Stabilization of Soft Soils by Soil Mixing

Abstract

The use of soil mixing for providing stabilization of soft or loose soils is considered a fairly new technology in the United States. Soil mixing has been successfully applied for liquifaction mitigation, steel reinforced retaining walls, groundwater cutoff walls, and stabilization of contaminated soils. Applications of this technology have recently been further expanded. Such applications have included settlement control of soils, slope stabilization and the formation of composite gravity structures. To design for these applications, the unconfined compressive strength, elastic modulus and shear strength of the soil and soil-cement columns must be determined or estimated. Settlement control of soft or loose soils under service loads can be sufficiently controlled with treatment ratios in the 20% to 35% range. On a recent project in Honolulu, Hawaii, loose soils were sufficiently stabilized with a 23% treatment ratio, and at a site in Lakeland, Florida, a very soft and compressible clay layer was sufficiently stabilized with only a 12% treatment ratio. In slope stability applications, soil mixing improves the overall shear strength of the soil formation to adequately increase the factor of safety, and also the soil-cement columns can force the potential failure surface deeper. Lastly, soil mixing has been applied to construct in-situ gravity structures where its composite action design assumption was confirmed with an instrumented test wall, and used in two recent commercial applications. Introduction and History In the United States, soil mixing was first developed by Intrusion-Prepakt, Inc. of Cleveland Ohio in the 1950’s (Liver et al. 1954) as “Intrusion Grout Mixed-in-Place Piles”. In the late 1960’s and early 1970’s the Swedes used a mixed-in-place lime stabilization process (Ryan et al. 1989). In the 1970’s, 1980’s and today, the Japanese and Scandinavians continue to refine the soil mixing technology in various foundation applications. 1 Regional Manager, Geo-Con, Inc., Monroeville, PA, USA, Email: kandromalos@geocon.net 2 Project Engineer, D’Appolonia, Monroeville, PA, USA, Email: yahegazy@dappolonia.com President, Geo-Con, Inc., Monroeville, PA, USA, Email: bjasperse@geocon.net Andromalos, Hegazy and Jasperse 1 Soil mixing was reintroduced into the United States by Geo-Con, Inc. in the late 1980’s. Its applications were for liquifaction mitigation, steel reinforced retaining walls, groundwater cutoff walls, and stabilization of contaminated soils. Soft and loose soils must be either bypassed by deep foundations, removed and replaced, treated for months or years by wick drains and pre-loading, or improved in order to build over them. The use of soil mixing for providing stabilization of soft or loose soils has not yet widely spread in the United States. Applications of this technology have recently been further expanded to include settlement control, slope stabilization and the formation of composite gravity structures in soft, loose and unstable soils.