Encased Stone Columns Research Articles

Uniaxial compression behavior of geotextile encased stone columns

The bearing capacity and failure mechanism of encased stone columns are affected by many factors such as encasement length, relative density, strength and stiffness of the encasement material. In soft soils where surrounding soil pressure is low, especially in the top section, the stone columns may be close to a uniaxial compression state, where the uniaxial compression strength controls the bearing capacity of the stone columns. A series of large-scale triaxial tests on ordinary stone columns and uniaxial tests on geotextile encased stone columns have been performed. The stone columns were 300 mm in diameter and 600 mm in height. Samples of four different relative densities, and five types of geotextiles were used in the tests to study the effect of initial void ratio and encasing materials on the uniaxial compression behavior of the stone columns. The results show the uniaxial compressive strength of the encased stone columns is not affected by the initial void ratio but mainly by the tensile strength of the encasing geotextiles. The stress strain curves of the encased stone columns under uniaxial loading condition are nearly liner before failure, which is similar to the tensile behavior of the geotextiles.

Numerical Simulation of the Response of Geosynthetic Encased Stone Columns Under Oil Storage Tank

Geosynthetic encased stone columns (GESCs) are a newly developed technique in which stone columns are wrapped with geosynthetic to overcome some of the limitations of ordinary stone columns (OSCs) through the additional confinement provided by the geosynthetic. This paper presents the behavior of GESCs under circular oil storage tank and its comparison with OSCs under the same in situ conditions using PLAXIS 3D. In this paper, initial studies were carried out to understand the mechanism of load carrying capacity of soils reinforced with stone columns and the later observations from the parametric studies supported the conclusions. The various parameters considered in this investigation include the effect of encasement stiffness and length on settlement and lateral deformation of stone columns. The results show that with an increase in stiffness value, there is a considerable reduction in the long-term settlement and lateral deformation of GESCs. It was found that settlement reduced by up to 55% and lateral deformation by up to 68% with an increase in geosynthetic stiffness from 1000 to 10,000 kN/m. Meanwhile the encasement length up to six times the diameter was found as the optimum encasement length to get the same performance as that of fully encased stone columns. Further a suitable arrangement of encased stone columns in terms of encasement length has been developed to economize the consumption of geosynthetic without compromising the performance of GESCs.

Behavior of foundation soil improved by stone column under cyclic load

This paper deals with using the stone column as a technique for the enhancement of the soft ground. The key goal of utilizing stone column is to decrease settlement and to increment the soil bearing ability, as well as decreasing the consolidation period. Nowadays, the current method concerns with various kinds of soil granular and cohesive. It is clear that the delicate soils (cohesive) possess a good settlement because of the disability of the ground to control the sidelong development and protruding of the stone sections. Moreover, the ways of utilization of the geosynthetic materials for encasement of the stone sections are other perfect ways to enhance the implementation, the quality, and firmness of stone segments. The present work investigates the behavior of the soft soil reinforced with ordinary and encased stone columns with geogrid under cyclic load. Six model tests were carried out on a soil with shear strength of about 15 kPa for both ordinary stone columns (OSC) and geogrid encased stone columns (ESC). For validating the enhanced method of utilizing stone columns, finite element model using the software PLAXIS 3D and field load exams had been applied. It was concluded that the models subjected to cyclic loading under the rate of loading 10 mm/sec reached the failure level faster than models tested under the rate of loading 5 mm/sec. The results of the finite element analyses of settlement compared with the records of settlement after the laboratory load tests seem to yield reasonably comparable values up to 50% of the design load. Afterwards, the recorded settlements show up to 60% higher values in compare with the results of the finite element analyses. This observation can be attributed to the occurrence of plastic failures under increasing load after an initial elastic response.

Evaluation of monotonic and cyclic behaviour of geotextile encased stone columns

Stone column installation is a convenient method for improvement of soft ground. In very soft clays, in order to increase the lateral confinement of the stone columns, encasing the columns with high stiffness and creep resistant geosynthetics has proved to be a successful solution. This paper presents the results of three dimensional finite element analyses for evaluating improvement in behaviour of ordinary stone columns (OSCs) installed in soft clay by geotextile encasement under monotonic and cyclic loading by a comprehensive parametric study. The parameters include length and stiffness of encasement, types of stone columns (floating and end bearing), frictional angle and elastic modulus of stone column\'s materials in comparison with OSCs. Also, encasing at the top portion of stone column up to triple the diameter of column is found to be adequate in improving its residual settlement and at all loading cycles, end bearing columns provide much higher resistance than floating columns.

