By Myron Washchyshyn, P.ENG.
INFRASTRUCTURE: Port of churchill wharfEngineering
MULVEY & BANANI INTERNATIONALThe long timber crib wharf in the remote port of Churchill in northern Manitoba was undermined by a dredging operation some years ago. The Hudson Bay Port Company called o...
MULVEY & BANANI INTERNATIONAL
The long timber crib wharf in the remote port of Churchill in northern Manitoba was undermined by a dredging operation some years ago. The Hudson Bay Port Company called on Wardrop Engineering of Winnipeg to help stabilize about 230 metres of the structure, which was built in 1930 and 1960.
The engineers considered various methods and materials for the stabilization project. In this remote area with extreme conditions their choices were constrained. The port is icebound eight months of the year, so the work had to be done quickly between July and October. There were challenges of constructability because access to the narrow wharf was constricted by existing buildings. The structure had to withstand the highly corrosive salt-water environment. And it had to have a minimum impact on marine life such as whales that frequent the area.
Not only did the structure have to be strong enough to resist ice floes and ice build up, but also the impact of large ships. The existing wharf was originally designed to accommodate ships in the range of 15,000 dead-weight tonnage (dwt). The owners wanted to increase its ability to load ocean going vessels in the range of 50,000 dwt. to create a deep loading berth that would compete with ports such as Vancouver and Thunder Bay.
Wardrop concluded that a combined sheet pile wall consisting of a king pile with two interlocking sheet piles provided the most economical and environmentally friendly solution. The Trade Arbed wall is designed not to corrode within 50 years. The new design allowed the port to increase the draught of the loading berth by 3.1 metres.
The steel wall system consists of king piles with two interlocking sheet structures. The king piles are socketed 3.5 metres into bedrock to enhance the flexural behaviour, and additional steel plates are used to optimize the flexural and shear resistance at the sockets. A fill of concrete (about 5,000 cubic metres) was poured between the new wall system and the existing timber crib face to provide added strength, and the structure was tied back with high-strength deformed steel bars (galvanized) to precast concrete anchor blocks. To provide a smooth surface and deflect the impact of the ice floes, armour plating, backed by concrete fill, shields the structure in the tidal zone.
Construction had to be done in 8 to 12 metres of water, depending on the tide, down through overburden material to reach bedrock. Despite the difficulties, the work was finished within one shipping season in 1999, and only 1.4% above the tendered cost. As a result of the changes, the port doubled its throughput during 2000 from the previous year. Local First Nations people were employed in the construction and there were considerable economic spin-off benefits for the community.
Owner: Hudson Bay Port Company, OmniTrax Inc.
Prime consultant (design and construction services):
Wardrop Engineering, Winnipeg (Rick Haldane-Wilsone, P.Eng.)
Other key players: Dyregrov Consulting (geotechnical)