The first stage of the construction of Queen’s Marque was to create a cofferdam for the district’s underground parking. A cofferdam is a watertight enclosure that allows space to be created below the waterline. For Queen’s Marque a steel sheet pile (SSP) cofferdam was constructed around the site, essentially encircling the parkade structure which will be constructed two levels below harbour level. The top of the SSP cofferdam has been set at approximately +9.5 feet above Canadian Geodetic Datum (CGD) which will allow for high tide, a slight storm surge and modest wave height. The maximum recorded water level in Halifax Harbour during Hurricane Juan was about +7 feet CGD. The SSP cofferdam was driven to bedrock level to facilitate the excavation activity and dewatering of the interior of the cofferdam. The SSP cofferdam was installed as follows:
- The perimeter of the site was pre-excavated with the use of a sliding-shore system to remove all materials from the area where the SSP cofferdam is currently installed. The pre-excavation removed any obstructions such as boulders, timbers, etc. that could impede the advancement of the SSP cofferdam which is to be driven to bedrock.
- The pre-excavated trench was back-filled with surge rock.
- A crane-mounted vibratory hammer drives the SSP’s to the bedrock level.
- A crane-mounted pile driver drives the SSP’s approximately two feet into the bedrock in order to provide the best possible water-tight seal.
- A total of 291 SSP pairs make up the perimeter of the site. The SSP vary in length, ranging from 17 feet long on the west side of the cofferdam to 50 feet long on the east (water) side of the cofferdam.
- Upon completion of the SSP cofferdam, the area inside the cofferdam was excavated sequentially and the interior structural steel walers and inclined post-tensioned rock anchors were installed to provide external soil and hydrostatic pressure resistance.
- A series of dewatering sump pumps are installed to keep the water level in the cofferdam to workable levels.
The cofferdam, now complete, will allow the Queen’s Marque district to provide over 300 parking stalls in a two-level underground parking structure. This nearly doubles the amount of parking stalls previously offered at the Queen’s Landing and Cable Wharf parking lots that were located on the site.