Seismic behavior of geosynthetic encased columns and ordinary stone columns

This study is concerned with evaluating and comparing the behavior of geosynthetic encased stone columns (GECs) and ordinary (conventional) stone columns (OSCs) during and after seismic excitations. For this purpose, well instrumented GECs and OSCs are installed in kaolinite clay beds consolidated in a large steel tank. In order to simulate the seismic behavior of columns supporting an embankment, surcharge loads are applied and the experimental setup is subjected to large-scale shaking table tests. The strains in the encasement are measured by making use of water-proof strain gauges during the course of the experiments. The vertical load capacities of GECs and OSCs after the seismic excitation were measured by a series of stress controlled column load tests. The experimental data at hand suggests that under the action of seismic loads there is a significant strain demand on the encasement confining the GECs. An almost linear relationship between the seismic energy input expressed in terms of IA (Arias Intensity) and reinforcement strain amplitude is observed. GECs in general have exhibited a superior performance both under static and seismic loads when compared to OSCs.

Bearing capacity of geogrid reinforced sand over encased stone columns in soft clay

Stone columns develop their load carrying capacity from the circumferential confinement provided by the surrounding soils. In very soft soils, the circumferential confinement offered by the surrounding soft soil may not be sufficient to develop the required load carrying capacity. Hence a vertical confinement would yield a better result. The load carrying capacity is further increased with the addition of a sand bed over the stone columns. In the present study, a series of laboratory model tests on an unreinforced sand bed (USB) and a geogrid-reinforced sand bed (GRSB) placed over a group of vertically encased stone columns (VESC) floating in soft clay and their numerical simulations were conducted. Three-dimensional numerical simulations were performed using a finite element package ABAQUS 6.12. In the finite element analysis, geogrid and geotextile were modeled as an elasto-plastic material. As compared to unreinforced clay bed, an 8.45 fold increase in bearing capacity was observed with the provision of a GRSB over VESC. The optimum thickness of USB and GRSB was found to be 0.2 times and 0.15 times the diameter of the footing. A considerable decrease in bulging of columns was also noticed with the provision of a GRSB over VESC. Both the improvement factor and stress concentration ratio of VESC with GRSB showed an increasing trend with an increase in the settlement. It was observed that the optimum length of stone columns and the optimum depth of encasement of the group of floating VESC with GRSB are 6 times and about 3 times the diameter of the column respectively.

Fully coupled solution for the consolidation of poroelastic soil around geosynthetic encased stone columns

The paper presents an extension of a recently developed fully coupled elastoplastic method (Pulko and Logar, 2016) for the analysis of a poroelastic thick-walled soil cylinder around an elastoplastic end-bearing stone column to account for the influence of an elastic geosynthetic encasement. The method was developed in the framework of Biot's consolidation theory (Biot, 1941) and is based on a unit cell concept, wherein the column encasement is modeled as a thin elastic membrane, which can only sustain tension and acts in the radial direction. Analytical closed-form expressions for excess pore pressures, stresses, strains, displacements and encasement forces were derived in the Laplace domain. The final elastoplastic solution in time domain was obtained numerically by using efficient numerical scheme for the inverse Laplace transform. The validity of the solution was checked against finite element analyses and compared with previously developed analytical methods. The results showing the influence of column encasement on transient state of settlements, strains, excess pore pressures and encasement forces under instantaneous or time dependent load are presented and discussed.

