Improvement of soft clay using installation of geosynthetic-encased stone columns: numerical study

The use of geosynthetic-encased stone columns as a method for soft soil treatment is extensively used to increase the bearing capacity and reduce the settlement of raft foundations and the foundation of structures like embankments. Pre-strain is an effect occurring in the encasement during stone column installation due to the compaction of the stone material. The present study uses the finite element program Plaxis to perform a numerical analysis of the soft clay bed reinforced by geosynthetic-encased stone columns. An idealization is proposed for simulation of installation of geosynthetic-encased stone columns in soft clay based on the unit-cell concept. In the analyses, initially, the validity of the analysis of the single column-reinforced soil in the unit-cell model was performed through comparison with the group columns. Then, by considering a unit-cell model, the finite element analyses were carried out to evaluate the stiffness of the reinforced ground to estimate the settlement. The results of the analyses show that the improved stiffness of the encased stone column is not only due to the confining pressure offered by the geosynthetic after loading, but the initial strain of the geosynthetic that occurred during installation also contributes to the enhancement of the stiffness of the stone column and the reduction of the settlement.     

An analytical solution for the settlement of stone columns beneath rigid footings

This paper presents a new approximate solution to study the settlement of rigid footings resting on a soft soil improved with groups of stone columns. The solution development is fully analytical, but finite element analyses are used to verify the validity of some assumptions, such as a simplified geometric model, load distribution with depth and boundary conditions. Groups of stone columns are converted to equivalent single columns with the same cross-sectional area. So, the problem becomes axially symmetric. Soft soil is assumed as linear elastic, but plastic strains are considered in the column using the Mohr–Coulomb yield criterion and a non-associated flow rule, with a constant dilatancy angle. Soil profile is divided into independent horizontal slices, and equilibrium of stresses and compatibility of deformations are imposed in the vertical and horizontal directions. The solution is presented in a closed form and may be easily implemented in a spreadsheet. Comparisons of the proposed solution with numerical analyses show a good agreement for the whole range of common values, which confirms the validity of the solution and its hypotheses. The solution also compares well with a small-scale laboratory test available in the literature.     

Settlement evaluation of soft clay reinforced with stone columns using the equivalent secant modulus

A method is proposed to evaluate settlement of soft clay reinforced with stone columns. Finite element analyses were carried out using 15-noded triangular elements with PLAXIS. A drained analysis was carried out using Mohr?CCoulomb??s criterion for soft clay, stones, and sand. The stress due to column installation has been considered in the analysis. At the interface between the stone column and soft clay, interface elements have been used. The settlement ratio (SR) of the soil has been estimated using the equivalent secant modulus. The results are compared with those available in the literature, and the advantages of the numerical analysis were highlighted. Based on the results of this analysis, the SR decrease with compaction surrounding soft soil, but decrease of SR is mainly due to a stiffer column material in soft clay.     

Response of multilayer geosynthetic-reinforced bed resting on soft soil with stone columns

In the present study, a mechanical model has been developed to study the behavior of multilayer geosynthetic-reinforced granular fill over stone column-reinforced soft soil. The granular fill and geosynthetic reinforcement layers have been idealized by Pasternak shear layer and rough elastic membranes, respectively. The Kelvin–Voight model has been used to represent the time-dependent behavior of saturated soft soil. The stone columns are idealized by stiffer springs and assumed to be linearly elastic. The nonlinear behavior of the soft soil and granular fill is considered. The effect of consolidation of soft soil due to inclusion of the stone columns on settlement response has also been included in the model. Plane strain conditions are considered for the loading and reinforced foundation soil system. An iterative finite difference scheme is applied for obtaining the solution and results are presented in nondimensional form. It has been observed that if the soft soil is improved with stone columns, the multilayer reinforcement system is less effective as compared to single layer reinforcement to reduce the total settlement as there is considerable reduction in the total settlement due to stone column itself. Multilayer reinforcement system is effective for reducing the total settlement when stone columns are not used. However, multilayer reinforcement system is effective to transfer the stress from soil to stone column. The differential settlement is also slightly reduced due to application of multiple geosynthetic layers as compared to the single layer reinforcement system.     

