By Westmar Consultants & Jacques Whitford
Voisey’s Bay WharfEngineering
Located in Edward's Cove on the northern coast of Labrador in the Canadian Arctic, the Voisey's Bay deep sea wharf was built to serve the huge mining facilities being developed at the site. First disc...
Located in Edward’s Cove on the northern coast of Labrador in the Canadian Arctic, the Voisey’s Bay deep sea wharf was built to serve the huge mining facilities being developed at the site. First discovered in 1993, the nickel and copper resources at Voisey’s Bay are considered to be one of the world’s richest deposits.
The wharf is used to land all the goods and materials required for the mine, including fuel oil for generating power. It is also used to ship out the nickel concentrate. Westmar Consultants and Jacques Whitford were commissioned to design the wharf by its former owner Inco. Inco was recently taken over by Brazil-based CVRD.
The remote Arctic site and other factors presented challenges to the engineers. There was a short construction season. The fragile environment had to be protected. The structure had to be relatively simple and economical to build, and construction had to involve the First Nations people. Perhaps most important, the structure has to withstand battering from ice floes and from the heavy icebreaker ships that use it to dock.
Coping with ice and icebreakers
Fednav, who received the contract to ship the mine’s metal concentrate, commissioned a new ice-breaking bulk carrier dedicated to the Voisey’s Bay operations.
The carrier uses its power and momentum to break up the ice within the berth and to attempt to break up the large “ice bustle” that adheres to the sheet piles. The wharf structure therefore needs to be robust and reliable under these harsh conditions. It was known that two other Arctic deep-sea ports had suffered severely from the abuse of winter shipping.
At Voisey’s Bay, the engineers used an innovative application of a conventional circular sheet pile structure. The structure could deal with the large ice forces, and it could provide the reliability to withstand potential ship collisions. It is also critical that the structure is robust and reliable because the shiploader is supported by a foundation constructed inside one of the cells.
By placing the main ship mooring points on the adjacent shoreline as simple spread footings, the wharf was reduced to approximately 100 metres in length, using only four 24.5-metre diameter sheet pile cells. The rock fill inside the wharf structure cells was processed from materials salvaged from the site in the preparation of the port storage building.
By paying careful attention to the soft seabed conditions and by reducing the height of the wharf deck, the engineers were able to design the structure to cope with the soft sediments without the need for dredging. This decision was made out of environmental concerns and saved dredging 80,000 tones of material. The wharf deck height criterion allows for a storm wave overtopping once a year.
Seabed conditions make construction delicate
Combining the heavy rock-filled sheet pile gravity structure with the thick soft seabed sediments presented a major technical challenge. In addition, the seabed was steeply sloping. As a result, the structure could have undergone significant distortions during construction. Unlocking any one of the 153 sheet pile interlocks could cause the cell to collapse.
To address the risks, careful monitoring procedures and staged filling were developed to continually assess the stability of the structure during construction. The instrumentation included 32 strain gauges to measure the strains in the piles, three inclinometers, two earth pressure cells to accurately assess the filling rate, and three piezometers to monitor pore water pressure in the clay.
Impact panels and armour
A unique system of precast concrete impact panels was developed to help the structure resist the thermal, icebreaking and ice floe forces. The panels needed to prevent the interlocks in the cells from being excessively deformed.
The panels were pre-fabricated off-site and quickly and simply installed inside the cells. A large concrete cope beam and slab was cast along the top edge of the wharf to tie the ice panels together and to increase the ability of the structure to survive a collision with the ice breaker bow.
In addition, a special riprap armour layer was designed in consultation with Fednav to help the wharf resist scour from the severe propeller washing that occurs with icebreaking.
Collaborating from opposite ends of the country
The simplicity in the construction of the main structure ensured the wharf could be constructed in a way that could maximize the use of labour from the local communities, especially First Nations people. The main wharf structure was completed in December 2004, just as the bay was beginning to ice up. The superstructure and shiploader foundation were completed in 2005 after the structure deformation had essentially stopped.
Westmar, whose head office is in Vancouver, planned and designed the wharf structure and provided technical assistance during construction. Their responsibilities also involved “port optimization,” and design of the shiploader and mechanical and electrical systems. Jacques Whitford, from its Halifax and St. John’s offices, had provided geotechnical expertise on the project since 1995. It provided geotechnical assistance to the wharf design and conducted inspections during the installation of the piles. Although the two consulting engineering companies were working from opposite ends of the country, they quickly developed a supportive and cohesive relationship.
Client: Voisey’s Bay Nickel Company
Marine structures specialist consultant: Westmar Consultants, Vancouver (Peter Acton, P.Eng., Harald Kullmann, P.Eng., Ryan MacPherson, P.Eng., Neil Campbell, P.Eng.)
Geotechnical consultant: Jacques Whitford, Halifax (Paul Deering, P.Eng., P.Geo., Dan McQuinn, P.Eng., Arun Valsangkar, P.Eng., Barry Power, CET)
EPCM contractor: SNC-Lavalin
Instrumentation subcontractor: Slope Indicator