Bearing capacity of footing supported by geogrid encased stone columns on soft soil

The stone columns are increasingly being used as a soil improvement method for supporting a wide variety of structures (such as road embankment, buildings, storage tanks etc.) especially built on soft soil. Soil improvement by the stone column method overcomes the settlement problem and low stability. Nevertheless, stone column in very soft soils may not be functional due to insufficient lateral confinement. The required lateral confinement can be overcome by encasing the stone column with a suitable geosynthetic. Encasement of stone columns with geogrid is one of the ideal forms of improving the performance of stone columns. This paper presents the results of a series of experimental tests and numerical analysis to investigate the behavior of stone columns with and without geogrid encasement in soft clay deposits. A total of six small scale laboratory tests were carried out using circular footing with diameters of 0.05 m and 0.1 m. In addition, a well-known available software program called PLAXIS was used to numerical analysis, which was validated by the experimental tests. After good validation, detailed of parametric studies were performed. Different parameters such as bearing capacity of stone columns with and without geogrid encasement, stiffness of geogrid encasement, depth of encasement from ground level, diameter of stone columns, internal friction angle of crushed stone and lateral bulging of stone columns were analyzed. As a result of this study, stone column method can be used in the improvement of soft ground and clear development in the bearing capacity of the stone column occurs due to geogrid encasement. Moreover, the bearing capacity is effected from the diameter of the stone column, the angle of internal friction, rigidity of the encasement, and depth of encasement. Lateral bulging is minimized by geogrid encasement and effected from geogrid rigidity, depth of encasement and diameter of the stone column.

Groups of encased stone columns: Influence of column length and arrangement

This paper presents a set of systematic 2D and 3D finite element analyses that study the performance of groups of encased stone columns beneath a rigid footing. Those numerical analyses show that, if the area replacement ratio, i.e. area of the columns over area of the footing, and the ratio of encasement stiffness to column diameter are kept constant, the column arrangement (both number of columns and column position) has a small influence on the settlement reduction achieved with the treatment. For high encasement stiffnesses, placing the column near the footing edges may be slightly more beneficial reducing the settlement; on the contrary, the maximum hoop force at the encasement is notably higher. Based on the minor influence of column arrangement, this paper proposes a new simplified approach to study groups of encased stone columns, which involves converting all the columns of the group beneath the footing in just one central column with an equivalent area and encasement stiffness. This simplified model is used to conclude that, for settlement reduction and fully encased columns in a homogeneous soil, there is a column critical length of around two or three times the footing width. The critical length of the encasement for partially encased columns is slightly lower than that of the fully encased columns.

Time-dependent behaviour of fully and partially penetrated geosynthetic encased stone columns

This paper presents the results of a numerical investigation performed to examine the time-dependent behaviour of fully and partially penetrated geosynthetic encased stone columns (GESC). The investigation studied the influence of geosynthetic encasement, the stiffness of the encasement and the length of the column on the deformation behaviour of GESC. The influence of these factors is compared and rated in terms of the efficiency of the system. Results indicate a greater reduction in the settlement, lateral displacement and time taken for dissipation of excess pore water pressure when the stone column was encased with a geosynthetic material. The effective stress concentration ratio (the ratio of the effective vertical stress on the column to that on surrounding soft clay, ESCR) of the stone column was observed to increase up to a particular consolidation period, beyond which this ratio decreases drastically. Interestingly, when the stone column was encased with a geosynthetic material, the ESCR was found to increase during the entire period of consolidation and the value was much higher than for an ordinary stone column (OSC). A higher value of ESCR indicates less load being transferred from the column to the surrounding soft clay layer, hence better performance. With an increase in the geosynthetic stiffness from 500 to 5000 kN/m, the efficiency of GESC over OSC, considering load transfer from the column to the surrounding clay, increases from 1.03 to 4.63. For partially penetrated GESC, a reduction in the length of GESC decreases the ESCR drastically.

Load transfer and arching analysis in reinforced embankment

In a previous study, a series of 9 model tests performed on embankment models resting on soft clay treated with and without ordinary stone columns (OSC), encased stone columns (ESC), and with horizontal layer of geogrid at interface and at 0·1 h height has been carried out to investigate the behaviour of piled (stone columned) and geogrid reinforced embankments. In this paper, these tests are simulated numerically by the finite element method using the program Geostudio (SIGMA/W). Firstly, reanalysis was done of experimental work of embankment models constructed on soft clay treated with OSC and ESC. Secondly, embankment models reinforced by a horizontal layer resting on soft clay strengthened with OSC and ESC were analyzed. A horizontal layer of geogrid is embedded at different levels of embankment height. It was concluded that in the embankment models constructed on soft clay treated with stone columns at a spacing s = 2·5 d with length to diameter ratio L/d ratio = 8, the maximum bearing improvement ratio (qt/qunt) equals 1·33 for the ordinary stone columns and 1·59 for the encased stone columns and about 1·13 and 1·23 when one layer of geogrid is embedded at interface or at 0·1 h height, respectively.

