Behaviour of Single and Group of Geotextile Encased Stone Columns in Single-Layered and Layered Soil

Stone columns (SC) have become a widely-utilized technique of enhancing the bearing capacity of soft soils. Although the SCs in single-layered soil have been studied extensively, SCs constructed in a ground consisting of varying soil layers are not fully understood. In this study, to investigate the behavior of single and group of SCs in different soil layering systems, small scale laboratory tests were conducted on non-encased (NEC) and encased (EC) floating and end bearing SCs installed in single-layered and layered soil consisting of loose sand overlying the soft soil. Different area replacement ratios (ARRs), which is the SC area to unit cell area ratio, 1.56% and 6.25% and the length to diameter ratios (L/D) of 5, 7.5, 10 and 15 were selected to evaluate the effect of these factors on the vertical stress-settlement behavior, bearing improvement ratio, subgrade modulus and bulging failure of NEC and EC-SCs in single-layered and layered soils. Test results indicated that bearing capacity of single-layered and layered soils was improved in all cases of single SC applications. In both soil layering systems, the single NEC floating SC (FSC) with smaller area replacement ratio (1.56%) and L/D of 10 gave much better results than all other NEC-SCs applications. The inclusion of geotextile encasement resulted in further improvement of both soil layering systems. The single encased FSC with smaller ARR (1.56%) had superior improvement among all single SC applications. However, in both soil layering systems, in case of single EC end bearing SC (EBSC) with higher ARR of 6.25% and L/D of 7.5, reduction in bearing capacity and subgrade modulus were obtained. For single SC applications, in single-layered soft soil, with smaller ARR of 1.56%, the inclusion of geotextile encasement resulted in a reduction in bulging diameter and increased the bulging depth of FSC which resulted in an increase in SC bearing capacity. While for single SC applications, in singlelayered soft soil, in case of encased floating and end bearing SCs with higher ARR of 6.25 % there was no bulging confronted. The bearing capacity and subgrade modulus of non-encased central column among group of SCs in both soil layering systems, were higher than the other SCs in the group. In addition to these, the bearing capacity and subgrade modulus of soils were significantly improved with reducing the spacing to diameter ratio of SCs. Moreover, the geotextile encasement provided an additional improvement to the SCs. In single-layered soil, non-encased floating central SC with spacing to diameter ratio of 2.5, bulging failure occurred and with decreasing the spacing to diameter ratio to 1.5, the bulging in the central SC was reduced. Whereas, with the geotextile encased floating group of SCs in single-layered soil, with smaller spacing to diameter ratio of 1.5, no bulging failure happened.