|
|
The local newspaper caption accompanying the photo said: “The earth rumbled as muddy water spilled from the cracks in the cliff face, moments before the three metre wide drill came crushing through the wall.”
The Tauranga City Council awarded the $8 million contract for a 400 metre stormwater outlet from the lowest point of the road out to the harbour, to alleviate flooding problems which had previously devastated the area. Harker took up the challenge and the new TBM, built in Pukekohe by Belcher Industries with a three-metre boring head, was lowered into position in September. “It’ll dig a hole big enough for a car,” was one comment when it was first sighted in the suburb. Each underground tunnelling job the company takes on has its own degrees of difficulty. Project manager Leigh Bishop explained that the Otumoetai area has some particular difficulty with regard to ground conditions and the quantity of very fine silt to be removed from the excavated slurry. This turned out to be more of a challenge than expected. “Something like 4500 tonnes of material had to be removed by the TBM. However, when the separation plant was started, the filtration system could not cope with the very fine particles in suspension.” So the Harker team, with assistance from Drilling Fluid Equipment of New Plymouth, bent to the task of solving the problem. It meant delaying the tunnelling for four months, but the solution put together by the New Zealanders led to a state-of-the-art solution. A painstaking review was taken on alternative disposal methods – they revisited their estimations and concluded that they needed to source a centrifuge larger in size and capability. This was not available in New Zealand – two had to be sourced and purchased. They bought and refurbished one and rented a further one from Australia. This new combination gave excess capability and meant they could make four times the original progress estimated for the drilling and extraction of the sub-strate and bring the programme back on track after the initial delays. Consequently, when drilling re-started on January 7, instead of the expected three to four month time span, the breakthrough was achieved in just five weeks. The original programme estimated 10 lineal metres being excavated per shift, but they ended up averaging 20 metres per day. Leigh Bishop said the one shift record drill of 40 metres was unheard before in New Zealand for this size and type of operation. “This was a great outcome and showed that if there is a problem with a job, then Harker has the ability and motivation to fix it, “ he says. With the separation plant working efficiently, material below 10 microns could be removed and trucked to landfill. Four to six trucks were working all day to remove the volume of slurry and silt. The unique slurry separation plant was situated in Rutherford Park off Pillans Road, and is one of the most sophisticated plants constructed for a job of this nature in the country “About 75 percent of the separation plant is new equipment – designed as a stand alone module so it can be utilised not only for micro-tunnelling contracts but for other applications where removal of silt from water is required,” says operations manager Mike Harker. “Water separation is actually quite an art to get the maximum out of your separation plant, as the water is being continually re-used or recycled – the whole system contains something around 70,000 litres. To my knowledge no other plant in the market can process the volume required and remove silts this fine.” There are two ways to control the separation plant. The total operation is remotely controlled from the surface and the TBM operator can pass control totally over to the water treatment plant supervisor operating the slurry treatment plant, or he can control it from the main cabin. “Leigh is the key man on the job – a qualified tunnel manager – a man who is dedicated to the contract,” says Harker. “He doesn’t ask for it but he deserves a lot of kudos for his efforts. “We put our heads down and got on with it – morale stayed strong,“ said Bishop. “Our new TBM (drilling head) worked above and beyond what we expected – it will be great for jobs where similar sub-strate has to be removed, such as sandstone and rotten rock.” What was the feeling when the breakthrough was about to occur? “The pipe had sat behind the final section for four days, so to see the TBM burst through just 20mm off target was a huge relief – an awesome feeling – it’s the best breakthrough I’ve ever been to,” says Bishop. The drilling head emerged into the Waikareao Estuary of the harbour on February 11, with local Mayor Stuart Clark, MP Bob Clarkson, HUC executives and Tauranga City officials in attendance. And about 150 local residents gathered at low tide on the harbour flat to see it – testimony to the excellent relationship built up by Bishop and his team during the time they have been ‘in residence’. “Some of the houses close by had some issues, especially when we were working into the early evening, but I made a point of visiting them all personally and discussing what was happening and why and where we were at,” said Bishop. “Nearby locals really took ownership of the project – they’d sit all day and watch proceedings, even when they couldn’t see much action – and they gave advice if they felt we were listening! I think they appreciated our work.” Several of the Harker team became neighbours because houses were rented in the locality for their families to live in while the project took place. The contract will be completed in this month, once the estuary outlet is built and the approach pipes to the tunnel are linked. Residents are looking forward to no more flooding concerns. The final word goes to Bishop: “I’ve had more difficult jobs but this has been the most frustrating because of the delay. Once the separation centrifuges were fixed, it became a dream job, and the reason for that was because of the attitude of our team on the site – they were great.” Harker Underground Construction was established in 1973 as a family business undertaking general civil construction. Today it has become a specialist microtunnelling and pipejacking company employing up to 50 staff through its Papakura based office and workshop, and operates throughout New Zealand. Contractor Vol.32 No.3 April 2008 |
The drill making the rumble was Harker Underground Construction’s Tunnel Boring Machine (TBM), a newly built three metre diameter slurry machine. For nearly 12 months, a team of nine from the Auckland-based company had been on-site in Pillans Road in the Tauranga suburb of Otumoetai, constructing preparatory works, outfall stabilisation and setting up ancillary equipment for operation of the TBM.