Performance of Stone Columns in Soft Clay: Numerical Evaluation

Stone columns (or granular piles) are increasingly being used for ground improvement. This study investigates the qualitative and quantitative improvement in soft clay by stone columns. Finite element analyses were carried out to evaluate the performance of stone columns in soft clay. A drained analysis was carried out using Mohr–Coulomb’s criterion for soft clay, stones, and sand. The interface elements were used at the interface between the stone column and soft clay. Analyses and calculations were carried out to determine equivalent parameters of soil/columns system. The bearing capacity ratio (BCR) of the soil has been estimated for homogeneous and heterogeneous soil. The results have shown that the values of BCR for homogeneous soil are obviously higher than those for heterogeneous soil.     

Finite Element Analysis of Geogrid Encased Stone Columns

Stone columns in soft soil improve bearing capacity because they are stiffer than the material which they replace, and compacted stone columns produce shearing resistances which provide vertical support for overlying structures or embankments. Also stone columns accelerate the consolidation in the native surrounding soil and improve the load settlement characteristics of foundation. In this paper, the finite element method is utilized as a tool for carrying out analyses of stone column–soil systems under different conditions. A trial is made to improve the behaviour of stone column by encasing the stone column with geogrid as reinforcement material. The program CRISP-2D is used in the analysis of problems. The program allows prediction to be made of soil deformations considering Mohr-Coulomb failure criterion for elastic–plastic soil behaviour. A parametric study is carried out to investigate the behaviour of standard and encased floating stone columns in different conditions. Different parameters were studied to show their effect on the bearing improvement and settlement reduction of the stone column. These include the length to diameter ratio (L/d), shear strength of the surrounding soil and, the area replacement ratio (a_s) and others. It was found that the maximum effective length to diameter (L/d) ratio is between (7–8) for Cu, between (20–40) kPa and between (10–11) for Cu?=?10?kPa for ordinary floating stone columns while the effective (L/d) ratio is between (7–8) for encased floating stone columns. The increase in the area replacement ratio increases the bearing improvement ratio for encased floating stone columns especially when the area replacement ratio is greater than (0.25). The geogrid encasement of stone column greatly decreases the lateral displacement compared with ordinary stone column.     

Finite element modeling of the consolidation behavior of multi-column supported road embankment

A multi-column composite foundation is a new concept utilizing different column types with varying lengths and diameters to support the embankment fill and to mobilize the strength and stiffness of the soil at shallow depths. This study presents the results of finite element analyses using the finite element software PLAXIS to investigate the consolidation behavior of a road embankment constructed on a multi-column composite foundation. The finite element results are calibrated for a period of 200 days. The settlement, horizontal displacement, differential settlement, column axial force, and the development and dissipation of excess pore pressure are presented and discussed in detail. It is concluded that a multi-column composite foundation allows a fast rate of consolidation and significantly increases the embankment stability. A multi-column composite foundation formed by CFG–lime columns is more effective than one formed by SC–lime columns. The CFG–lime columns improve the long-term stability of the embankment because the compression modulus of CFG columns is significantly greater than that of SC columns.     

软粘土层上的油罐差异沉降

根据某一大型油罐软基加固处理工程方案设计和优选需要,按照离心模型相似律,开展了三组模型试验,分别模拟了天然地基、土工合成材料袋装碎石垫层和既在填土层中设置袋装碎石垫层又在淤泥质粘土层设置土工合成材料排水板三种情况,以研究这一加固布置形式对减小高压缩性软土层地基上油罐罐底的差异沉降效果反应。模型油罐地基采用原型土重塑制备,现场土工合成材料袋装碎石采用柔性机织玻璃纤维细管塞装粗砂条模拟,并在不停机运转条件下模拟了多次充放水预压加载。试验结果表明,油罐软弱地基经土工合成材料袋装碎石加固后,罐底总沉降值和差异沉降值均明显小于天然地基情形下对应的沉降值,罐底畸变得到显著减小,就本文所述的土质条件、土层厚度和预压荷载强度,地基经加固处理后,油罐罐底畸变减小了近50 %。最后就土工合成材料在加固油罐地基布置形式的合理性进行了初步探讨。     

