61 truckloads, was the first of three concrete pours to form walls for the treatment plant’s outfall pump station. About a dozen people were involved in the pour that continued in brilliant autumn weather from 6.30am until mid-afternoon.

Gisborne’s Aitkens Concrete provided the concrete itself and McLeods Concreting Contractors ensured the concrete was distributed and compacted correctly.

The concrete is reinforced by 40 tonnes of steel.

HEB Structures project manager Colin Newbold said yesterday’s pour was the largest of three pours to be made in the pump station.

“The second pour will take place in about a fortnight with the final lift, reaching two metres above the sheet piling in place, to be poured about three weeks later. The base of the tank has a 600mm slab of concrete comprising 94 cubic metres. Two of the walls are about three metres thick, and the other two are just under a metre wide.”

The concrete is compacted as it is poured. The wooden shutters holding the concrete in place were expected to come off today [subs:Friday]. The top surface of the concrete was to be water blasted to expose the aggregate, enabling a better join for the next layer of concrete. The mass weight of concrete is necessary to stop the pump station floating like a boat due to high groundwater levels.

Several months’ work has gone into the pump station to date. Thick sheet metal piles were driven in to form the pump station’s external walls, and 1000 cubic metres of soil excavated. A network of wood panels and steel bracing was built as the formwork for the concrete. The inside surfaces of the pump station walls have a thick layer of plastic attached to the concrete to protect the surface from corrosion by hydrogen sulphide gas released from the raw sewage.

Treated wastewater from the biological trickling filter tank will be held in this tank before being pumped through the existing outfall 1.8km to sea. 

Mr Newbold said the good weather had been a boost for site progress. All 1300 stone columns built to strengthen the ground under the various buildings are now complete.

Work has begun on foundations for the pre-treatment and control buildings, and the biological trickling filter tank. Precast panels for the buildings and BTF tank are being produced off site.

About a quarter of the 20 or so workers on site are local. Gisborne’s Currie Construction has been sub-contracted to build the above-ground structures.