BEARING CAPACITY and BULGING BEHAVIOR of GEOGRID ENCASED STONE COLUMNS

In recent years, geogrid encasement has been used to extend the utilization of stone columns to improve soft soil’s properties. This paper investigates the influence of geogrid encasement on the performance of stone columns in soft clay deposits. Experimental and numerical studies were conducted to figure out behavior of stone columns with and without geogrid reinforcement. The small scale laboratory tests were carried out. In addition, PLAXIS which is a finite element package intended for the two dimensional analysis of deformation and stability in geotechnical engineering was used to numerical studies. Before conducting the numerical studies, the validation between results of the experimental studies and numerical analysis was performed. After validation, the numerical studies were performed with different parameters such as the rigidity effect of geogrid, depth of geogrid reinforcement and lateral bulging of the stone columns. In addition, group of stone column was investigated. As a result of this studies, load carrying capacity and bulging behavior of the geogrid encased stone column was significantly influenced from the depth and the stiffness of the geogrid.

Effects of geogrid encasement on lateral and vertical deformations of stone columns in model tests

Geosynthetic encasement of individual stone columns can provide additional confinement to the columns thus increasing their load capacities and reducing lateral and vertical deformations. Most of past studies have been focused on the load capacities and settlements (i.e. vertical deformations) of the encased stone columns. However, the load transfer mechanism and the lateral bulging deformation pattern of the encased stone columns are not thoroughly understood. In the present study, four series of laboratory model tests in a large-scale testing tank were performed to investigate the effect of geogrid encasement on the lateral and vertical deformations of stone columns installed in a clay bed. For comparison purposes, ordinary stone columns were also tested and evaluated. The main objective of this research is to investigate the lateral and vertical deformation patterns of the encased stone columns and the reinforcement mechanisms of the geogrid encasement with different encasement lengths. In addition, the stress–strain characteristics of the encasement were measured and analysed. The test results show that the ultimate load capacity of the soft soil was greatly increased by the geogrid-encased stone columns. The effective length of the encasement was three to four times of the diameter of stone columns based on the consideration of performance and economy. In comparison with the analytical solution based on the unit cell concept with full encasement of columns, the experimental tests on composite foundations with partially encased columns, which allowed lateral deformations of columns and soils and slippage along the column-soil interfaces (geogrid–soil, stone column–geogrid, and stone column–soil if the column is not encased), resulted in larger settlements, especially at higher vertical pressures.

Планирование крупномасштабного моделирования фундаментов на основании, улучшенном грунтовымит сваями в оболочке из геосинтетических материалов

The article provides information about improvement of the weak clay subgrade using geotextile encased stone columns. The article also presents overview of the technology, the state of knowledge on this topic. Article gives materials about relevance of the study of this technology for applying in Perm region which is characterized by the presence of weak bases represented by water-saturated clay soils. The overview and analysis of existing experimental studies of various scholars on the subject, which allows to identifying the area of the current study which aim is applying this technology to improve the weak bases of the foundations of buildings and structures, are presented. Also it considers the problem of planning of the experimental studies of geotextile encased stone columns in the laboratory conditions. The paper presents developed experimental schemes. Conclusions on the relevance and novelty are given. The objectives of the planned experimental studies are made.

Experimental Analysis of Embankment on Ordinary and Encased Stone Columns

AbstractThis work investigated the behavior of embankment models resting on soft soil reinforced with ordinary and encased stone columns (ESCs). Model tests were performed with different spacing distances between stone columns and two length-to-diameter ratios (L/d) of the stone columns, in addition to different embankment heights. A total of 39 model tests were performed on soil with an undrained shear strength of ≈10 kPa. The system consisted of a stone column–supported embankment at different spacing-to-diameter ratios (s/d) of stone columns. Earth pressure cells were used to measure directly the vertical stress on the column for all models, and another cell was placed at the base of the embankment between two columns to measure directly the vertical stress in reinforced soft soil. For embankment models constructed on soft clay reinforced with ESCs, it was found that whether a column was floating or end bearing (resting on a rigid stratum), encasement of the column by a geogrid was most effective in im...

