Ordinary Stone Columns Research Articles

Model tests on ordinary and geosynthetic encased stone columns with recycled aggregates as filler material

PurposeSincethe availability of natural aggregates is very sparse, recycled industrial and construction waste provides a sustainable alternative to ground improvement using vibro replacement method. Utilizing recycled building waste caters the requirement for its disposal and offers an effective remedy for the scarcity of natural resources. The aim of this study was to give a sustainable alternative for the natural aggregates as the material for stone column.Materials and methodsA good stone column material should be hard, dense, chemically inert and must comply with the size requirement. The utilization of construction debris and spent railway ballast as column material has been the subject of numerous researches. This work focuses on finding the suitability of railway ballast and concrete debris as alternatives for stone column material. A detailed laboratory testing of these materials has been carried to judge their strength requirements as the material for both Ordinary Stone Columns (OSCs) and Geosynthetic Encased Stone Columns (GESCs). The improvement in capacity of both OSCs and GESCs is evaluated by performing California Bearing Ratio (CBR) test in laboratory by creating unit cell stone column models of different recycled aggregates and comparing their load settlement behavior with natural aggregates.Results and discussionRailway ballast, natural aggregates, concrete debris and virgin soil were found to show decreasing order in CBR test results. Loading required for causing settlement in both OSCs and GESCsshowed remarkable increase as compared to that of virgin clay and the maximum load settlement improvement was observed for railway ballast in both the types of stone columns. The CBR values for GESC made from railway ballast, natural aggregates and concrete debris were 54, 49 and 38% respectively. On the other hand, CBR for OSC made from railway ballast, concrete debris and natural aggregates were found to be 25.5, 20.4 and 24% respectively and CBR of virgin clay was found to be just 11%.ConclusionThe demonstrated application of sustainable sources in place of natural aggregates provides a crucial pathway for utilizing the recycled aggregates as stone column filler material. Up on encasing the OSC with geotextile the performance of stone columns has improved appreciably in terms of load capacity. Railway ballast and concrete debris can be adopted as an alternate for the natural stone column materials to improve the bearing capacity of site consisting mainly of soft clays.

Numerical Analysis of a 3D Unit Cell Model for Soft Soil Reinforced with Different Granular Columns

Recycled aggregates have been increasingly considered in recent years, owing to the limited supply of natural aggregates coupled with the corresponding carbon footprint. Recycled aggregates are aggregates prepared from construction and demolition waste. Their use aims to reduce energy consumption and contributes to reducing waste harmful to the environment. This study is based on a number of numerical tests using the finite element method of PLAXIS 3D software with the elastic-perfectly plastic behavior model and the Mohr flow criterion for all materials. A unit cell model of soft soil treated with three types of granular columns was loaded to failure: ordinary stone columns (OSCs), sand-fiber mix (SFM) and recycled aggregate porous concrete pile (RAPP). An extensive parametric study was conducted to investigate the effect of column type, friction angle, elasticity modulus, column length and geotextile effective stiffness on the behavior of soft soils. The results of numerical tests indicated that the bearing capacity of the recycled aggregate columns is three times greater than that of columns of natural aggregates. The findings of this research are given in the form of load-settlement graphs, which made it possible to release recommendations to carry out works using this technique. KEYWORDS: Soft soil, Granular column, Numerical analysis, Unit cell, Bearing capacity, PLAXIS 3D software

An Experimental Analysis of Embankment on Stone Columns

When embankment is constructed on very soft soil, special construction methods are adopted. One of the techniques is a piled embankment. Piled (stone columns) embankments provide an economic and effective solution to the problem of constructing embankments over soft soils. This method can reduce settlements, construction time and cost. Stone columns provide an effective improvement method for soft soils under light structures such as rail or road embankments. The present work investigates the behavior of the embankment models resting on soft soil reinforced with stone columns. Model tests were performed with different spacing distances between stone columns and two lengths to diameter ratios of the stone columns, in addition to different embankment heights. A total number of 21 model tests were carried out on a soil with undrianed shear strength ≈ 10 kPa. The models consist of stone columns embankment at spacing to diameter ratio equal to 2.5, 3 and 4. Three embankment heights; 200 mm, 250 mm and 300 mm were conducted. 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 reinforced soft soil directly. The embankment models constructed on soft clay treated with ordinary stone columns at spacing ratio equal 2.5 revealed maximum bearing improvement ratio equals (1.21, 1.44 and 1.7) for 200 mm, 250 mm and 300 embankment heights, respectively and maximum settlement improvement ratio equals (0.78, 0.67 and 0.56) for 200 mm, 250 mm and 300 embankment heights, respectively.

