Numerical Analysis of Geosynthetic-Encased Stone Columns under Laterally Loads
Out of all methods for ground improvement, stone column became more popular these days due to its simple construction and economic consideration. Installation of stone column especially in loose fine graded soil causes increasing in load bearing capacity and settlement reduction. Encased granular stone columns (EGCs) are commonly subjected to vertical load. However, they may also be subjected to significant amount of shear loading. In this study, three-dimensional finite element (FE) analyses were conducted to estimate the shear load capacity of EGCs in sandy soil. Two types of different cases, stone column and geosynthetic encased stone column were studied at different normal pressures varying from 15 kPa to 75 kPa. Also, the effect of diameter in two cases was considered. A close agreement between the experimental and numerical curves of shear stress horizontal displacement trend line is observed. The obtained result showed that, by increasing the normal pressure and diameter of stone column, higher shear strength is mobilized by soil; however, in the case of encased stone column, increasing the diameter had more dominated effect in mobilized shear strength. Keywords—Ordinary stone column, validation, encased stone column, laterally load.
Numerical simulations of stone column installation
This paper describes the results of numerical simulations investigating the installation effects of stone columns in a natural soft clay. The geometry of the problem is simplified to axial symmetry, considering the installation of one column only. Stone column installation is modelled as an undrained expansion of a cylindrical cavity. The excess pore pressures generated in this process are subsequently assumed to dissipate towards the permeable column. The process is simulated using a finite element code that allows for large displacements. The properties of the soft clay correspond to Bothkennar clay, modelled using S-CLAY1 and S-CLAY1S, which are Cam clay-type models that account for anisotropy and destructuration. Stone column installation alters the surrounding soil. The expansion of the cavity generates excess pore pressures, increases the horizontal stresses of the soil, and most importantly modifies the soil structure. The numerical simulations performed allow quantitative assessment of the post-installation value of the lateral earth pressure coefficient and the changes in soil structure caused by column installation. These effects and their influence on stone column design are discussed.
Installation effects of vibro replacement stone columns in soft clay
The installation of vibro replacement stone columns in any soil causes significant changes to the stress regime around each probe location. In soft clays, the principal effects are caused by a cavity expan- sion mechanism as the probe is introduced into the soil and the stone column constructed. This paper presents observations of changes in soil volume and pore water pressure during stone column installation. The implica- tions for the most suitable column construction techniques and the soil strength limiting column construction arising from the observations are discussed. The data presented are also of interest to those validating numerical models of stone column installation, and issues relevant to the practical application of numerical modeling are considered. This paper considers the observed effects of both sin- gle and multiple stone column installation in soft clay and demonstrates that successful ground improvement can be achieved with this technique. A detailed expla- nation of the various Vibro Replacement and Compac- tion techniques available is provided by Sonderman & Wehr (2004) and is not repeated here. This paper con- centrates on stone columns installed using the dry bottom feed method, which is the most widely used technique in the UK for routine projects on soft clay sites, often preferred nowadays to the wet method. A significant advantage of the dry bottom feed method over the wet method is that a supply of water flush is not required with the attendant requirement of handling and disposal of the wet spoil. Numerous case studies illustrating the use of the dry bottom feed method for a wide range of projects have been published, for exam- ple, Cooper & Rose (1999), Wehr (2006), McNeill (2007), Castro (2007) and Egan (2008). Three aspects of the installation process are con- sidered in this paper: (a) the ground response during the installation of a dry bottom feed stone column, (b) changes to the soil stress field and pore water pressure during the equalization period following stone column construction and (c) the interaction effects resulting from the construction of both small groups and large grids of columns.