Stone Column Installation
Cornforth Consultants (CCI) was retained by PGE to provide geotechnical and foundation design services for a new combined-cycle gas power plant on the banks of the Columbia River near Clatskanie, Oregon. Several potential sites were under consideration by PGE and exploratory drillholes revealed highly variable conditions ranging from very soft, organic silts to interbedded, predominantly sandy silts (Columbia River Alluvium). CCI determined that a sandy site nearer the river would be less expensive to develop than the highly compressible silt further inland, which would have required extensive preloading to mitigate potential settlement. In addition, heavier turbine structures would still require deep foundation support.
The sandy site, however, contained potentially liquefiable foundation materials. Several mitigation options were considered including soil-cement columns, blast-induced densification and deep, dynamic compaction. Ultimately, 4,000 stone columns were considered most effective. CCI developed acceleration time histories and performed ground response analyses prior to stone column treatment. Estimates of cyclic shear stress were used to provide a target improvement density for the ground between stone columns. Pre- and post-explorations, cone penetrometer and SPT blowcounts, were performed to assess the success of ground densification. Cost estimates indicate that mitigating the sandy site for liquefaction saved several million dollars over preloading the alternative sites.