Stone columns have been used as an effective technique for improving the engineering behaviour of soft clayey grounds and loose silt deposits. The soil improvement via stone columns are achieved from accelerating the consolidation of weak soil due to shortened drainage path, increasing the load carrying capacity and/or settlement reduction due to inclusion of stronger granular material. This paper discusses the techniques, methods of construction of stone columns, mechanisms of stone column behaviour under load and associated design philosophies along with some practical findings from recent research programs.

A Critical Review of Construction, Analysis and Behaviour of Stone Columns

This paper describes the soil layering effects on response of the stone column and stone column improved ground through a series of small scale laboratory tests and numerical analyses. Two types of layering systems, i.e. soft clay overlying stiff clay and vice versa are considered for the present study. The entire laboratory tests were carried out on 88 mm diameter stone columns installed in a two layered soil systems. Unit cell concept is used to idealize the behaviour of a single column within an infinite group of stone columns. Entire unit cell and only the stone column area were loaded to evaluate the stress versus settlement response of the entire improved ground and that of the stone column. Effects of the top soft and stiff clay layer thickness on the axial stress of the whole improved ground and stone column only are evaluated through laboratory tests. A detailed parametric study using finite element based software Plaxis was also carried out. Elastic-perfectly plastic Mohr–Coulomb failure criterion with drained conditions was used for the soil and stone columns in the numerical analyses. Result shows that the limiting axial stress of the stone column is found to be influenced by the top clay layer thickness up to two times the diameter of the stone column beyond which it remains constant for both the layering systems. The limiting axial stress of the whole improved ground is found to be influenced by the presence of the top layer up to a depth of four times the diameter of the stone column for both the layering systems. The stiffness improvement factor of the improved ground increases with increase in the thickness of the top soft clay layer and attains maximum value for the full depth of soft clay whereas it remains constant for different depth of the top stiff clay. The vertical extent of the bulging increases with increase in the thickness of the top soft clay up to two times the diameter of the stone column for both the layering systems.

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Experimental and Numerical Studies on Response of the Stone Column in Layered Soil

Stone column use has gained popularity around the globe as an effective ground improvement technique to improve the load-bearing capacity of soft soils. End-bearing stone columns are commonly used, but occasionally floating stone columns are adopted since they have been proven to achieve the necessary load-carrying capacity and serviceability requirements. However, stone column parameters are often estimated without much basis for simulation and design purposes due to a lack of practical research and real-life case studies in this area. Hence, in order to develop a fundamental understanding of the load-settlement responses of floating stone columns in soft clays, detailed back-analysis was performed in this study. Throughout the back-analysis, all the numerical models were validated by comparing the analysis results with field measurements. This study highlights the existence of some critical interpreted parameters, namely soil cohesion c′, friction angle ϕ′, and dilation angle ψ′, as well as the soil–structure interaction mechanism that influences the load-settlement responses of the floating stone columns. The successful characterization of the stone column materials and the modeling technique of stone columns in finite element analyses show great potential in assessing the real-life performance of floating stone columns in soft clays.

/pdf/performance-of-field-and-numerical-back-analysis-of-floating-59ynqigk2q.pdf

Performance of Field and Numerical Back-Analysis of Floating Stone Columns in Soft Clay Considering the Influence of Dilatancy

The Stone-column is a useful method for increasing the bearing capacity and reducing settlement of foundation soil. The prediction of accurate ultimate bearing capacity of stone columns is very important in soil improvement techniques.Bulging failure mechanism usually controls the failure mechanism. In this paper, an imaginaryretaining wall is used such that it stretches vertically from the stone column edge. A simple analytical method is introduced for estimation of the ultimate bearing capacity of the stone column using Coulomb lateral earth pressure theory.Presentedmethodneeds conventional Mohr-coloumb shear strength parameters of the stone column material and the native soil for estimation the ultimate bearing capacity of stone column. The validity of the developed method has been verified using finite element method and test data. Parametric studies have been carried out and effects of contributing parameters such as stone column diameter, column spacing, and theinternal friction angle of the stone column material on the ultimate bearing capacity have been investigated.

A simple analytical method for calculation of bearing capacity of stone-column

The application of stone columns increases the stiffness of soft soils which contributes to its load carrying capacity and accelerates the process of consolidation leading to reduction in settlement. However, under external loading, squeezing of adjacent soil into the columns not only compromises the integrity of the columns but also reduces its stiffness, strength and drainage properties. The present study investigates the use of vertically and horizontally reinforced stone columns, as a remedial measure for ordinary unreinforced stone columns. The vertical reinforcement is done by encasing the stone columns in geotextile and horizontally by placing geotextile circular discs within the columns at regular interval. Model tests on group of 3 and 4 unreinforced and reinforced stone columns have been conducted in weak sandy soil. The load–settlement response and failure modes for both reinforced and unreinforced groups have been studied. It is observed that reinforced group of stone columns depict better load bearing capacity as compared to unreinforced group. Moreover, bearing capacity for vertically encased and horizontally reinforced is almost similar, with horizontally reinforced group of 4 stone columns depicting slightly higher (1–2%) bearing capacity for a settlement of 30 mm. The experimental results are also validated with theoretical results and are found to be in good agreement.

Experimental study of ground improvement by using encased stone columns

The unit cell idealization has been long adopted in the settlement prediction of stone column-reinforced soils. This paper tests the accuracy of this modeling concept against trusted settlement values of engineering foundations. It is believed that in order to bestow the outcome of this study adequate generality different soil properties and foundation geometries need to be considered. It was, nevertheless, found impracticable to collect field settlement records for all the analyzed cases. The authors, therefore, appealed to the back analysis concept to construct a reliable mathematical model, calibrated against settlement records of full-scale field load test. This model, which is capable of reproducing the real field settlements, is then employed as a generic tool to obtain trusted settlement values for a variety of cases with essential geometrical similarity. The investigation revealed that the unit cell analysis may, in some cases, lead to erroneous estimation for the settlements of foundations with limited extents. Correction factors, dependent on the treated soil properties as well as the foundation size, are introduced.

Reliability of Conventional Settlement Evaluation for Circular Foundations on Stone Columns

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Reliability of Conventional Settlement Evaluation for Circular Foundations on Stone Columns