Stress Concentration Ratio Research Articles

Influence of gravel density in the behaviour of soft soils improved with stone columns

Stone columns are frequently employed to improve the bearing capacity of soft soils, to reduce settlements, and to increase the speed of consolidation. Their behaviour depends on several factors, such as the density of the aggregate that forms the column and the area replacement ratio. This paper presents a study of the influence of the density of the gravel forming the columns on the deformation and stresses around end-bearing stone columns installed in soft soils. For this purpose, the behaviour of a horizontal slice of a unit cell has been analyzed by small-scale laboratory tests performed in a Rowe–Barden cell. Tests have been performed with a gravel relative density of Dr= 30% and with two area replacement ratios. Their results have been analyzed along with those from similar tests performed with a gravel density of Dr= 100%. The study is focused on the soil–column stress concentration ratio and the reduction of settlements. Finally, the experimental results are compared with numerical simulations. The results show that a reduction of settlements around 10% occurs when the relative density of the gravel increases from Dr= 30% to 100%. Numerical analyses reproduce well the behaviour of stone columns and are in good agreement with the experimental results.

Laboratory investigation of two basic configurations for inhomogeneous soils

Lumpy soils are inhomogeneous materials which can be encountered in land reclamation and open pit mining. The lumps are randomly distributed in the reconstituted soil but on a mesoscale their configuration can be expressed by two simplified configurations of both constituents. In order to investigate the behaviour of these two basic configurations, isotropically consolidated drained triaxial shear tests were performed on artificially prepared specimens with parallel and series structures. The laboratory tests show that the failure plane of the series specimens is located in the constituent with the lower strength; the specimens with the parallel structure have a failure plane which crosses both constituents. As a result, the shear strength of the series specimens is only slightly higher than that of the constituent with the lower strength and the strength of the parallel specimens lies between those of the constituents. Furthermore, the maximum volumetric strain of the series specimens is lower than that of its constitutes, which is significantly different from that of the parallel specimens. The stress concentration ratio of the parallel specimens , defined as the ratio of stress in the stiffer constituent to that of the weak one, increases with the consolidation stress; while, the strain concentration ratio of the series specimens , defined as the ratio of strain in the stiffer constituent to that of the weak one, is not sensitive to the consolidation stress.

STATIC RESPONSE ON LIME COLUMN AND GEOTEXTILE ENCAPSULATED LIME COLUMN (GELC) STABILISED MARINE CLAY UNDER VERTICAL LOAD

Marine clay, which is widely encountered in coastal area in Malaysia, is a problematic base material. Previous researchers reported that deep lime stabilisation can significantly improve clay. However, insufficient confining pressure from surrounding soil normally lead to the inferior performance on the upper part of column such as column head crushing and larger deformation on the surrounding soil at toppart of column. Therefore, geotextile encapsulation was proposed for lime column in this study. Static response and stress distribution are essential in the understanding on behaviour of columnar stabilised soil under vertical load. Multi stages loading tests were conducted onPontian marine clay, with and without geotextile encapsulation.Stress concentration ratio (σmid/ σsoil) was examined in each loading stage, where it is defined as stress on column (σmid) divided by stress on surrounding soil (σsoil). The samples were cured for 14, 28 and 56 days before tested. It was found that stress concentration ratio was dependent on column materials strength properties and applied loading. Geotextile encapsulation increased the stress concentration ratio on lime column.Stress concentration increment effect by geotextile encapsulation was further enhanced by the confining pressure of surrounding soil; however, the effect reduced with increase of applied loading. Higher stress concentration ratio indicated lesser load on surrounding soil and therefore the soil settlement could be reduced

Settlement behavior of embankment on geosynthetic-encased stone column installed soft ground – A numerical investigation

This paper presents the results of a numerical investigation into the settlement behavior of embankment constructed on geosynthetic-encased stone column installed in soft ground. A three-dimensional finite element model was employed to carry out a parametric study on a number of governing factors such as the thickness and consistency of soft ground, the geosynthetic encasement length and stiffness, the embankment fill height, and the area replacement ratio. It is shown among other things that unlike an isolated geosynthetic-encased stone column, full encasement of stone column is necessary to minimize the settlement for the embankment loading condition. Also revealed is that the efficiency of increasing encasement stiffness is construction condition dependent and is maximized for cases with which the stress intensity in the soft ground is larger. Based on the results of this investigation, design charts for use in estimating the maximum settlement and the stress concentration ratio during preliminary design are presented.