Sustainable utilisation of steel slag as granular column for ground improvement in geotechnical projects

Granular column is an attractive mitigation method which is widely used in geotechnical engineering to enhance the bearing capacity, reduce settlement and accelerate consolidation. Utilisation of industrial wastes such as steel slag in soil stabilization can be an environment-friendly and cost-effective extraction technique to the disposal of solid waste materials. Previous studies investigated the bearing capacity and settlement of ground improved with granular columns with or without the geosynthetic encasement. However, very limited number of studies have investigated the response of ordinary stone columns without encasement and geosynthetic encased steel slag columns under lateral loading. In this paper, the lateral load capacity of steel slag granular column-soil composites has been investigated. For this purpose, a series of large direct shear tests were performed on the column-soil composites with the steel slag column and the ordinary stone column with or without the geosynthetic encasement. The effect of type of column materials (steel slag and sand), column diameter, number of columns, columns arrangement, and geosynthetic encasement on the shear strength parameters of column-soil composites have been studied. The experimental results show the effectiveness of using the steel slag columns to improve the lateral load-bearing performance of soil.

Numerical analysis on load-settlement response of reinforced granular blanket over ordinary stone column

Stone columns are a more economical and efficient method to enhance the strength of expansive soils. Using a granular blanket over the top of the ordinary stone columns (OSC) improves the drainage and distribution of the applied stress impending from the superstructure. The present study studied the effect of geogrid layers in a granular blanket (GB) over the top of the OSC numerically using 'PLAXIS 2D'. 'Mohr-Coulomb failure criterion was deliberated for the stone column, granular blanket, expansive soil, and elastoplastic behavior is considered for geogrid layers as reinforcement. Present review results are validated with the experimental results and agree greatly. Numerical results show that the construction of a GB with a geogrid layer over the stone column increases stress transformation to the depth of OSC. Thus 'stress concentration' is decreased in the higher zone of the OSC. Likewise, assessing the impact of geogrid layers in a granular blanket on the 'bearing capacity and settlement of OSC, it was observed that it reduces the lateral bulging, settlement and increases the ground's bearing capacity.

Influence of positions of the geotextile on the load-settlement behaviour of circular footing resting on single stone column by 2D Plaxis software

Purpose: This study aims to study the load – settlement behaviour of circular footing rested on encased single stone column. Design/methodology/approach: The effect of vertical, horizontal and combined verticalhorizontal encasement of stone column on the load carrying capacity were examined numerically. The effect of stone column dimension (80 mm and 100 mm), length (400 mm and 500 mm), and spacing of reinforcement on the load carrying capacity and reinforcement ratio were assessed. Findings: The obtained results revealed that the load carrying capacity of geotextile encased stone columns are more than ordinary stone columns. For vertically encased stone columns as the diameter increases, the advantage of encasement decreases. Whereas, for horizontally encased stone column and combined vertical- horizontal encased stone column, the performance of encasement intensifies as the diameter of stone column increases. The improvement in the load carrying capacity of clay bed reinforced with combined verticalhorizontal encased stone columns are higher than vertical encased stone columns or horizontal encased stone column. The maximum performance of encasement was observed for VHESC1 of D = 80 mm. Research limitations/implications: For this study, the diameter of footing and stone column was kept same. The interface strength factor between stone column and clay bed was not considered. Practical implications: The encased stone column could be use improve the laod bearing capacity of weak soils. Originality/value: Many studies are available in literature regarding use of geosynthetic as vertical encasement and horizontal encasement of stone column. The study on combined effect of vertical and horizontal encasement of stone column on load carrying capacity of weak soil is very minimal. Keeping this in view, the present work was carried out.

