Performance Evaluation of Encased Stone Column Supported Embankments With Geosynthetic Materials as Basal Reinforcement

Abstract

Lithomargic clay is extensively found along the Konkan belt in peninsular India and serves as a foundation for most structures. The reduction in strength under saturated conditions makes lithomargic clay problematic, causing many engineering problems such as uneven settlements, erosion, slope failures, and foundation problems. The effect of column configuration (i.e., equivalent number of columns with reduced diameter for the same surface area) on the performance of lithomargic clay reinforced with geogrid encased stone columns, and the basal geogrid layer was studied. The investigations were performed both experimentally through small-scale models and finite element analyses. The results were compared with the performance of lithomargic clay reinforced with ordinary and encased stone columns. A single geogrid encased stone column with a basal geogrid layer improved the load- carrying capacity of lithomargic clay by 180%. In contrast, the percentage increment in a group of three geogrid encased stone columns with a basal geogrid layer having the same surface area was 210%. It was also observed that the geogrid encasement of stone columns reduced the maximum column bulging by 38%. In comparison, geogrid encased stone columns along with basal geogrid layer reduced the bulging by 82% compared to ordinary stone columns. Geocells are a superior form of reinforcement due to their cost-effectiveness and three- dimensional confining properties. Numerical modeling of geocell is always challenging due to its three-dimensional honeycomb structure. The limitations of the equivalent composite approach (ECA) led to the recent development of full 3D numerical models, which consider geocell-infill material interaction. The present work discusses the time- dependent performance of geocell reinforced encased stone column supported embankment considering the actual 3D nature of geocells using the finite element program ABAQUS. Parametric studies were carried out to study the stress transfer mechanism, vertical deformation of the foundation soil, arching behavior, and stress- strain variation inside the geocell pockets. It is found from the analyses that with the provision of a geocell layer on top of Geosynthetic Encased Stone Columns (GESC), the stress concentration ratio improved by 47% at the end of consolidation compared to GESC alone. Also, with geocell-sand mattresses, an 80% reduction in foundation surface settlement is observed. Analysis results showed that the arching behavior is not predominant in geocell reinforced columnar structures. The proposed model's numerical results show an overestimation of stress concentration ratio and bearing capacity by ECA. The geocell-sand mattress reduced the vertical settlement of foundation soil due to the embankment construction by 80%. The vertical settlement reduction was 78% and 79% for single and two-basal geogrids, respectively. The basal geogrids and geocell-sand mattress decreased the bulging of the stone columns, and the maximum bulging was visible at a depth of 3.5 D in both cases, where D is the diameter of stone columns. 69% reduction in the lateral bulging occurred in GESC than the ordinary stone column when a single basal layer was placed. The reduction is 52% and 54% for two basal layers and geocell, respectively. When the geocell-sand mattress was placed, almost 80% of the stone column bulging occurred by the end of the embankment construction. Among the various infill materials analyzed, the aggregates were the best suited considering stress concentration ratio and vertical settlement. The mobilized tensile stress in geocell due to embankment loading was maximum for aggregates and minimum for quarry dust. Multiple layers of geosynthetic can be replaced by a single layer of geocell, considered to be a superior form of reinforcement because of the three-dimensional confinement offered to the infill material. The proposed system from this research work encased stone columns with geocells as basal reinforcement at the embankment base, serves like a Geosynthetic Reinforced Piled Embankment System (GRPES). Time-dependent analyses on encased stone columns supported basal geogrid reinforced embankment were carried out using the developed full 3D model. Compared to an unreinforced embankment, 87% reduction in lateral deformation near the toe was observed for GESC+ One basal layer at the end of consolidation, and 90% reduction in the lateral deformation was obtained when two layers of geogrids (stiffness equivalent to that of a single layer) was provided at the base. In multiple basal geogrids, the tensile force at the top layers was less compared to the bottom layers. The variation of stress reduction ratio with embankment height from different analytical methods and 3D numerical model for the encased stone column with single basal geogrid follows the same trend. Among the different design methods, Guido et al. (1987), Low et al. (1994), and Abusharar et al. (2009) significantly under-predicted the stress reduction ratio. Terzaghi’s method (1943) and BS 8006 (2010) exhibited a closer range of Stress Reduction Ratio values than that from full 3D analysis for low height embankments. These methods over-predicted the tensile force in the basal reinforcement. 3D column analysis gave lesser tensile forces compared to full 3D analyses. Guido et al. (1987) method shows good agreement with full 3D results for basal geogrid tension compared to other methods.