H-pile로 보강된 Sheet pile 흙막이 벽체의 적용을 위한 수치해석

This paper is results of numerical study for application of sheet pile retaining wall reinforced with H-pile as sheet piles are needed in field for a cutoff wall and are limited to use because of driveability in the ground condition of having a larger strength than a weathered rock. Extensive 101 cases of numerical approach were conducted to investigate the behavior of sheet pile retaining wall reinforced with H-pile, changing installing members of two types of sheet pile and three types of H-pile, the embedded depth of sheet pile and H-pile, the horizontal space between H-piles and excavation conditions. As the results of numerical analysis, combined use of the sheet pile SP-IIIA with H-Pile H250 and the sheet pile SP-IV with H-Pile H350 among precast products was found to be efficient since two members tended to reach allowable stresses simultaneously or have similar stress concentration ratios. Increased stiffness in reinforced sheet pile showed reduction of lateral displacement of wall. Embedded depth of sheet pile did not affect stability of wall significantly so that driving the penetrable depth of sheet pile should be enough to maintain stability of wall and satisfy purposes of cutoff and stiffness increase of wall.

Lateral displacement of silty clay under cement-fly ash-gravel pile-supported embankments: Analytical consideration and field evidence

Based on back analysis of lateral displacements measured in situ by using the analytical solution, a useful method for estimating stress concentration ratio of geosynthetic-reinforced and pile-supported (GRPS) embankments was proposed. In order to validate the proposed method, a full-scale high-speed railway embankment (HSRE) with four instrumented subsections over medium compressibility silty clay was constructed in three stages. The soil profile, construction procedure and monitoring of settlements and lateral displacements of the four test sections were described. The field deformation analysis results show that 1) the combined reinforcement of CFG piles and geosynthetic layer perform well in terms of reducing lateral displacements; 2) the development of lateral displacements lags behind the increase of fill load, which can be attributed to the vertical load transfer mechanism of the pile foundation; and 3) pile length has a dominant effect on the stress distribution proportion between piles and surrounding soils. The comparison between predicted and experimental results suggests that the proposed analytical solution and the back analysis-based method are capable of reasonably estimating the lateral deformation and the stress concentration ratio, respectively, if the appropriate soil elastic modulus is chosen.

Shear Stress Discontinuities in Adhesive Joints

This paper deals with the stress discontinuities in shear stress distribution of adhesive joints. The three-dimensional finite element analysis (FEA) software was used to model the joints and predict the shear stress distribution along the whole beam. The FEA results indicated that there are stress discontinuities existing in the shear stress distribution within adhesive layer and adherends at the lower interface and the upper interface of the boded section. The numerical values of the shear stress concentration at key locations of the joints and the stress concentration ratio are discussed.

Influence of permeability of soft clay on the efficiency of stone columns and geosynthetic-encased stone columns – a numerical study