Numerical analysis of foundation columns to support widening of embankments

Increased traffic volume has made it necessary to increase highway capacities by widening embankments and pavements. Adding a new embankment to an existing embankment induces additional stresses and deformations beneath the widened and existing portions of the embankment. Differential settlement may develop between and within the new and existing portions of the embankment, especially over soft soils. This differential settlement often causes pavement distress, such as longitudinal cracks or the drop-off (or sinking) of pavement sections. Different techniques have been adopted to remedy these problems, including the use of foundation columns, such as deep mixed columns, vibro-concrete columns, stone columns, and aggregate piers. However, design procedures for foundation columns constructed for this purpose are not well developed. The analyses of eight cases of column-supported widened embankments and two untreated foundations are presented in this paper. The factors considered include the consolidation of foundation soils under existing embankments and the spacing, region, and modulus of foundation columns. Two-dimensional finite difference software was used after the calibration of the model against a field case study and numerical analyses were conducted to investigate stresses and deformations of the widened embankments over soft soil with or without the remediation of foundation columns. The results presented in this paper include the vertical and the horizontal displacements, the maximum settlements, the transverse gradient change, and the distribution of the additional stresses induced by the widening. Recommendations are made for the design of foundation columns to remedy roadway pavement failure due to widening of embankments.     

Finite element analyses of bridge abutments on firm clay

In finite element analysis of soil-structure interaction problems involving firm to stiff overconsolidated clay, there have been difficulties in modelling the stress-strain response of the soil. Non-linearity and anisotropy of the soil depend on the inherent anisotropy of its particle structure and the induced anisotropy of its stress history and current stress path. In CRISP modelling of the centrifuge test of an abutment wall and its backfill of sand on the surface of a firm to stiff overconsolidated kaolin, the clay foundation was divided into 6 broad zones in accordance with the stress history and stress path. Undrained movements of the abutment and its subsoil were closely modelled in two analyses; one with a non-linear elastic model and the other with the Schofield model with shear modulus G assigned to the foundation zone in accordance with the estimated strain level as well as stress history and stress path. In the prediction of consolidation movement, there is a difficulty in the current critical state soil model in CRISP. The fe solution incorrectly predicted that substantial horizontal movement would accompany settlement due to consolidation, whereas the centrifuge test showed mainly vertical movement. This is attributable to the pronounced anisotropy separately observed in element tests.     

Three-dimensional analysis of bearing behavior of piled raft on soft clay

The piled raft has proved to be an economical foundation type compared to conventional pile foundations. However, there is a reluctance to consider the use of piled rafts on soft clay because of concerns about excessive settlement and insufficient bearing capacity. Despite these reasons, applications of piled rafts on soft clay have been increased recently. Current analysis methods for piled rafts on soft clay, however, are insufficient, especially for calculating the overall bearing capacity of the piled raft. This study describes the three-dimensional behavior of a piled raft on soft clay based on a numerical study using a 3D finite element method. The analysis includes a pile–soil slip interface model. A series of numerical analyses was performed for various pile lengths and pile configurations for a square raft subjected to vertical loading. Relatively stiff soil properties and different loading types were also used for estimating the bearing behavior of the piled raft. Based on the results, the effect of pile–soil slip on the bearing behavior of a piled raft was investigated. Furthermore, the proportion of load sharing of the raft and piles at the ultimate state and the relationship between the settlement and overall factor of safety was evaluated. The results show that the use of a limited number of piles, strategically located, might improve both bearing capacity and the settlement performance of the raft.     

