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1.
Numerical modelling of the improvements to primary and creep settlements offered by granular columns 总被引：4，自引：3，他引：1
Analytical vibro-replacement design approaches typically quantify the settlement reduction using a dimensionless settlement improvement factor, defined as the ratio of the settlements without and with treatment. Most approaches do not explicitly consider the improvement to creep settlement. This is unfortunate because the ‘stone column’ technique is being increasingly used to reduce settlement and improve bearing capacity in soft normally consolidated and lightly overconsolidated cohesive soils (in which creep settlement tends to account for a significant proportion of the total settlement). Analytical design approaches typically consider primary settlement only. In this study, two-dimensional axisymmetric analyses have been carried out using PLAXIS 2D to establish the variation of improvement factor with time using different soil models, one of which incorporates creep behaviour. Two different approaches have been used to establish the influence of creep on predicted settlement improvement factors. The first approach is based on a direct comparison of two different soil models (one of which incorporates creep) whereas the second approach is based solely on the model incorporating creep. The settlement improvement factors have been evaluated for different area-replacement ratios, modular ratios and column lengths. The primary settlement improvement factors are in good agreement with some of the more popular analytical design methods while the creep settlement improvement factors are either equivalent or lower (depending on the approach used). The primary settlement improvement factors show a dependence on the modular ratio whereas it appears that the corresponding creep settlement improvement factors are relatively independent of it. 相似文献
2.
Kousik Deb 《国际地质力学数值与分析法杂志》2008,32(10):1267-1288
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. 相似文献
3.
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 (as) 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. 相似文献
4.
Response of multilayer geosynthetic-reinforced bed resting on soft soil with stone columns 总被引：2，自引：0，他引：2
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. 相似文献
5.
Souhir Ellouze Zeineb Bensalem Mohamed Nadhir Znaidi 《Geomechanics and Geoengineering》2017,12(2):73-85
This paper addresses the installation effects of stone columns in soft soils. Focus is made on the lateral expansion of stone material using the vibro displacement and substitution techniques by means of numerical simulations. The behaviour of reinforced soil after stone column installation is investigated to show how the properties of soft soils can be improved prior to final loading. The effect of such an improvement on the prediction of reinforced soil settlement is evaluated. The axisymmetric unit cell model (UCM) served for the comparison between numerical predictions made by the Mohr-Coulomb and hardening soil constitutive laws adopted for the soft soil. An equivalent group of end bearing columns model was investigated in the axisymmetric condition to predict the settlement of reinforced soil by adopting the Mohr-Coulomb constitutive model for soft clay. The reduction of settlements predicted by the unit cell and group of columns models, due the improvement of the Young’s modulus of soft clay, were compared. It is concluded that a significant reduction of settlement is expected when the group of columns model is considered. 相似文献
6.
Settlement evaluation of soft clay reinforced with stone columns using the equivalent secant modulus 总被引：3，自引：2，他引：1
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. 相似文献
7.
Panagiotis Andreou Vassilis Papadopoulos 《Geotechnical and Geological Engineering》2014,32(5):1175-1185
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. 相似文献
8.
A. J. Choobbasti A. Zahmatkesh R. Noorzad 《Geotechnical and Geological Engineering》2011,29(5):675-684
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. 相似文献
9.
M. R. Dheerendra Babu Sitaram Nayak R. Shivashankar 《Geotechnical and Geological Engineering》2013,31(1):1-22
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. 相似文献
10.
Axi-symmetric Analysis of Geosynthetic-reinforced Granular Fill-soft Soil System with Group of Stone Columns 总被引：1，自引：1，他引：0
The paper presents a mechanical model to predict the behavior of geosynthetic-reinforced granular fill resting over soft soil
improved with group of stone columns subjected to circular or axi-symmetric loading. The saturated soft soil has been idealized
by spring-dashpot system. Pasternak shear layer and rough elastic membrane represent the granular fill and geosynthetic reinforcement
layer, respectively. The stone columns are idealized by stiffer springs. The nonlinear behavior of granular fill and soft
soil is considered. Consolidation of the soft soil due to inclusion of stone columns has also been included in the model.
The results obtained by using the present model when compared with the reported results obtained from laboratory model tests
shows very good agreement. The effectiveness of geosynthetic reinforcement to reduce the maximum and differential settlement
and transfer the stress from soft soil to stone columns is highlighted. It is observed that the reduction of settlement and
stress transfer process are greatly influenced by stiffness and spacing of the stone columns. It has been further observed
that for both geosynthetic-reinforced and unreinforced cases, the maximum settlement does not change if the ratio between
spacing and diameter of stone columns is greater than 4. 相似文献
11.
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. 相似文献
12.