A large-scale shaking table model test for acceleration and deformation response of geosynthetic encased stone column composite ground

Effective mitigation of seismic-induced ground hazards requires an improved understanding of ground response in terms of earthquake wave propagation and ground deformation. Here, this paper examines the effects of geosynthetic-encased stone columns (ESCs) and ordinary stone columns (SCs) on the acceleration amplitude and frequency content responses of sand profiles, and the deformation of the ground using a large-scale shaking table model test. The model was excited by 15 shaking events including El Centro motion, Wenchuan Qingping motion and Kobe motion with peaks ranging from 0.1 to 0.9 g. The results indicate that the ESCs more significantly amplify surface accelerations compared to the SCs in the frequencies ranging from 10 to 17 Hz and from 2.5 to 9 Hz. The horizontal peak acceleration values in the ESCs composite ground are approximately twice those of the SCs composite ground. The acceleration response of the ground is influenced by the applied acceleration peak and frequency content, reinforced type, and structure. After the seismic excitation, the ESCs composite ground develops much narrower surface cracks distributed in a larger area compared to the SCs.

Critical length of encased stone columns

Encased stone columns are vertical inclusions in soft soils formed by gravel wrapped usually with a geotextile. Their critical length is the one where further lengthening of the column provides a negligible improvement and it is therefore not effective to build columns longer than it. This paper aims to obtain common values of the critical length using simplified two-dimensional axisymmetric and full three-dimensional finite element analyses. A uniform soft soil layer with a linear elastic perfectly plastic behaviour is considered for the sake of simplicity. For the studied cases, the critical column length is around 1.3–2.5 times the footing diameter for encased stone columns, and slightly lower for ordinary stone columns, namely around 1.1–1.9. The critical length of the encasement is found to be slightly lower than the critical column length. The value of the critical column length is related to the extent of plastic deformation and that may be used to decide the column length in the design phase without the need of parametric analyses. As a first approximation, a general value of the critical column length of 2 and 2.5 times the footing diameter may be considered for ordinary and encased stone columns, respectively.

Utilization of stone column for improving seismic response of foundation on soft clay- Numerical study

Nowadays, some projects have to be constructed in areas having thick layers of soft clay. These soft soils with low shear strength and high voids ratio, lead to excessive settlements even if subjected to low vertical or lateral loads. For this reason, soft clays are considered problematic soils for foundation purposes. Several improvement techniques have been done to enhance such clay. Stone columns have been used to improve soft soils by increasing it carrying capacity and reducing the settlement. In this paper, a numerical study on seismic behavior of stone column in soft clay subjected to earthquake loading has been performed. A two-dimensional plane strain program PLAXIS (dynamic version) is used for the present numerical modeling. A series of modeled stone columns were simulated with different diameters and spacing between columns. Also, the influence of geotextile encasement on the performance of stone columns, foundation systems in soft clay is investigated and compared with the behavior of ordinary stone columns without casing. The results showed that the ordinary stone column with small spacing and larger diameter has a greater bearing capacity and give a smaller settlement, compared to the column with large spacing. In addition, the geotextile encasement for stone column can be provided a significant increase in stone column capacity as well as a huge reduction in settlement is considered with increasing the encasement strength.

Performance enhancement of encased stone column with conductive natural geotextile under k0 stress condition

In this study, the performance of the stone column encased with conductive jute geotextile in improving the characteristics of soft clay under k0 stress condition was evaluated using a custom-designed large-scale consolidation test setup. The electrokinetic-encased stone column (e-ESC) was designed as a cathode, while mild steel rods were chosen as anodes, and by applying a voltage gradient of 0.1 V/mm between the electrodes, electrokinetic processes were initiated. The efficacy of using the e-ESC in improving the performance of soft clay was assessed by comparing the results with an ordinary stone column (OSC) and encased stone column (ESC) reinforced soft clay. The results from the study reveal that the inclusion of stiffer material like OSC and ESC in soft clay has significantly increased undrained shear strength and modulus of subgrade reaction of the composite ground with a reduction in settlement and compression index. Due to the application of the voltage gradient between the electrodes, the rate of consolidation settlement of composite ground has significantly increased when compared to the OSC/ESC case. Further, it was confirmed from the chemical and mineralogical analysis that the pH, chemical composition, mineral phases, and microfabric of soft clay were altered with the voltage gradient.

