DFI Technical Committee Project on the Effect of Overburden Stresses (During Curing) on the Mechanical Properties of Soil-Cement Mixed Materials

Deep soil mixing (DSM) has become a widely used ground improvement technique globally, producing soil-cementitious materials for various complex civil and environmental applications. In the field, the soil-cement materials form under confining stress conditions, which are often overlooked, as most wet grab specimens are cured and tested under ambient atmospheric conditions. This oversight has led to an underestimation of the engineering and mechanical properties of the soil-cement mixed material. With support from the DFI Technical Committee Project Fund, the authors initiated a multi-phased comprehensive research study in collaboration with industry experts to gain a fundamental understanding of these effects. This study investigates the curing conditions to which in-situ materials are exposed and how these conditions influence the strength of deep mixed materials. The findings reveal that the curing conditions for in-situ materials, including applied stresses and temperatures, significantly differ from those under which typical quality control specimens are cured. Neglecting these conditions can lead to a substantial underestimation of in-situ material strength. The data compiled for this study encompass strength and temperature data obtained from specially designed laboratory bench tests, field-conducted modified oedometer tests, in-situ instrumentation tests within deep mixed columns, and a review of independent coring data from deep mixed projects across North America. Analysis of these collective studies reveals a correlation between curing stress and unconfined compressive strength (UCS). The rate of strength gains follows a linear pattern and is influenced by the soil type being treated. Fine-grain soils, in particular, exhibit a higher rate of strength gain when subjected to in-situ curing stress (with depth) compared to granular soils. The paper delves into fundamental processes such as drainage, consolidation, and interparticle behaviors of deep mixed materials from their fresh state until the treated column has cured. It also presents empirical and analytical methods to correct in-situ strength measurements to account for the effects of curing stresses.