Ground Improvement for Foundation Support in Organic Soils

Twenty years ago, most buildings in the United States were supported by either shallow spread footings or deep foundations. Since then, “intermediate” ground improvement options have increased in use, providing building owners, contractors, and engineers with a host of options each with its own unique design and construction characteristics. Loose sands are now often densified, and soft cohesive soils are reinforced with displacement or replacement aggregate piers prior to shallow foundation construction. For heavier loads and softer soils, the subsurface materials are often now reinforced with “rigid inclusions”, typically consisting of cementitious columnar elements installed by either driven-displacement or drilling methods. Organic soils, such as those typically found in New England, the Upper Midwest, and the Pacific Northwest almost always require “improvement” prior to shallow foundation construction because of their inherent compressibility under even small loads. In certain conditions, the organic materials can be reinforced by non-cemented aggregate piers, provided care is used in the appropriate selection of pier stresses to avoid the pier “bulging” and long-term settlement performance issues. In other conditions, such as when the thickness of the organic soils is large, when the loads are heavy, or when the consequences of settlement are severe, more robust solutions, such as those provided by rigid inclusions are necessary. This paper provides the results of multiple case histories in which organic soils have been reinforced by both confined aggregate and cemented ground improvement elements.