This paper presents the results of a series of numerical studies performed to evaluate the time-dependent behavior of ordinary stone column (OSC) and geosynthetic-encased stone column (GESC) under embankment loading with a special attention on permeability of soft clay. The results from the study indicate that during the embankment construction stage, the excess pore pressure generated in the surrounding soft clay for both OSC- and GESC-treated grounds was found to be the same. However, when the ground was allowed to consolidate, the load-carrying capacity of soft soil was improved because of the presence of stone column. The performance has been improved further when the stone column was encased with geosynthetic layer. The percentage reduction in the ultimate settlement of OSC and GESC when compared to soft clay was found to be 43 and 61%, respectively. A considerable reduction in the time taken for consolidation of GESC-treated ground over OSC-treated ground was also noticed. Effective stress concentration ratio (ESCR) (i.e. ratio of effective vertical stress on stone to that on the soft clay at the same depth) for GESC-treated ground was found to increase gradually during the entire period of consolidation; however, without geosynthetic encasement, it was found to increase up to certain consolidation period beyond which it started decreasing irrespective of soft clay permeability. A reduction in the value of ESCR implies stone column has failed to take the additional load and hence transferred the stress to the surrounding soft clay layer. This signifies the better performance of GESC compared to OSC. The result from the study indicates that the permeability of soft soil has greater influence on the generation and dissipation of excess pore water pressure and hence has a significant influence on the post-construction settlement.

Stress Transfer Mechanism in 2D and 3D Unit Cell Models for Stone Column Improved Ground

Stone column has been used widely to improve the foundation for many structures. Many designs of stone column are based on the unit cell concept. However, the intrinsic mechanism of stress transfer between the column and the surrounding soil has not been investigated thoroughly. This paper presents the important features of stress sharing mechanism in unit cell concept under an embankment loading. The arching effect, the deformation mode, the stress concentration ratio and the plastic straining in the unit cell are the main focus of this paper. Finite element software PLAXIS was used to examine these features. Unit cell was simulated as a two-dimensional (2D) axisymmetrical model and a representative three dimensional model in the numerical analysis. Drained loading condition was analyzed in this study in which the embankment is assumed to be built slowly with no excess pore pressure buildup. The change of the stress concentration ratio as the embankment height increases was also studied. From this study, it was found that the bulging happened near the column head accompanied by multiple shear bands progressing along the column. Generally, stone column in the unit cell shared about 4–5 times more the loads than the surrounding soils throughout the column depth. In most cases, 2D and 3D models give results that are similar to each other especially on the settlement performance and the failure mechanism.

Displacements of column-supported embankments over soft clay after widening considering soil consolidation and column layout: Numerical analysis

The common challenges for constructing embankments on soft clay include low bearing capacity, large total and differential settlements, and slope instability. Different techniques have been adopted to improve soft clay, such as the use of foundation columns including stone columns, deep mixed columns, and vibro-concrete columns, etc. Due to increased traffic volume, column-supported embankments may be widened to accommodate the traffic capacity need. Adding a new embankment to an existing embankment generates additional stresses and deformations under not only the widened portion but also the existing embankment. Differential settlements between and within the existing embankment and the widened portion may cause pavement distresses. Limited research has been conducted so far to investigate widening of column-supported embankments. In this study, a two-dimensional finite difference numerical method was adopted. This numerical method was first verified against field data and then used for the analysis of widened column-supported embankments over soft clay. The modified Cam-Clay model was used to model the soil under the existing embankment and the widened portion. Mechanically and hydraulically coupled numerical models were created to consider the consolidation of the foundation soil under the existing embankment and the widened portion. Different layouts of foundation columns under the existing embankment and the widened portion were investigated. The numerical results presented in this paper include the vertical and horizontal displacements, the maximum settlements, the transverse gradient changes, and the stress concentration ratios, which depended on column spacing. The columns installed under the connection side slope were most effective in reducing the total and differential settlements, horizontal displacement, and transverse gradient change of the widened embankment.