The paper presents numerical simulations investigating the settlement reduction caused by stone columns in a natural soft clay. The focus is on the influence of the soft soil alteration caused by column installation. A uniform mesh of end-bearing columns under a distributed load was considered. Therefore, the columns were modelled using the “unit cell” concept, i.e. only one column and the corresponding surrounding soil in axial symmetry. The properties of the soft clay correspond to Bothkennar clay, which is modelled using S-CLAY1 and S-CLAY1S, which are Cam clay type models that account for anisotropy and destructuration. The Modified Cam clay model is also used for comparison. Column installation was modelled independently to avoid mesh distortions, and soft soil alteration was directly considered in the initial input values. The results show that the changes in the stress field, such as the increase of radial stresses and mean stresses and the loss of overconsolidation, are beneficial for high loads and closely spaced columns but, on the contrary, may be negative for low loads, widely spaced columns and overconsolidated soils. Moreover, whilst the rotation of the soil fabric reduces the settlement, in contrast the soil destructuration during column installation reduces the improvement. 相似文献
13.
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. 相似文献
14.
R. Shivashankar M. R. Dheerendra Babu Sitaram Nayak R. Manjunath 《Geotechnical and Geological Engineering》2010,28(5):695-706
This paper presents results from a series of laboratory plate load tests carried out in unit cell tanks to investigate the
improvement in stiffness, load carrying capacity and resistance to bulging of stone columns installed in soft soils. A new
method of reinforcing the stone columns with vertical nails installed along the circumference of the stone column is suggested
for improving the performance of these columns. Tests were carried out with two types of loading (1) the entire area in the
unit cell tank loaded, to estimate the stiffness of improved ground and (2) only the stone column loaded, to estimate the
limiting axial capacity. It is found that stone columns reinforced with vertical nails along the circumference have much higher
load carrying capacity and undergo lesser compression and lesser lateral bulging as compared to conventional stone columns.
The benefit of vertical circumferential nails increases with increase in the diameter, number and depth of embedment of the
nails. The improvement in the performance of stone column was found to be more significant, even with lower area ratio. It
is found that reinforcing stone column with vertical circumferential nails at the top portion to a depth equal to three times
the diameter of stone columns, will be adequate to prevent the column from excessive bulging and to improve its load carrying
capacity substantially. 相似文献
15.
《Geomechanics and Geoengineering》2013,8(3):185-194
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. 相似文献
16.
Wissem Frikha Mounir Bouassida Jean Canou 《Geotechnical and Geological Engineering》2013,31(2):739-752
The paper describes the behavior of remolded kaolin clay reinforced by stone column as investigated in laboratory. The installation of stone column was simulated by performing lateral expansion at different rates within hollow cylindrical remolded kaolin specimens initially subjected to K0 consolidation path. These specimens were, then, subjected to classical consolidated undrained triaxial tests while recording excess pore pressure values. The objective of the experimental programme was to quantify the effects of consolidation stress and stone column on the undrained Young’s modulus and shear strength of kaolin clay. Obtained results showed a significant improvement in Young’s modulus when the cavity expansion ratio and the consolidation stress increase. It was also found that the increase in undrained shear strength of improved kaolin clay mainly occurred at lower consolidation stress. Another important finding of this study is that the ratio of undrained Young’s modulus to undrained shear stress increases when the consolidation stress decreases. Finally, the paper presents a model developed for the design of stone columns. 相似文献
17.
18.
Stone columns are found to be effective and economical ground improvement technique in soft grounds. Understanding its behaviour
when they are installed in stratified soils, in particular when the upper layer consists of weak soil, will be of great practical
significance. This paper presents results from a series of laboratory plate load tests carried out in unit cell tanks to investigate
the behaviour of stone columns in layered soils, consisting of weak soft clay overlying a relatively stronger silty soil,
for various thicknesses of the top layer. Tests were carried out with two types of loading (1) the entire area in the unit
cell tank loaded, to estimate the stiffness of improved ground and (2) only the stone column loaded, to estimate the limiting
axial capacity. Laboratory tests were carried out on a column of 90 mm diameter surrounded by layered soil, for an area ratio
of 15%. It is found that the depth of top weak layer thickness has a significant influence on the stiffness, load bearing
capacity and bulging behavior of stone columns. 相似文献
19.
软土的固结与蠕变是土力学研究和工程建设中需要考虑的重点问题, 但是传统的固结理论, 例如太沙基固结理论没有考虑软土的流变特性, 对软土的次固结沉降无法做出准确的解释。为了研究软土的固结与蠕变以及次固结沉降特性, 掌握其应力和变形规律, 本文对传统的固结理论进行了改进。在太沙基一维固结理论的基础上, 利用次固结系数和修正的Singh-Mitchell经验蠕变模型, 建立了考虑次固结效应的一维流变固结微分方程。在太沙基三维固结理论和比奥固结理论的基础上, 利用修正的Singh-Mitchell经验蠕变模型, 建立了考虑次固结效应的三维流变固结微分方程。新建立的流变固结微分方程能够更加准确地反映软土固结和蠕变性状的应力-应变-时间关系, 为软土固结沉降研究提供新的理论依据。 相似文献
20.
This work studies the effect of the curing time on the creep behaviour of a stabilised soft soil, using volumetric and deviatoric creep laws associated with constitutive models. Results of unconfined compressive strength tests for several curing times are used to define the time evolution of the mechanical and creep properties. The models/creep laws are validated by oedometer and triaxial creep tests, for 28 and 90 days of curing. The long-term behaviour of an embankment built on a soft soil reinforced with deep mixing columns shows that the effect of curing time decreases the settlement and increases the improvement factor. 相似文献