Factors Affecting the Settlement Estimation of Stone Column Reinforced Soils

The ground improvement technique of stone columns has been effectively used over the last decades in order to accelerate the consolidation rate of soft soils by providing a drainage path, reduce foundation settlements, improve the bearing capacity of the soil, and limit the risk of liquefaction due to seismic activity. Because of the time consumption of a three dimensional (3D) calculation, most stone column projects are presently studied by axisymmetric finite element computations. Otherwise, it is the analytical method of Priebe that is commonly used to predict the final settlements of a single stone column and the corresponding settlement reduction factor. Both methods, axisymmetric finite element analysis and Priebe’s analytical method make use of the axisymmetric theory of unit cell. The main objective of this study is to examine and compare the three dimensional response of stone column reinforced soils with the axisymmetric analysis results. A proposed analytical method, governed by the failure mechanism of a cylindrical cone is also presented. The comparison between these different approaches is studied and evaluated in conjunction with in situ measurements. It is proved that the beneficial effect of this ground improvement technique, especially on the foundation soil with the lower strength characteristics, is more pronounced. Also, it is concluded that due to unit cell theory, the settlement reduction factor is usually under predicted and the failure of the composite system is impossible, even under high applied pressures. The proposed analytical method of the cylindrical cone failure mechanism is proved to be in good agreement with the 3D results.     

Numerical analysis of stone column supported foundations

In this paper, settlement and failure load of rafts resting on stone column reinforced soft clays are analyzed. The influence of the stone columns is assumed to be uniformly and homogeneously distributed throughout the reinforced region. It is also assumed that both columns and surrounding soil undergo the same total strains i.e. no slip occurs on the soil-column interface. A constitutive model is presented for an equivalent material. It combines different elasto-plastic laws, namely the Critical State model for clay and the Mohr-Coulomb criterion for gravel. Continuity of radial stresses is ensured by an additional pseudo-yield criterion. The model is incorporated in a finite element code and results for a circular footing are presented. The influence of dilatancy of the columns is highlighted together with the differences in the behaviour for columns situated at the centre or at the outer boundary of the footing. Flexible as well as rigid foundations are considered. It is emphasized that the finite element mesh is independent of the column spacing leading to considerable advantages in carrying out parametric studies.     

模糊聚类在不均匀场地土分类归并中的应用

在含有砂性土、淤泥质粘土的吹填土场地建设大型储罐,采用高能级强夯置换进行地基处理。由于吹填土在块石强夯置换后,流塑状的淤泥质粘土被置换或被扰动,砂性土被振密或挤密。根据多种检测手段获得的墩间土指标,具有较大的离散性,墩间土土性分布在空间上极不均匀,难以对复合地基沉降变形作出预估分析。采用模糊聚类方法,对获得的所有测点土性指标和分布区域进行聚类归并后,得出按不同平面区域分布的场地土类别,在一定置信度上,保证了归并后的指标合理性,使储罐基础的沉降变形预估更符合实际情况。     

Nonlinear analysis of footings on granular bed–stone column improved soft soil

In this paper, a model for the analysis of footings having finite flexural rigidity resting on a granular bed on top of stone columns improved saturated soft (clayey) soil has been proposed. Soft soil has been modeled as a Kelvin–Voigt body to represent its time dependent behavior. Pasternak shear layer has been used to represent the granular layer and the stone columns have been idealized by means of nonlinear Winkler springs. Nonlinear behavior of granular fill, soft soil and stone columns has been invoked by means of hyperbolic constitutive relationships. Governing differential equations for the soil–foundation system have been obtained and finite difference method has been adopted for solving these, using the Gauss-elimination iterative scheme. Detailed parametric study for a combined footing has been carried out to study the influence of parameters, like magnitude of applied load, flexural rigidity of footing, diameter of stone column, spacing of stone column, ultimate bearing capacity of granular fill, poor foundation soil and stone column, relative stiffness of stone columns and degree of consolidation, on flexural response of the footing.     

Modeling of granular bed‐stone column‐improved soft soil

The present study pertains to the development of a mechanical model for predicting the behavior of granular bed‐stone column‐reinforced soft ground. The granular layer that has been placed over the stone column‐reinforced soft soil has been idealized by the Pasternak shear layer. The saturated soft soil has been idealized by the Kelvin–Voigt model to represent its time‐dependent behavior and the stone columns are idealized by stiffer Winkler springs. The nonlinear behavior of the granular fill has been incorporated in this study by assuming a hyperbolic variation of shear stress with shear strain as in one reported literature. Similarly, for soft soil it has also been assumed that load‐settlement variation is hyperbolic in nature. The effect of consolidation of the soft soil due to inclusion of the stone columns has also been included in the model. Plane‐strain conditions are considered for the loading and foundation soil system. The numerical solutions are obtained by a finite difference scheme and the results are presented in a non‐dimensional form. Parametric studies for a uniformly loaded strip footing have been carried out to show the effects of various parameters on the total as well as differential settlement and stress concentration ratio. It has been observed that the presence of granular bed on the top of the stone columns helps to transfer stress from soil to stone columns and reduces maximum as well as differential settlement. Copyright © 2007 John Wiley & Sons, Ltd.     