Evaluation of the Effect of Penetration Ratios on Composite Grounds Improved with Sand Compaction Piles

In this paper, centrifuge model tests were conducted in order to understand the deformation characteristics and behavior of sand compaction piles (SCPs) reinforced grounds in relation to area replacement ratios and penetration ratios. To simulate ground stress conditions, preliminary compaction was conducted to form grounds that maintained a certain level of strength. SCPs were installed in the grounds using compaction methods, and the relationship between loads and settlement as well as stress under rigid loading conditions were compared and analyzed. In addition, finite element analyses were conducted in order to verify the results of the centrifuge model tests and assess the effects of penetration ratios and depths on variations in stress. According to the results of the analyses, stress concentration ratios gradually decreased as depths increased, and the decreasing rate increased as penetration ratios decreased. However, in regions close to the surface layer in depth in which SCPs were installed, stress concentration ratios showed almost the same range regardless of penetration ratios. Stress concentration ratios showed proportional relations with penetration ratios. However, they showed similar values in regions close in depth to the surface of the ground. In particular, they showed very close ranges at penetration ratios of 100% and 80%.

Model test study on the behavior of geotextile-encased sand pile in soft clay ground

When constructing structures receiving high vertical and horizontal force such as port facilities, caissons and breakwaters on a soft ground on the sea or seashore, it is very important to increase the strength of the ground and inhibit excessive settlement. Recently, there have been numerous studies on the method of wrapping the outer wall of granular piles with geotextile or geogrid that has a certain level of tensile strength. Since the geotextile increases the strength of granular piles by confinement effects, those confined granular piles more advantages than the Conventional Sand Compaction Piles (SCP) in terms of the bearing capacity and settlement. Furthermore, this method consumes less aggregate compared to the conventional Sand Compaction Pile (SCP) or Gravel Compaction Pile (GCP) method. This study proposed to determine the characteristics of mechanical behaviors such as bearing capacity and stress concentration ratio of a composite ground improved using SCPs reinforced with geotextile (Geotextile-Encased Sand Pile, GESP) as an alternative to the conventional SCP method. With this purpose, several loading tests were conducted on the artificially sedimented clay grounds reinforced using conventional SCPs and GESPs with geotextiles of three different tensile strengths with various replacement ratios. Based on the results of this study, the bearing capacity of the soft clay ground reinforced by the GESPs is larger than that of the soft ground reinforced by the conventional SCPs and the failure mode of the GESPs is buckling different from the bulging of the SCPs. The tensile strength of the geotextile has little effect on the bearing capacity in the buckling failure of GESPs.

Influence of column yielding on degree of consolidation of soft foundations improved by deep mixed columns

Laboratory and field data showed that deep mixed (DM) columns accelerated the rate of consolidation of the soft foundations. Most analyses of consolidation of DM column-improved foundations so far have been based on the elastic theory. In reality, the DM columns may yield due to the stress concentration from the soft soil and its limited strength. The influence of column yielding on the degree of consolidation of the soft foundation improved by DM columns has not been well investigated. A three-dimensional mechanically and hydraulically-coupled numerical method was adopted in this study to investigate the degree of consolidation of the DM column foundation considering column yielding. A unit cell model was used, in which the soil was modeled as a linearly elastic material. For a comparison purpose, the DM column was modeled as an elastic or elastic-plastic material. This study examined the aspects of stress transfer, settlement, and degree of consolidation of the foundations without or with the consideration of the yielding of the DM column. A parametric study was conducted to investigate the influence of the column yielding on the stress concentration ratio, settlement, and average degree of consolidation of the DM column foundation. The stress concentration ratio increased and then decreased to reach a constant value with the increase of the column modulus and time. A simplified method was proposed to calculate the maximum stress concentration ratios under undrained and drained conditions considering the column yielding. The simplified method based on a composite foundation concept could conservatively estimate the consolidation settlement. An increase of the column modulus, area replacement ratio, and/or column permeability increased the rate of consolidation.

