Vancouver Island Conference Centre Foundation

The Vancouver Island Conference Centre is under construction in downtown Nanaimo, B.C., a city on the southeast coast of the island, opposite Vancouver.

The downtown location is vital to the success of the conference centre when it is completed in the spring of 2008. It will take advantage of Nanaimo’s picturesque harbourfront and the retail and restaurant businesses that have flourished in adjacent historic buildings.

However, the preferred site presented several geotechnical and environmental challenges. It was originally a sandstone inlet used as a harbour. Dating back to the late 1800s, the inlet was filled in with heterogeneous soils containing natural and man-made materials such as loose sandy soils, boulders, wood and metal debris, concrete, brick and blast rock. The in-fill material would be susceptible to liquefaction and large-scale movement in the event of an earthquake, posing a significant risk of the structure settling or collapsing.

As Nanaimo is located in a region which has one of the highest risks of a major earthquake in Canada, it was necessary to find a means to improve the ground conditions and address the seismic issues. In addition, an environmental site assessment indicated the presence of contaminants in the soil at levels exceeding B.C. standards.

Various ground improvement solutions were considered, including stone columns and steel piles. Their costs, however, ranged from $5 to $10 million, well beyond the budget. These conventional solutions also required completely excavating and relocating thousands of tonnes of soil that was contaminated by the industrial businesses that once stood on the banks of the inlet. The soil would require disposal at a remediation site, likely adding to the costs. A solution that was timelier and cost effective needed to be developed.

European cutter soil mixer

Golder Associates and Golder Associates Innovative Applications (GAIA) proposed a foundation design using a European cutter soil mixer (CSM) and deep soil mixing at select footing locations.

Deep soil mixing (DSM) is an in situ technology by which cement is blended with soil at depth by mechanical or rotary tools.

The cutter soil mix technology was developed by a German construction and machinery manufacturer. It makes use of two sets of cutting wheels that rotate about a horizontal axis to produce rectangular panels of soil mixed with cement. What made the technology viable for this project is the ability of the cutter soil mixer to key into bedrock or dense strata at depth, something conventional auger arrangements are not capable of.

Golder and GAIA proposed importing the technology from Europe to construct soil-cement walls through the loose site soils, keying the walls into competent ground at depth.

An innovative design was developed that resists large lateral ground movements with a cellular structure of strengthened soil. The in situ structure, which acts as the building foundation, provides adequate shear resistance and confines adjacent liquefiable soils within its cells.

The soil-cement structure provides vertical foundation support by transferring the load of the building foundations to the underlying competent ground (till or bedrock).

In addition to providing the geotechnical solutions, the CSM approach allowed the existing soils to remain on the site rather than having to be removed and disposed of. Further, the reduced permeability of the soil-cement structure after CSM treatment was able to contain the existing contaminated groundwater on the site.

Over 2,000 tonnes of cement were used to build the cellular foundation structure, which is 150 metres long, 40 metres wide, and of varying depths. Cast on-site wet samples and drilled core samples were tested in concrete laboratories. The results indicated that the treated soil had a strength of 1.5 MPa, meeting specifications.

Goals achieved

The cutter soil mixing solution addressed all the on-site issues at a total cost of $3 million, substantially less than the other options. The project was successfully completed on budget and on schedule in August 2006.

The project represents the first North American application of this type of deep soil mixing technology, and possibly the first application of this design approach for a commercial application in an urban environment anywhere.

The cutter soil mixing technology has subsequently been used to address seismic and settlement concerns on another site in B.C.’s Lower Mainland and there are currently other projects in a proposal phase, including mine tailings enclosures.

A unique aspect of the project was Golder and GAIA’s ability to provide all the necessary expertise in-house, from environmental investigations, to presenting the design-build option, engineering analysis and design, and finally construction. CCE

Name of project: Vancouver Island Conference Centre Deep Soil Mixing — Ground Stabilization Foundation

Award-winning firms: Golder Associates, Burnaby, B.C. (prime geotechnical foundation consultant) (Herb Hawson, P.Eng., Randy Williams, P.Eng., Dawn Flotten, P.Eng., Robert McLenehan, P.Eng.)

Golder Associates Innovative Applications (GAIA), Burnaby, B.C. (prime foundation contractor) (Brian Wilson, P.Eng., John Scholte, P.Eng., Megan Atkinson, EIT, Bruce Lawson)

Owner/client: City of Nanaimo

Other key players: Read Jones Christoffersen Consulting Engineers (structural engineer); Rapid Impact Contractors (ground improvement)