Analysis of Geocell-Reinforced Stone Column-Supported Embankment Considering Soil-Structure Interaction

Geocells are a superior form of reinforcement due to their cost-effectiveness and three-dimensional confining properties. The effect of the basal geocell layer on the performance of the stone column-supported embankment was evaluated in this paper by carrying out time-dependent coupled analyses considering the geocell-infill soil interactions. The various parameters, such as arching, stress concentration ratio, surface settlement, column bulging characteristics, and the influence of various types of infill materials and drainage blanket thickness on column-supported embankment behavior, were analyzed. The load transfer mechanism was quantified using the term stress concentration ratio, and with the use of geocell mattress above stone columns, the stress concentration ratio increased by 1.5 times that of ordinary stone columns. Also, it was observed that compared to ordinary stone column-supported embankment, the combination of stone columns with geocell-sand mattress resulted in a further reduction of the foundation settlement by 15%. Analysis results showed that the arching behavior is not predominant in geocell-reinforced columnar structures. The various analytical methods like Guido et al. (1987), Low et al. (1994), and Abusharar et al. (2009), which were developed for geosynthetic reinforced columnar embankments, were found to significantly under-predict the stress reduction ratio values with geocells.

Hybrid Continuous-Discrete Modeling of an Ordinary Stone Column and Micromechanical Investigations

An innovative method for modeling a complicated geotechnical problem combining the coarse and fine zones is to use the hybrid methods. The granular zone of a geotechnical system can be modeled by the discrete element method (DEM) and the fine zone by continuum-based methods. The advantage of this approach is the use of the capabilities of both methods. In the present research, by combining DEM and finite difference method (FDM), a 2D numerical laboratory framework is constructed. Simulation of a vertically loaded stone column installed in clay was chosen to examine the capabilities of the present approach. The contrast between the stone column aggregates, which usually consist of crushed stone with distinct behavior, and the host medium, which is clay with continuum behavior, makes the stone column suitable for this kind of coupled simulation. The coupled numerical model is validated by comparing the load–settlement response of the numerical model and the reported experimental results. Afterward, the failure mechanism of the stone column is evaluated. The column bulging was captured properly, and it was found that in addition to the bulging, the contraction occurs at the upper portion of the column. It was also found that at the early stages of loading, the compaction occurs in the column. However, at later stages of loading, the bulging will happen. Furthermore, the results showed that bulging decreases the internal stresses of the column. All of these achievements were obtained by micro and macro investigations. The obtained results indicate that the coupled DEM–FDM method is a robust approach to simulate the behavior of some geotechnical problems.

Numerical Modeling of Horizontally Layered Geosynthetic Reinforced Encased Stone Columns Supporting Embankment on Sabkha Soil

The present research work is concerned with the construction of road embankments on a Sabkha soil in Algeria. This soil is not only soft and very humid during the flooding seasons but also has frequent small areas of very soft soil which are called locally weak zones (LWZ) in the context of this study. LWZ are characterized by low strength and high compressibility. Two-dimensional axisymmetric analyses were carried out using PLAXIS 2D 2017. The study demonstrated that ordinary stone columns (OSC) are ineffective given the nature of these soils due to the excessive bulging caused by the lack of lateral pressure. On the other hand, the reinforced stone columns with external and internal reinforcements called as vertical encasement and horizontal strips (VESC+HRSC) are one of the best improvement methods of locally weak zones (LWZ), especially to increase the stability of embankment on the highway, namely, a much reduced bulging and a reasonable settlement, so that it is possible to build safe and very high embankments (indeed, numerical results showed for a (VESC+HRSC) combination, a vertical settlement of 0.74 m and a lateral deformation of 20.02 mm vs. 1.56 m and 221.16 mm for an OSC). Besides, an extensive parametric study was conducted to investigate the effect of the spacing of the horizontal reinforcing strips and of the column reinforced length. The influence of stone column diameter, depth of locally weak zone, and the effective stiffness of the geosynthetic, on the performance of the (RSC) - embankment composite were also investigated. The computational results are presented in the form of tables and graphs, and compared with previous published results available in the literature.

