Fit for a king

Fletcher Construction’s six-year project to build three berths at Marsden Point features pile driving of a kind not seen before in Australasia.   BY GAVIN RILEY

Northport.jpgA six-year-plus task by Fletcher Construction, spanning two projects, to design and build a deep-water port at Marsden Point to replace Port Whangarei was successfully wrapped up at Christmas with the completion of a third berth.

A fourth berth will eventually be built at the port, which is operated by Northport Ltd and is designed to handle 2.8 million tonnes of cargo a year.

Construction of the third berth featured the biggest king piles driven in Australia or New Zealand to date, as well as innovative design aspects.

Fletcher began construction of Northport’s two-berth first stage in mid-2000 when it assembled and laid quarry boulders on the harbour seabed to form the bund walls which outlined the hardstand area.

Some 2.4 million cubic metres of dredgings were used to form a 32 hectare marshalling area behind these walls. The dredged area formed a 600-metre turning basis with a depth of 13 metres.

The finished stage-one wharf face is 400 metres long and consists of a rear sheet-pile retaining wall and two rows of piles, which support the concrete deck. The rear sheet-pile wall was tied back to the deadman sheet-pile wall within the reclamation. Fendering completes the berth face.

The hardstand area initially was filled to a height of two metres above the bund wall to aid compaction and then smoothed over before the final paving was carried out and the various services laid.

Stage two of the port project, the recently completed 2790 square metre third berth, is connected to the first two berths and extends to the east towards the Marsden Point refinery jetties. It is able to accommodate ships up to nearly 200 metres long with a maximum draft of 13 metres.

Fletcher began work on the $21 million, 15.5-metre-deep third berth at the beginning of August 2005 and spent till Christmas clearing away rock and bund material. Simultaneously, the company had all its pile guides and crane frames designed and manufactured ready for a piling start in January last year.

However, the rock proved more difficult to move out of the way than anticipated and that put the programme six weeks behind schedule.

“It wasn’t a lot of rock – about 1000 cubic metres of armour and about 2000 cubic metres of core material – but it was just difficult to get out,” says project manager Tony Pike.

Fletcher used a different method of construction for the third wharf – twin combination HZ-AZ sheet-pile walls, where king piles are spaced along the wharf at intervals with an infill sheet between. The king pile carries the load and the infill sheet creates the integral structure that holds back all the sand and the reclamation.

The king piles are at 1.8-metre centres and required 2.2-metre-diameter oversize casings to be used for the first 20-plus metres of rock core.

“We had to put the oversize casing right through it, clean out the inside and remove all the rock, backfill it with sand to the same level, then extract the casing so that we actually had a clean area to drive the king pile through,” Pike explains.

“That was quite an effort to do and is unusual in this type of construction. 

“We were then able to set up our pile-guide frame over it and drive the king piles into these pre-prepared zones of sand and then drive the sheets down in between them.”

The king-pile sections Fletcher was driving are 33 metres long and weigh 7.5 tonnes. “Nobody’s driven king piles of this size before in Australia or New Zealand and the advice we had was that it would be extremely difficult, even if possible, to achieve the accuracy that was required within the design,” Pike says.

“Ordinarily, even in high-production China, they would only drive 15-metre lengths and would splice another length on.

“There were a few serious doubts, mainly from the manufacturer from Singapore. He watched, and ultimately we ended up educating him. He’s taken on board a lot of what we’ve done and is trying to make it more of a standard practice.

“We drove the king piles in at full length because to have made a structural splice every time we drove one would have been very time-consuming and very expensive.”

Pike says even the design is not traditional. The first two berths were based around putting a deck on top of piles – “a standard sort of wharf where you can go underneath”. But at the third-berth end of the wharf the ground conditions deteriorate and wouldn’t support such loads. Piles would have had to be 50-60 metres long to carry on the same design.

“So this design was born of needing to get around those issues,” Pike says.

“This is a mass-gravity wall where we’ve got two rows of sheet piles, one at the front of the wharf and one at the back, and they’re connected by tie rods which are installed under water. There are two rows of those, one at minus-2 chart datum and one at minus-10.

“The walls are 12.5 metres apart and the tie rods connect into them at those two levels. There are two tie rods for each set of king piles. You fill the box you’ve made with sand and that creates the mass-gravity wall that’s floating there. That holds back your reclamation, and the king piles take the load from the wharf down to the ground.”

The Fletcher team had other challenges to overcome. The site had only one access (from the edge of the existing wharf), making the job “linear” and unforgiving on the programme; working in a live-port situation required flexibility and adaptability; the 250-tonne crane needed on the project demanded a massive frame consisting of four 20-tonne units; and the piles had to be placed within 100mm horizontally, and no tolerance.

“They had to be driven to a level vertically, and a 33-metre-long pile weighing 7.5 tonnes is notoriously difficult to drive,” Pike says.

“They tend to get into the ground and spring off, and so at the top end it could be a couple of metres out of position by the time you’ve finished driving it in.

“So we came up with a pile-guide system that countered that and a methodology that held everything in place until we had the tie rods in. Then it all just locked into place.”

Piling was completed in early September, just a week behind schedule despite progress having been hampered by winds (anything above 10 knots meant a down day).

Placing of the deck then began and was completed in late November after eight pours of 300 cubic metres each. Each pour lasted about eight hours and all the concrete was supplied by Firth’s Waipu plant.

Because the deck is self-supporting and spans 12.5 metres, the construction required “heavy” use of reinforcing steel – “a grillage that you couldn’t even get your fingers through in places”, according to Pike. “It was not easy to construct.”

Pike joined Fletcher Construction eight years ago from what was then Works Civil and was project engineer for bunding and reclamation on stage one at Marsden Point. Between stages one and two he worked as a project engineer on Auckland’s Grafton Gully roading project and the Upper Harbour Bridge scheme.

He said he and his Marsden Point stage-two team, which peaked at 35, were very pleased to have completed the project just before Christmas, a week or so inside the target date.

“You don’t get better than this. Even though it was a difficult job, it was a beautiful location. It was also out of the way – we were not dealing with traffic.

“And Northport were a fantastic client to work with because they’re pragmatic, they know what they want, and they know the value of quality.”

Contractor Vol.31 No.1 February 2007
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