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Decoration

Construction of new deepwater port : Northport, New Zealand

Northport is situated on the north-east coast of New Zealand’s northern island. It is here where the country's most modern and deepest port was constructed. Offering permanent shipping access and guaranteeing a continuous increase in traffic, the harbour is undoubtedly a key factor in the development of the New Zealand economy.

The idea of building a new port in this region goes back several decades. The Northland Port Corporation, which operates the nearby port of Whangarei, purchased the site in the 1960s when New Zealand’s forestry was still in its infancy. With the passing of the years some 190000 ha of pine forest have reached maturity, resulting in sustained growth of exports of forest products from Whangarei. The export tonnage was predicted to increase fourfold between 2000 and 2004. Port Whangarei, whose wharf structures are not designed to carry today's heavy handling equipment is incapable of handling this growth. The port accepts draughts of merely nine metres, which is inconsistent with the evolution of modern shipping fleets. Furthermore this capacity can only be assured at the cost of regular dredging. The port lately faced a shortage of dumping sites for the dredged material. These arguments made the construction of a new deepwater port unavoidable.

Northport, a joint venture between Northland Port Corporation and Port of Tauranga, teamed up with Australia’s largest forest-products company Carter Holt Harvey for land operation at the planned new port. Following nearby construction of a very large laminated veneer lumber plant aimed principally at the export market, Carter Holt Harvey undertook a firm commitment to ship one million metric tonnes of wood and derivative products from the new port in its first five years of operation. A growing range of other exports were soon handled by the port. The annual cargo volume reached 2,8 million tonnes in 2005 including agricultural produce, fertilisers, cement and containers as well as wood.

Thorough studies concerning hydrographic and environmental aspects of the site were already carried out in the 1970s. The area where the new port was built is situated between two inactive faults; its favourable geological conditions facilitated the construction works. The soil consists of alluvium, essentially sand, with rare peaty areas, all overlying deep greywacke and argillite bedrock. This meant not only that reclamation could go ahead without any preparatory dredging but also that the dredged soil from the turning basin could be used as backfill material: several million cubic metres of sand were placed behind a 1700 m embankment standing between 3 and 16 meters high. The embankment was also built from readily available local materials, mainly sand and was protected by heavy rocks. Consolidation of the structure was achieved by loading the top of the embankment up to a height two meters with backfill material before levelling it off to build the pavement.

Construction of the new deepwater port began in October 2000. Less than two years later, in June 2002, the first ship was able to dock at Northport. The second berth was completed only a few months after the first berth came into service. The project was built under a US$ 36 million fixed-price design including a 50 ha cargo marshalling area (32 of which were to be on reclaimed land) and a 390 m wharf with two mooring spots able to handle 65000 t bulk carriers with maximal lengths and draughts of  230 respectively 13 m.

The designer Beca Carter Hollings & Ferner Limited (BCHF) originally proposed a HZ/AZ combined sheet pile wall to the contractor FIetcher Construction Company Ltd. The retaining wall was intended to both contain the adjacent reclamation and support the rear of the wharf deck, a cast-in-place reinforced-concrete flat slab supported on the seaward side by two rows of concrete-filled pipe piles. Invited by BCHF to analyse the suggested layout, Arcelor’s engineering team suggested to optimise the HZ/AZ sheet pile solution. The proposed study allows a substitution the combined sheet pile wall by a more cost-efficient AZ 36 wall. The design takes the present high loads (dead load of 85 kN/m, live load of 240 kN/m) into account. The considerable width of the chosen AZ piles (1,26 m per double pile) is an advantage in terms of construction productivity. Although manufactured on the other side of the world, the 3000 t of sheet piles used for the Northport project were supplied to the site within 3 months.

The 30 m high retaining wall made of AZ 36 sheet piles penetrates 16 m into the prevalent sand, the lower five meters being of very compact nature. Two 15 m high retaining walls connect the main quay wall to the backfilled embankments. The AZ 36 sheet piles are anchored by 26 m tie rods to a deadman wall with a height of 7.5 m made of double AZ 26 sheet piles.

The main retaining wall and the deadman wall were connected by 60 mm diameter steel tie rods provided by Anker-Schroeder, consisting of two 13,45 m bars joined by a threaded coupler. Continuous steel-channel walers were bolted to the sheet piles by a team of divers who also installed the tie rods.

The site’s geology and the depths involved meant there were no particular problems regarding pile driving. The main retaining wall was driven from a Flexifloat S-70 jack-up barge using a PTC 50 HL vibratory hammer operated with a P&H 5100 crane. In case of refusal prior to design penetration into the compact sand, a nine metric tonne Junttan hydraulic hammer was used to complete the driving process. A guide frame with two supporting levels 8 m apart guaranteed the required alignment. The template was held in place with steel tubes that were driven until refusal with the same driving gear that installed the sheet piles. Nine pairs of AZ 36 piles were inserted into the guide frame and driven in panels to assure good verticality. The deadman’s AZ 26 sheet piles did not require driving: they were simply set up from the embankment and propped until enough sand had been placed on both sides.

The design life was set at 50 years, wherefore special care was afforded to the corrosion protection of the wall’s most critical parts: the intertidal zone and the top of the immersion zone, which coincides with the level of the tie rods. The sheet piles were coated with two 175 mm tar-free epoxy coatings called Altra Tar. Once assembled, the tie rods were wrapped with Denso Ultraflex corrosion-protection tape with heat-shrink sleeves around the couplers.

Right from the outset, plans were made for two further berths drawing 14,5 m. Construction of Berth No 3 started end of 2005 using a combined sheet pile solution delivered by ArcelorMittal.

 Owner  Northland Port Corporation, Port of Tauranga and Carter Holt Harvey
 Designer  Beca Carter Hollings & Ferner Limited (BCHF)
 Contractor  FIetcher Construction Company Ltd
 Steel grade  S 430 GP
 Sheet piles  AZ 36, L = 30 m; AZ 26, L = 7,5 m
 Total tonnage  3000 t



Published: 2006

 

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