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Posted 15 February 2012
Stone column ground improvements have become common practice for waterfront structures to provide greater stability for the slopes beneath the wharf or pier structure. For the Port of Tacoma, Washington United Terminals wharf extension project, ground improvements consisting of stone columns on the waterside and landside of the bulkhead extension were installed to stabilize the slope and reduce seismic displacement demands. The stone column installation provides a mechanism for groundwater pore pressures to be relieved and densifies the ground locally.
The preferred practice is to install stone columns before and at some distance from locations where wharf piles or sheet pile walls are driven. This eliminates the potential for pile and wall movement as a result of stone column installation, but may cause reduced driveability in soils already densified by tight stone column grids. Because pile driving and dredging are controlled by annual in-water work windows, the preferred sequencing of stone column and in-water pile installation may not be possible. Driving piles into a dense grid of ground improved with stone columns proved to be a significant concern for the wharf extension project, as the driveability of long precast concrete piles to deep tip elevations within the stone column grid was unknown. However, because of the way the stone columns were spaced and arranged around locations where piles were to be driven, and the sequencing of the dredging work, it was possible to install the stone columns before pile driving and dredging began. As a result, only a limited increase in driving effort was realized due to the careful selection of stone column installation patterns. Also, because the stone columns were installed prior to slope dredging, enabling column installation in the upper reaches of the slope while in the dry, increased slope construction and stone column installation productivity was achieved.
Awarded a silver “Best in State” award for Complexity by the American Council of Engineering Companies of Washington, this project was the first known attempt to drive piles in ground densified by a tight grid of stone columns, demonstrating that piles can be driven after stone column installation without significant increase in driving effort. In addition, the project illustrated that this sequence of work when properly planned will not cause detrimental impacts to pile-driving operations.