INFLUENCE OF GEOSYNTHETIC ENCASEMENT ON THE PERFORMANCE OF STONE COLUMN GROUPS IN CLAYEY SOIL

Abstract

Geotechnical engineers have challenges in building due to the presence of soft soils in coastal areas, weak subsurface conditions, and poor fill soils. In order to overcome these challenges, there are several options for enhancing the quality of the soil. Granular stone columns are frequently employed to provide structural support in challenging site circumstances. Nevertheless, the inadequate lateral confinement provided by the poor soil around the stone columns results in their failure when the stone column material is compressed into the neighbouring soil. This weakens the overall strength of the technique being used and results in a reduced ability to handle heavy loads and settle properly. Therefore, the use of encasement for granular columns has been employed to address the aforementioned issue. The current study examines a common soil condition frequently encountered by site and geotechnical engineers, which has a layer of weak, cohesive soil overlying a somewhat more rigid underlying soil layer. These soil profiles have been documented in the literature and are frequently observed in the Indian coastal region, as well as certain areas of the mainland and other countries. The literature extensively examines and documents the application of stone columns in soft cohesive soils to enhance load capacity. However, among the limited number of studies on strengthening cohesive soil using rammed stone columns, the results of this study will make a significant contribution to accurately understanding and confirming the load capacity and failure mode of ordinary end-bearing stone columns when installed in cohesive soil conditions. The current work contributes to the existing body of research by examining the impact of vertically and horizontally enclosing/reinforcing stone columns to reduce bulging failure experienced by ordinary end-bearing stone columns under compressive load in a cohesive soil medium. In the current study, model testing for a single conventional stone column for diameter (D) = 50, 75 and 100mm was conducted. The load-settlement analysis and failure pattern were investigated. The analysis was further carried forward by using vertical as well as horizontal encasement for the stone column. For vertically encased stone column (VESC) four different variations of length of reinforcement (Lr) was used (Lr=L, Lr=0.75L, Lr=0.5L and Lr=0.25L). For horizontally reinforced stone column (HRSC), three variations were employed where in first case horizontal discs were employed at 100mm spacing throughout the length of the column. Secondly, discs were provided only for the top half of the column i.e., from column head to the centre of the column and last it was reinforced only for the ix bottom half of the column i.e., from column’s centre to its end. All the above experiments in the single stone column were done for three different diameters of stone column i.e., D= 50mm, 75mm and 100mm to understand the effect of area replacement ratio. Also, two different types of geotextiles (G1 and G2) were used for each of the experiments explained earlier to understand the effect of stiffness of geotextile material. The tests were also conducted for the stone columns in the group arranged in a triangular and square pattern for varying S/D ratio of 2, 3 and 4. The group analysis was also conducted for three different diameters of the column i.e., 50, 75 and 100mm. Both vertical encasement and horizontal reinforcement by a disc were used to study the encasement effect similar to that done in the analysis of a single stone column. The length of encasement (Lr) was used as Lr=L for VESC and when horizontal discs were employed at 100mm spacing throughout the length of the column for HRSC tests. Also, only G1 type geotextile was used as an encasement material for both VESC and HRSC group analysis. Figure 3.6 represents the variation of both vertical and horizontal reinforcement used for the single stone column analysis. Table 3.4 and 3.5 shows the outline of the various experiments performed for single stone column and stone column in groups respectively. Also, one of the industrial wastes i.e., steel slag is used as the column filler material which can act as a sustainable material and will also address the current environmental concern as the utilisation of steel slag for stabilisation of soil can be an eco-friendly and economical extraction method for getting rid of solid waste. A numerical analysis was done to study the various behavioural characteristics of virgin soft clay bed, when it was installed with ordinary steel slag column (OSSC) and also with encased steel slag column (ESSC). A comparison between was made among all for studying various parameters such as settlement, stress concentration ratio and excess pore water pressure. The model testing findings showed that the load carrying capability was higher when geotextile reinforcement was used compared to conventional columns. Vertical reinforcement provides greater load capacity and reduced settlement compared to horizontal encasement. The shown failure mechanism shows that reinforced stone columns are more resistant to bulging than unreinforced stone columns. The VESC for full-length encasement with G2 type of geotextile for a 100 mm stone column diameter was most desirable among the various tests conducted on single stone column. For group analysis, with increasing S/D ratio, load bearing capacity decreases for both triangular and square arrangement. Model testing was also validated by the help of numerical investigations. In comparison to Ordinary Steel Slag Column (OSSC), the settling of soft clay at conclusion of embankment building phase has x been high of selected configuration and model specifications. By encasing column in a sufficiently stiff geosynthetics material, further settlement decrease can be seen. Underneath the embankment, where soft clay produces the maximum excess pore water pressure (PWPexcess), the amount of PWPexcess diminishes as the embankment slopes.