Treatment of Iraqi collapsible soil using encased stone columns

This study examined the load-settlement characteristics and mechanism of failure of a footing-type foundation resting on untreated soil, soil treated with an ordinary stone column (OSC), and soil treated with an encased stone column (ESC) into artificial loess deposits subjected to inundation. The investigation was carried out by means of conventional geotechnical laboratory work and the electrical resistivity tomography method. In addition, an analytical solution using a MATLAB script was presented to determine the load carrying capacity of the reinforced foundation and to validate the experimental results. Outcomes of the physical model demonstrated the efficiency of using an encased stone column over the ordinary stone column and the untreated collapsible soil. Results of the resistivity tomography system offered a valuable window into the soil-column interface. The soil conductivity was very sensitive to load and moisture variations during collapse. The analytical model illustrated that the enhancement in the ultimate bearing capacity of the reinforced column is influenced in proportion to variations in both the degree of saturation and matric suction. It increases with increasing the tensile strength of the geotextile and the angle of internal shear resistance of the fill material. However, it decreases when the diameter of the column expands.

Developing a Performance Criteria for Stone Columns to Improve Surface Pavement for Weak Subgrade Conditions

Soft, saturated, fine grained subgrade soils are distinguished by their low undrianed shear strength and high compressibility. Such soils cover most of the middle and southern parts of Iraq. The effect of using stone column, encased in geogrid and steel mesh to improve pavement's performance is experimentally investigated and evaluated. To compare the experimental and analytical outputs, three dimensions finite element throughout elastic and elasto-plastic models using ABAQUS ver.6.12.3 software are developed to simulate and analyze the relations between the cycling load and deformation of the suggested pavement modes. Based on the results and the limitation of this study it is concluded that, using encased stone columns, is more practical and suitable alternative to improve weak subgrade against permanent deformation as compared with the other simulated pavement modes. The results of ABAQUS program are very close to results of laboratory tests.

Experimental and Numerical Studies of Geotextile Encased Stone Columns in Geological Conditions of Perm Region of Russia

The paper provides information about improvement of the weak clay soil with geotextile encased stone columns. The paper presents materials about the relevance of this technology for applying in Perm region of Russia. The article presents an overview and analysis of existing experimental studies on the subject, which allows identifying the area of the current study which aim is applying this technology to improve the weak bases of foundations of buildings and structures in geological condition of Perm region. Also paper considers the problem of estimating of deformations of improves soil base. Experimental studies consist of tests with use of triaxle equipment and stamp tests of big-scale models of geotextile encased stone columns. The conclusions about the effectiveness of geotextile encased stone columns were made on base of the results of experimental studies and analysis of existing materials. Also paper presents areas of possible cost-effective application of technology and the results of numerical simulation of test cases with using the finite element method.

Experimental and numerical modelling of geosynthetic encased stone columns subjected to shear loading

Experimental and numerical modelling of geosynthetic encased stone columns subjected to shear loading

Soil arching analysis in embankments on soft clays reinforced by stone columns

The present work investigates the behavior of the embankment models resting on soft soil reinforced with ordinary and stone columns encased with geogrid. Model tests were performed with different spacing distances between stone columns and two lengths to diameter ratios (L/d) of the stone columns, in addition to different embankment heights. A total number of 42 model tests were carried out on a soil with undrianed shear strength ≈10 kPa. The models consist of stone columns embankment at s/d equal to 2.5, 3 and 4 with L/d ratio equal 5 and 8. Three embankment heights; 200 mm, 250 mm and 300 mm were tested for both tests of ordinary (OSC) and geogrid encased stone columns (ESC). Three earth pressure cells were used to measure directly the vertical effective stress on column at the top of the middle stone column under the center line of embankment and on the edge stone column for all models while the third cell was placed at the base of embankment between two columns to measure the vertical effective stress in soft soil directly. The performance of stone columns embankments relies upon the ability of the granular embankment material to arch over the „gaps‟ between the stone columns spacing. The results showed that the ratio of the embankment height to the clear spacing between columns (h/s-d) is a key parameter. It is found that (h/s-d)<1.2 and 1.4 for OSC and ESC, respectively; (h is the embankment height, s is the spacing between columns and d is the diameter of stone columns), no effect of arching is pronounced, the settlement at the surface of the embankment is very large, and the stress acting on the subsoil is virtually unmodified from the nominal overburden stress. When (h/s-d)