The Novel Usage of EPS Geofoam as Column Material: A Laboratory Study

The technique of stone column to improve the performance of soft soil is well established. However, an alternative material to enhance the performance of the soft soil by reinforcing with geofoam materials is suggested. Expanded polystyrene (EPS) geofoam is a superlight weight geosynthetic material used in various geotechnical engineering applications. This study deals with the innovative use of geofoam as a column material in soft soil for improving the bearing capacity. The method was developed in small-scale laboratory tests, and a series of loading tests were carried out on various single floating geofoam columns (normal geofoam and hollow geofoam) with two different diameters and the length-to-diameter ratio of 5. Next, a comparison was made with the results of ordinary stone columns and reinforced stone columns to obtain the benefits of geofoam columns. According to the results and by considering the bearing capacity, geofoam columns could be a good alternative material for improving the bearing capacity of soft soils. It was also found that the efficiency of the geofoam columns is almost similar to that of the ordinary stone columns and the usage of the geofoam is easy and economical. However, encasing the stone columns with geotextile results in further growth in the bearing capacity.

Small scale behavior of stone columns encased by tires

Stone columns are one of the best-suited methods of ground improvement for sites consisting of soft clays, silts and silty sands. Stone columns need to be supported against bulging failure as the main reason that reduces their bearing capacity. Disposal of scrap tires is one of the principal environmental problems around the world. Therefore, the reuse of scrap tires has grown globally. This paper investigates the behavior of stone columns encased by scrap tires which has the potential of replacing other encasements. Stone columns with diameters of 66, 80 and 92 mm which are one-tenth of original tire sizes have been tested in a large box, and the load-carrying characteristics of them are analyzed. Based on the results, by increasing the diameter of the encasement stone column, the benefit of the encasement increases. In addition, tests on the groups of stone columns with a diameter of 66 mm were carried out to investigate the presence effects of neighboring columns on the reference load. Furthermore, a comparison has been made with the results of ordinary stone columns and encased stone columns with geotextile to obtain the benefits of scrap tire columns. The bearing capacity of both single and group of stone columns encased with the scrap tire increases more than the ordinary stone columns. However, the bearing capacity of geotextile encased stone column is more than the other groups. Further, numerical analysis has been conducted to implement full-scale reinforced columns. The results illustrate that using scrap tires reduces bulging failure and increases the bearing capacity of stone columns. Accordingly, scrap tires replace the geotextile (as usual encasement) because their amount of bearing capacities are similar to each other especially in columns with larger diameters.

Centrifuge model tests on the deformation behavior of geosynthetic-encased stone column supported embankment under undrained condition

A series of centrifuge model tests were carried out to investigate the performance of geosynthetic-encased stone columns (GESCs) supported embankment under undrained condition. The influence of stiffness of encasement, basal reinforcement and embankment loading on the deformation behavior of GESCs were also assessed. The centrifuge test results reveal that under undrained condition, compared to ordinary stone column (OSC) supported embankment, the settlement of column has reduced by 50% and 34% when columns were encased with high and low stiffness geogrids respectively. Moreover, under identical embankment loading condition, the stress concentration ratio has increased significantly upon inclusion of basal reinforcement in the GESCs supported embankment. In case of OSCs supported embankment, columns experiences bulging in the top portion, inward bending in the central portion and a noticeable shear at the bottom portion. However, when columns were encased with geogrid layer, bulging in the top portion was significantly reduced but the inward bending of columns were noticed. With the inclusion of basal reinforcement, bending curvature of columns increases thereby inducing higher settlement in columns and relatively lesser settlement in surrounding soil. The differential settlement between the encased column and the surrounding soil under embankment loading has been considerably reduced with the inclusion of basal reinforcement.

Shear Response of Pervious Concrete Column Improved Ground

This study deals with numerical analysis of the shearing resistances of pervious concrete column improved ground vis-a-vis ordinary stone column improved ground. Analysis is done by numerically simulating a large shear test model, representing pervious concrete column improved ground. The parameters varied in this study are the depth of pervious concrete column/pile, floating and end-bearing piles, diameter, single pile and two-pile group and distance from the edge of loading area in the model. The shear response of improved ground is quantified by the applied strain controlled vertical load to the entire width of large shear test model that induces shear movements within the tank model. It is observed that the pervious concrete column improved ground shows better shear performance than ordinary stone column improved ground. It is also found that the pervious concrete column undergoes very small lateral deflections. It is also observed that more number of pervious concrete columns, and closer they are to the loaded area, better is the shear performance.

