Case History: Use of Vibro-Replacement Stone Columns for both Liquefaction Mitigation and to Improve Static Settlement-Paper

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The paper describes the use of vibro-replacement stone columns to improve foundation conditions for a two-story office building in San Diego. The site had variable geology: relatively competent Mission Valley Formation in the south and loose, compressible alluvial/colluvial soils in the north. Some northern layers were susceptible to liquefaction during the design earthquake, and the site also had potential for static settlement and minor lateral spreading.<br /><br />Stone columns were selected as a cost-effective alternative to deep foundations, enabling the building to use spread footings across the site. Design analyses evaluated both seismic settlement and static settlement. Predicted seismic performance indicated that treatment depths of about 11 feet would meet the differential seismic settlement criterion; however, static settlement governed the final design. Columns were installed to varying depths of approximately 18 to 30 feet, adjusted to footing loads and local ground conditions.<br /><br />Columns were constructed using a bottom-feed vibro system and #57 crushed rock. Construction quality control recorded installation depth, amperage, and stone volume to assess column dimensions and replacement ratio. A field modulus (plate load) test on an early production column exceeded the design modulus, supporting the assumed bearing performance and static settlement calculations. Post-treatment CPTs were not performed; the seismic assessment relied on pre-treatment investigations, predicted improvement, conservative soil assumptions, and the achieved replacement ratio.<br /><br />The project demonstrates how stone columns can address liquefaction-related differential settlement and improve static foundation performance while accommodating variable site conditions. The completed treatment supported the use of spread footings and met the project’s settlement, safety, and cost objectives.

Keywords

vibro-replacement

stone columns

foundation improvement

liquefaction mitigation

differential settlement

static settlement

spread footings

San Diego geology

bottom-feed vibro system

construction quality control