Stone column effectiveness in soils with creep: a numerical study

While it is well established that vibro stone columns reduce primary settlement and improve bearing capacity, their effect on creep compression has largely been overlooked to date. However, with increasing pressure to develop marginal sites underlain by soft organic soils, the effect of ground treatment on creep is an important emerging issue in geotechnical engineering. In this paper, a series of axisymmetric unit cell analyses have been carried out using the PLAXIS 2D finite element program in conjunction with the Soft Soil Creep (SSC) model. Examination of the evolution of settlement improvement factor with time has indicated that the presence of creep leads to a lower ‘total’ improvement factor than would be obtained for primary consolidation settlement alone. Separate ‘primary’ and ‘creep’ improvement factors have also been derived; the latter are much lower than the former, but are nevertheless greater than unity. Creep results in a stress transfer process; as the soil creeps, vertical stress is transferred from the soil to the stone column. The additional load carried by the column induces additional yielding and shear-plane formation in closely-spaced columns. The additional increment of stress transferred to the already yielded column reduces its efficacy.     

Consolidation and deformation around stone columns: Numerical evaluation of analytical solutions

Coupled finite element analyses of the consolidation and deformation around stone columns have been performed to assess the accuracy of different analytical solutions. The numerical model reproduces the hypotheses of the closed-form solutions. In the model, a rigid load is applied to a unit cell formed by a fully penetrating column and its surrounding soil, and simple elastic or elasto-plastic soil models are used. The surface settlement, the dissipation of the pore pressure and the vertical stress concentration on the column are studied. These soil responses are accurately estimated with closed-form solutions that properly include the radial and plastic strains in the column. However, the surrounding soil does not yield for usual conditions, which reasonably justifies the elastic soil behavior assumed in the analytical solutions. The differences between drained and consolidation analyses are also evaluated. Comparing the numerical results with the closed-form solutions illustrates the implications of the assumptions of each approach.     

Finite element simulations of the behavior of piled‐raft foundations using a multiphase model

Finite element simulations of the behavior of a piled raft foundation have been carried out using a multiphase model conceived as an improved homogenization approach. According to this model, the ground reinforced by a group of piles is treated as a homogeneous continuous medium. In this approach, no specific interface elements are necessary to account for the mechanical interaction between the piles and the ground: this interaction is described by means of two scalar parameters, one stiffness parameter and one which can easily be derived from the maximum ground‐pile friction. The implementation of the model into a finite element code provides an efficient tool for the analysis of the influence of the pile number or length on the settlement and bearing capacity of a square piled raft foundation and of the way the total applied load is shared between the raft and the piles. Results are compared with a standard 3D finite element analysis. The comparison highlights the fact that the proposed approach remains to be improved to account for tip resistance. Copyright © 2012 John Wiley & Sons, Ltd.     

Modeling of Stone Column-Supported Embankment Under Axi-Symmetric Condition

In the present work, a simplified model has been developed to study the behavior of stone column-supported embankment under axi-symmetric loading condition. The rate of consolidation of stone column-reinforced soft ground under axi-symmetric condition has also been presented in the paper. Mechanical model elements such as Pasternak shear layer, spring–dashpot system are used to model the different components such as granular layer, soft soil, stone columns etc. The governing differential equations are solved by finite difference technique. Parametric study has also been carried out to show the effect of different model variables on the settlement, stress concentration ratio of the foundation system. It is observed that for lower diameter ratio, at a particular time, the degree of consolidation predicted by the present method for axi-symmetric loading condition is almost same or lower than the degree of consolidation obtained by unit cell approach, but as the diameter ratio increases present analysis predicts higher degree of consolidation as compared to the unit cell approach. The maximum settlement decreases as the modular ratio increases and beyond the modular ratio value 30, the reduction rate of settlement decreases.