Numerical investigation on factors for deep-seated slope stability of stone column-supported embankments over soft clay

Stone columns have been commonly used as an alternative to solve deep-seated slope stability problems. Due to the complexity of a three-dimensional (3-D) arrangement of multiple columns, a 3-D problem has been commonly converted into a two-dimensional (2-D) model which has equivalent properties and dimensions, by the column-wall method and the equivalent area method. In this paper, two column-wall approaches based on matching either column geometry or column properties were compared and verified by 3-D numerical results in the stability evaluation of the stone column-supported embankment over soft soils. This study also investigated the 2-D numerical models using the column-wall method and the equivalent area method considering the factors of stress concentration, area replacement ratio, and soil conditions under short-term and long-term conditions. The numerical results show that the equivalent area method resulted in a continuous critical slip surface in the stone column-supported embankment over soft soil; however, no continuous slip surface developed using the column-wall method. Under the short-term condition, the computed factor of safety by the equivalent area model with or without considering the stress concentration effect was greater than that computed by the column-wall model. However, their difference became smaller under the long-term condition. The columns at certain locations along a prescribed slip surface from the equivalent area method did not mobilize their shear strengths under the short term condition. A reduction factor of 0.9 is suggested to correct the calculated factor of safety by the equivalent area method without considering the stress concentration ratio to that by the column-wall method under the short-term condition. No reduction factor (or the reduction factor of 1.0) is proposed under the long-term condition.

Experimental Studies on Bearing Capacity of Geosynthetic Reinforced Stone Columns

The stone column is a useful method for increasing the bearing capacity and reducing the settlements of foundations. The confinement provided by the native soft soils has an effective influence on the stone column bearing capacity. In this paper, laboratory tests were performed on unreinforced and reinforced geotextile-encased stone columns. Tests were performed on columns with diameters of 60, 80, and 100 mm and a length to diameter ratio of 5. Vertical encased stone columns (VESC) and horizontal reinforced stone columns (HRSC) were used for stone column reinforcement to investigate the effect of reinforcement type on the bearing capacity. The main objective of this research is to study the efficiency of VESC and HRSC under the same conditions for diameters of 60, 80, and 100 mm. Experimental results show that the bearing capacity of stone columns increases using vertical or horizontal reinforcing material. Moreover, the bearing capacity of reinforced stone columns increases by increasing the strength of reinforcement in both VESC and HRSC. Results show that bulging failure mechanism governed in all tests and that lateral bulging decreases using geotextiles and increasing strength of reinforcement. In addition, for both VESC and HRSC, the stress concentration ratio of the columns also increases.

Stress Discontinuities in Normal Stress Distribution of Adhesively Bonded Beams

This paper deals with the stress discontinuities in normal stress distribution of adhesively bonded beams. The 3-D finite element analysis (FEA) software was used to model the beams and predict the normal stress distribution along the whole beam. The FEA results indicated that there are stress discontinuities existing in the normal stress distribution within adhesive layer and adherends at the lower interface and the upper interface of the boded section. The numerical values of the normal stress concentration at key locations of the beams and the stress concentration ratio are discussed.

Experimental evaluation of stress concentration ratio of model stone columns strengthened by additives

The main objective of this paper is to find an experimental base for the value of the stress concentration ratio by manufacturing a model of a single stone column with rigid instrumented loading plates such that the total load applied to the model footing, and the load applied to the stone column can be measured alone. Model tests of soil treated with a single stone column and groups of two, three and four stone columns were tested. In addition, model tests were prepared with 10% cement as additive to backfill material for the columns. The backfill material was also modified by mixing 30% sand by weight with 70% crushed stone. It was found that the addition of the sand to the crushed stone indicated a marginal effect on stress concentration ratio in soil of shear strength cu = 12 kPa, but no clear effect was noticed during addition of sand to stone in treated soil having a shear strength cu = 6 kPa.

