Small Stone-Column Groups: Mechanisms of Deformation at Serviceability Limit State

Abstract

Stone columns are a popular form of ground improvement that can be applied to a variety of foundation solutions involving soft soil deposits. Many design methods are based on an assumption that columns with a length-to-diameter ratio greater than 4 bulge uniformly along their length. This mode of deformation is not valid for small groups of columns in which interaction between individual columns and the footing gives rise to more complex behavior. The deformational behavior of stone columns at serviceability limit state is of utmost importance, because settlement rather than bearing capacity criteria are generally more onerous in soft soils. The results of some high-quality laboratory studies have been informative in this respect, but the extrapolation of results from small-scale tests comes with the caveats associated with single-gravity laboratory testing. The FEM, in combination with advanced constitutive models, can provide a link between the deformational behavior and settlement performance of discrete column groups at working stress levels. A series of analyses was conducted to examine the influence of key design parameters such as column length, spacing, and the number of columns. The influence of a stiff crust was also examined, because the reference soil profile adopted for these analyses was that of the well-characterized Bothkennar test site. The following three modes of deformation were identified by examining the distribution of stress and strain within columns and the surrounding soil: individual punching, block punching, and bulging. Individual and block punching developed in short or closely spaced columns, whereas block punching was more prevalent in larger groups. Bulging develops in long, widely spaced columns and generally occurs at the point of lowest lateral restraint. In addition, novel parameters referred to as compression and punching ratios were developed to validate the in-depth analysis and identify the principal design parameters that influence the behavior of small groups of stone columns. The results indicate that the mode of deformation is governed by column spacing and length rather than the number of columns. This finding is important, because it indicates that settlement design methods that are developed for large groups of columns are applicable to discrete groups at similar spacings and lengths. It was also observed that the influence of the stiff crust forces bulging to occur deeper and might have led authors in the past to suggest the existence of a critical length.