Performance of High Confined Stone Columns in Pond Ash Fills: an experimental and numerical study

Abstract

Vertical encased stone columns are a ground modification technique that improves the bearing capacity and reduces the total displacement of soft and loose cohesionless soils, which gain strength through the skin friction of surrounding soils and end-bearing of the column. However, these stone columns (SCs) are limited in their ability to carry vertical and lateral loads due to the low radial pressure offered by the adjoining soft or cohesionless soil. The SCs experience bulging due to the squeezing of column aggregates. The present study investigates the performance of highly confined stone columns, which are encased with vertical and horizontal reinforcement of the aggregates, pervious concrete columns with under-reamed single bulbs constructed in loose cohesionless soil. The performance of these highly confined columns was studied under varying conditions such as tensile strength of the encased material (geosynthetics), pervious concrete without and with under-reamed single bulbs, slope stability, group action, and piled-raft on soil improved with the combined effect of ordinary and encased columns. A physical examination was conducted in the laboratory using a model tank, model footing, a load cell, and dial gauges for load and deformation measurements. The materials used were pond ash as loose cohesionless soils, recycled concrete aggregates as column fill material, geotextile for vertical encasement, geogrid discs for aggregate reinforcement, and pervious concrete with no fines. The preliminary geotechnical parameters of all the materials were studied to achieve the objectives. Long stone columns (SCs) with a diameter of 80mm and a length of 500mm were constructed in the model setup, which was filled with a pond ash bed. The various types of SCs included ordinary stone columns (OSCs), aggregates reinforced with horizontal geogrid layers (HGL), vertical geotextile encased stone columns (GTX), and high confined columns (HC=GTX+HGL). The studied parameters included applied pressure vs. total settlement, stiffness improvement factor, and modulus of reaction, among others. The load carrying ratio of HC is 3.19 times higher than untreated pond ash fills. To understand the performance of high confinement columns, pervious concrete was prepared without fine aggregates (zero sand) to maintain the designed permeability and void content that may have equivalent hydraulic properties of stone columns. The mix proportion used had a water to cement ratio of 0.3 and a binder to aggregate ratio of 5. Cement binder was used to confine the aggregates that may have equivalent or less binding than the HC.Two types of columns were constructed in the model tank, including circular and under-reamed pervious concrete columns (CPCC & UPCC with single bulb), each tested after 3 and 7 days of curing period. The experimental results showed that the load-bearing capacity increased in the order of OSC<GTX<HC<CPCC<UPCC. The total deformation of pond ash fill treated with PCC was controlled to 61% in comparison to straight shaft PCC. For end-bearing CPCC and UPCC, failure was characterized by lateral column deformation occurring at a distance of 4D from the ground due to the non-uniform lateral soil movement governed by the cavity expansion developed during displacement installation. This non-uniform lateral soil movement is negligible beyond 5D. Theoretical equations were developed for CPCC and UPCC by introducing non-dimensional parameters, including bulb ratio (Br), area replacement ratio (Ar), under-reamed bulb ratio (Br), and column length ratio (RL). These parameters were used to analyze the bearing capacity of loose cohesionless soils and settlement reduction. Load bearing was calculated using the analytical equations, and the results were in close agreement with the measured values. The LCR values increased considerably (up to six times higher) with variations in the area replacement ratio and column length ratio, when the bulb ratio was 2.5. This increase may be due to the area replacement ratio being computed at relatively higher values (12.3%, 16%, and 20%). The numerical analysis was conducted to validate the experimental results using the three-dimensional Plaxis program. All cases were studied with equivalent properties of the materials measured from the experimental work, and wherever scaling was applied, dimensional scale rules were followed for geosynthetic tensile strength and model footing sizes. All results obtained from the FEM were validated. Three-dimensional finite element method (FEM) analysis was further used to investigate the time rate of consolidation in pond ash fills (loose cohesionless soils). The consolidation studies were conducted for OSC, GTX, HC, and CPCC cases to observe the generation and dissipation of excess pore water pressure, degree of consolidation, and settlement underneath the embankment. To understand the effect of CPCCs on the acceleration rate of consolidation, they were installed underneath the embankment in pond ash fills. The pervious concrete column creates a pathway for the expulsion of pore water. It was observed that using CPCC, the maximum excess pore pressure generated was found to be 24kN/m2, and the total time for dissipation was recorded as 25 days for CPCCs. Hence, these pervious concrete columns are effective in accelerating consolidation. However, they are less effective in comparison with the OSC and GTX cases. A three-dimensional numerical analysis was conducted using the Plaxis program for the second parametric study on piled-raft foundations resting on cohesionless soils modified with OSCs and GESCs. The purpose was to understand the performance of heavily loaded superstructures with piled-raft foundations resting on OSCs and GESCs while varying the area replacement ratio. Twenty-seven cases were studied with varying area replacement ratios of 10%, 15%, 20%, 25%, 30%, and 35%. The mobilized shear strength increased by 45% when using encased stone columns compared to ordinary stone columns due to the constraints provided by the encasement. The square arrangement of GESCs of various diameters and piles beneath the raft carried the maximum load among the other combinations, and the load-carrying capacity increased up to 71% when using GESCs compared to raft alone. Piled-raft foundations resting on soils treated with OSCs and GESCs performed better in terms of the percentage of load shared by the piled-raft with OSCs and GESCs, which was up to 40%. Therefore, using piled-raft foundations with OSCs and GESCs effectively reduces total settlement, increases the bearing strength of loose cohesionless soils, and reduces the cost of the foundation system compared to using piles alone.