Bearing capacity of geosynthetic encased stone columns

The benefit of using stone columns in low strength soil has been proved as an efficient method to improve load-carrying characteristics of shallow footings. The stone column bearing capacity mainly depends on circumferential confinement providing by native soft soils. In this paper, some large body laboratory tests were performed on stone columns with diameters of 60, 80, and 100 mm and a length to diameter of 5. Both unreinforced and encased geotextile reinforced stone columns were tested. Vertical encased stone column (VESC) have been considered to investigate the effect of reinforcement on the footing load-carrying characteristics. The main objective of this research is to compare the effectiveness of vertical encapsulating of stone columns in the same conditions for various stone column diameters. In addition, tests on groups of stone columns with 60 mm diameter were carried out to investigate the effect of presence of neighboring columns on the reference loaded stone column. Results show that the stone column bearing capacity increases by using vertical reinforcing material. With increasing the length and strength of reinforcement in VESC, the stone column bearing capacity increases. In addition, the stress concentration ratio of columns also increases. Moreover, the lateral bulging decreases by using geotextiles. Numerical analysis based on finite element method (FEM) was also conducted to study scale effects on small stone columns tested and how to expand reinforcement effectiveness to large reinforced stone columns.

축소모형실험에 의한 반복하중 작용시 지오그리드 감쌈 쇄석말뚝의 비배수 거동 특성

본 논문에서는 반복하중 작용시 연약지반에 설치된 토목섬유 감쌈 쇄석말뚝의 비배수거동특성을 다루었다. 모형실험을 통해 점성토 지반에 적용된 지오그리드 감쌈 쇄석말뚝의 침하량, 과잉간극수압, 응력분담비 그리고 지오그리드 변형률의 변화특성을 기존의 쇄석말뚝과 비교하여 분석하였다. 실험결과를 통하여 연약지반에 설치된 GESC는 단기하중 재하시 쇄석기둥의 변형을 토목섬유의 인장력으로 구속 시켜 변형저항력의 증가와 침하저감 효과가 있음을 알 수 있으며 지오그리드의 최대 변형률은 1.2D와 1.5D에서 발생하는 것으로 나타났다. 본 논문에서는 반복하중 재하시 지오그리드 감쌈 쇄석말뚝에서의 지오그리드 감쌈 효과와 치환율이 쇄석말뚝에 미치는 영향을 고찰하였다. This paper presents the results of a laboratory investigation into a study on undrained characteristics of a geogrid-encased stone column (GESC) installed in soft clay under cyclic load. In order to analyze behavior of settlement, pore water pressure, stress concentration ratio and strain of the GESC compared to a stone column, a series of reduced-scale laboratory tests were performed. The model tests show that GESC provides a simple and effective method of deformation resistance and settlement restraint when a short-term cyclic load is applied. The maximum strain of geogrid occurred at 1.2D and 1.5D from the top of the column. This paper highlights the importance of considering overlay effect and replacement ratio on cyclic load supporting GESC.

Numerical analysis of consolidation of soft soils fully-penetrated by deep-mixed columns

Deep mixing is a common ground improvement technology that can be used to increase bearing capacity and stability and reduce total and differential settlements of foundations constructed on soft soils. Field data have shown that deep mixed column foundations had a higher consolidation rate than untreated foundations even though deep mixed columns had similar or lower permeability than untreated soils. The consolidation mechanisms and behavior of deep mixed column foundations have not been well understood. In this study, the consolidation of soft soils fully penetrated by deep mixed columns was investigated using a mechanically and hydraulically coupled three-dimensional finite element method. One quarter of a unit cell was used considering its symmetry and the column and the surrounding soil were modeled as elastic materials. A case study for a stone column foundation in the literature was first used to verify the numerical model and then this model was adopted to analyze the stress transfer, settlement, and consolidation of the soft soil fully penetrated by a deep mixed column. A parametric study was conducted to evaluate the influence of four key factors, soft soil thickness, area replacement ratio, column modulus, and column permeability, on the stress concentration ratio (the ratio of the average vertical stress on the column to that on the soil), settlement, and average degree of consolidation of the deep mixed column foundation. The numerical results show that the average degree of consolidation calculated based on the settlement or the excess pore water pressure was identical. The stress concentration ratio increased with the column modulus and time, but the effect of the soft soil thickness, area replacement ratio, and column permeability was not significant. The simplified method based on a composite foundation concept could conservatively estimate the consolidation settlement. An increase of the column modulus, area replacement ratio, and/or column permeability increased the rate of consolidation.