Stability Analysis of Geotextile Encased Sand Columns
Kadhim, Shaymaa Tareq
University of Kansas
Civil, Environmental & Architectural Engineering
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Global instability or deep-seated failure of roadway embankments constructed on soft soils is a serious concern in the field of geotechnical engineering. Different ground improvement techniques, such as stone columns, have been widely implemented to avoid deep-seated failure. Stone columns derive their bearing capacity from the passive resistance provided by the native surrounding soil, therefore, inclusion of stone columns in very soft soils may not be sufficient to yield the desired level of improvement. As a result, geosynthetic encased stone columns (GESC) have been introduced to improve soft soils with low undrained shear strengths. The objective of this study is to quantify the contribution of GESC to vertical and global stability. In this study, model GESC with geotextile sleeves with three different diameters: 10, 15, and 30 cm, were tested as part of the experimental program. Kansas River Sand of 70% relative density was used as the infill material of GESC. CD triaxial compression tests were conducted on both ordinary sand and geotextile encased sand columns. The results showed that using geotextile encasement increased the strength of column by providing an apparent cohesion and increasing the friction angle beyond the peak friction angle of the ordinary sand column. The vertical stability of GESC was investigated through a series of loading tests. The loading tests were conducted on columns having various diameters and lengths, installed both in air and in very weak surrounding soil (i.e. loose sand with 30% relative density). The performance of GESC in air and with loose sand surrounding soil was studied with regard to bearing capacity, radial strain, and axial strain relationships. The results of both cases showed that columns of smaller diameters and shorter lengths exhibited higher bearing capacities compared with those of larger diameters and longer lengths. GESC with surrounding loose sand exhibited lower radial and axial strains compared with those in air at the same applied pressure. In addition, GESC with soil confinement had higher bearing capacities than those in air at the same diameters and length to diameter ratios. The experimental findings were verified using the finite difference method within the software program FLAC3D 5.01. The numerical results matched well with the experimental data. A parametric study was conducted to assess the factors that may have an impact on the performance of GESC, such as column diameter and length, soil thickness, geotextile encasement length, geotextile stiffness, and friction angle of infill material. The results showed that increasing the size and length of end-bearing GESC reduced its bearing capacity and increased its lateral deformation, while shorter, partially penetrating GESC had lower bearing capacities as compared with longer ones. The effective geotextile encasement length was found to be approximately five times the column diameter. Geotextile stiffness had a substantial influence on the performance of GESC, and the friction angle of infill sand had a less significant effect on the behavior of the GESC. Finally, a two dimensional finite difference method using FLAC2D 6.0 was used to investigate the effect of ordinary stone columns and GESC on the short-term stability of an embankment constructed over soft soil. Two different models were adopted in this study: column walls and an equivalent improved area. A parametric study was conducted by varying some parameters such as the spacing and size of stone columns, cohesion of the soil deposit, and stiffness of the geosynthetic. The results showed that the equivalent area method yielded higher factors of safety than the column wall method. The stability factor of safety decreased when the center-to-center spacing between columns was increased, and increased when the soil cohesion was less than 25 kPa. Increasing the stiffness of geosynthetic encasement up to 2000 kN/m significantly increased the stability factor of safety.
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