Pioneer performed a geotechnical investigation for a proposed 105,000 square-foot warehouse facility. The investigation identified very poor subsoils consisting of urban fill to a depth of 16 feet below existing grade, followed by soft clay to approximately 25 feet. Below this layer, the silty clay exhibited a stiff consistency to a depth of 30 feet and bedrock was encountered at 36 feet below existing grade.
Pioneer’s settlement analysis predicted excessive differential settlements of the proposed foundations and floor slabs if placed on these existing soils. To further complicate site conditions, the surficial soils were environmentally contaminated which made it too costly to use an excavation and replacement strategy to support the proposed floor slabs. Therefore, various deep foundation and soil amendment alternatives were evaluated. The foundation alternatives included deep foundations such as H-piles, pipe piles, auger cast piles, and caissons, as well as soil amendment alternatives, such as Deep Dynamic Compaction (DCC), Geopier® or Rammed Aggregate Piers, and vibro-replacement stone columns. Based on cost analyses, the client selected the installation of vibro-replacement stone columns spaced at 8-foot intervals, extending to approximately 20 feet below design grade, providing an improved allowable bearing pressure of 4,000 psf across the entire building area. A geogrid membrane system was placed across the slab area. This recommended soil amendment technique reduced the calculated differential settlements and minimized the haul-off of contaminated soils during construction.