Loose Ash Fills Reinforced With the High Confined Encased Stone Columns: Experimental and Numerical Investigation

The use of industrial waste (ash) in large-scale earthworks for civil engineering applications such as construction of highway embankments, the rise of ash dykes, the filling low-lying areas, fillings of buildings, solves the problem of waste disposal besides in addition to the conservation of the natural soils. It is has a low density, low consolidation, lower potential for bearing and higher settlement. To use these areas for construction purposes, it is necessary to reinforce them at deeper depths. The present study uses highly confined stone columns to investigate the strengthening of ash fills. A total of six cases are investigated, including slope reinforced with an encased stone column. Model tests were conducted with pond ash at 40% relative density with a circular footing on untreated and treated ash fills. The enhanced cases include ordinary stone columns,vertically encased columns with geotextile, vertically encased columns with geogrid, highly confined vertically encased columns with both geotextile and geogrids (HC-1) and highly encased columns with both vertical encased geotextile and horizontal geogrid (HC-2) layers. The parametric study includes an examination of load-settlement behavior, load-carrying ratio, settlement reduction ratio, and stiffness factor, modulus of subgrade reaction and post-failure behavior of slopes reinforced with encased stone columns. In all cases, the high confined encased system yields better results. In each case, the stone column failure pattern of was observed at a distance range of 1–3 times the stone column diameter. Post-failure behavior indicates that for steep slopes where massive slip failures are expected, highly confined encased stone columns can be useful. Results are validated by the numerical modelling analysis of Plaxis.

Centrifuge modeling of geosynthetic-encased stone column-supported embankment over soft clay

Geosynthetic-encased stone column (GESC) has been proven as an effective alternative to reinforcing soft soils. In this paper, a series of centrifuge model tests were conducted to investigate the performance of GESC-supported embankment over soft clay by varying the stiffness of encasement material. The enhancement in the performance of stone columns encased with geosynthetic materials was quantified by comparing the test with ordinary stone columns (OSCs) under identical test conditions. The test results reveal that by encasing stone columns with geosynthetic material, a significant reduction in the ground settlement, relatively faster dissipation of excess pore pressure and enhanced stress concentration ratio was noticed. Moreover, with the increase in the encasement stiffness from 450 kN/m to 3300 kN/m, the stress concentration ratio increased from 4 to 6.5, which signifies the importance of encasement stiffness. In addition, a relatively lower value of soil arching ratio observed for GESCs compared to OSCs indicate the formation of a relatively strong soil arch in the GESC-supported embankment. Interestingly, under embankment loading, GESCs fail by bending while OSCs fail by bulging. The stress reduction method can be used to calculate the settlement of GESC-supported embankment with larger stress reduction factor than that in the OSC-supported embankment. Finally, the limitation of the construction of the embankment at 1 g was addressed.

Three Dimensional Numerical Study on Behavior of Geosynthetic Encased Stone Column Placed in Soft Soil

Ordinary stone column (OSC) reinforced soft soils undergo excessive settlements under vertical stresses due to the lack of adequate lateral support from the surrounding native soil. To overcome this issue, stone columns are suitably encased by a geosynthetic material having high axial stiffness, which provides the required additional confinement. A numerical analysis aimed at analyzing the effect of geosynthetic encasement on the load settlement behavior of geosynthetic encased stone columns (GESC) under vertical stresses is presented. Three dimensional (3D) models were developed in PLAXIS3D to simulate the behavior of stone column reinforced soft soils using the unit cell idealization concept. The numerical models were first validated with the help of experimental data of model tests on GESCs from literature. Various parameters were varied to quantify their impact on the load settlement behavior under column only loaded condition. The parameters varied include the diameter of GESCs, spacing to diameter (S/D) ratio, pattern of stone column installation, geosynthetic encasement stiffness, length of encasement, length of floating column, cohesion of soil and friction angle of stone column infill. Increase in the diameter of GESCs led to increased settlement for a particular vertical load intensity. The bearing capacity improved with increase in the geosynthetic stiffness, encasement length, length of floating GESCs, the cohesion of soil and the friction angle of the stone column infill. Increase in S/D ratio decreased the bearing capacity and triangular pattern of stone column installation was found to be more efficient. Moreover, the lateral bulging indicated a reducing trend upon increasing the axial stiffness of encasement.