Behaviour of Stone Columns Research Papers (original) (raw)

2025, Proc ICSMGE 2005: …

A significant part of the consolidation of soft clay fitted with vertical drains (or stone columns) occurs during a step by step embankment loading. In order to schedule the loading program, it is necessary to predict the settlement evolution during the construction
process. A unit cell model, made up with one column surrounded by saturated soft clay overlaid by drained layer, is considered with
oedometric conditions. An analytical poroelastic solution is derived that provides, in addition to the degree of radial consolidation and
excess pore water pressure dissipation, the evolution with time of the reinforced soil settlement. The latter is predicted by introducing
the concept of “equivalent membrane” assuming a uniform excess pore water pressure in the soft clay. The Barron’s factor time of radial consolidation is also adopted that provides the determination of the permeability of this “equivalent membrane”. The results of
the proposed method are illustrated as a function of the substitution factor.

2025, ASCE

The performance of floating stone columns (FSCs) has been recently adopted to improve thick soft clay deposits, but their behavior is not yet fully understood. To provide a comprehensive understanding, field load tests were conducted with varying area replacement ratios and length-to-diameter ratios (L/D) at Misono-cho, Suzuka City, Mie Prefecture, Japan. Bulging failure modes were observed using strain gauges, and stress sharing between the stone columns and the surrounding soil was monitored using earth pressure cells. The PLAXIS 3D FEM program was used to conduct finite-element computations on 10-noded tetrahedral elements. The FEM model was calibrated and adjusted to account for the installation effects of FSCs and aligns closely with the outcomes of the field test. The settlement improvement factor for FSCs was found to be slightly higher in the long term compared to the short-term condition. Moreover, the settlement improvement factor increases with the applied stress and over time. The ultimate bearing capacity of FSCs depends on the additional confinement provided by adjacent columns. An improved model and design charts were proposed to estimate the ultimate bearing capacity and settlement of FSC-improved ground, showing lower relative error compared to other published results. Finally, a design procedure was anticipated to assist geotechnical engineers in designing FSCs constructed in soft clay.

2024, Earth and Environmental Science

Soft clayey soils present a geotechnical challenge due to their low shear strength and high compressibility. The finite element package, ABAQUS, is used to perform a comprehensive numerical analysis on stone columns. The study investigates the bearing capacity and vertical and lateral deformations of soft clayey soils due to installing stone columns in them. The numerical model using the unit cell concept investigates the influence of varying two main physical parameters, i.e. the stone column material strength (ϕ), and the native soil cohesion (c). Moreover, this study utilizes variable numerical models to study the effect of the ratio between column length and its diameter, (L/d), on the efficiency of stone columns as a soil-enhancing method for weak clayey soil conditions. The influence of the stone column length on the occurred failure mode was studied. The outcomes demonstrated that using stone columns improves the behaviour of soft clayey soil, where the bearing capacity is enhanced and the soil surface settlement is reduced. Furthermore, using stone column filling materials with higher friction angles enhanced the stability of the system, and the optimum values of native soil cohesion and stone column materials friction angle are 30 kPa and 40°, respectively. Also, the outcomes illustrated that increasing the applied pressure controls the bulging length of SC, where the bulging length was 2.5d, 3d, and 3.5d for the applied pressure of 50.0 kPa, 100.0kPa, and 150.0 kPa, respectively.

2024, International Journal of Geosynthetics and Ground Engineering

2023, IOP Conference Series: Earth and Environmental Science

This study investigates the effectiveness of encapsulated polypropylene (PP) column in enhancing the undrained shear strength of kaolin (soft clay). The usage of PP in treating problematic soil is a more sustainable and cost-effective alternative compared to other materials. The installation of granular column can be done by using vibro-replacement method. Several geotechnical tests to determine the properties of materials were conducted. The shear strength of treated kaolin sample was examined by using Unconfined Compression Test (UCT). There are seven (7) batches of soil sample in total which included a control sample, three (3) batches of 14 mm and three (3) batches of 20 mm diameter PP column. Different diameters of PP column were examined with 60 mm, 80 mm and 100 mm height, respectively with soil sample of 50 mm in diameter and 100 mm in height. The shear strength improvement of kaolin is 33.82%, 46.51%, and 49.88% when implanted with a PP column with a 7.84 area replacement r...

2023, E3S Web of Conferences

This study used a finite element analysis approach employing Plaxis 3D to analyze the stress concentration ratio, a critical parameter in geotechnical engineering, to examine stresses operating on stone columns and soft soils. This study also looked at the effect of the stiffness ratio between the stone column and the neighboring soil. With the same length and three different diameters, 0.8 m, 1.0 m, and 1.2 m, or three area replacement ratios ranging from 7% to 16%, respectively, floating and end-bearing stone columns were used. The influence of soft soil undrained cohesion, cu ranging from 6 kPa to 40 kPa, was also considered in the current study. The stiffness ratio for columns to adjacent soil, end bearing or floating stone column, and area ratio all have a significant impression on the performance of the stone column in treating soft soil and stress transmission mechanisms in the enhanced soil body, according to parametric studies. The average stress concentration ratio in soil...

2023, International Journal of Geotechnical Engineering

Stone columns are one of the common types of ground improvement methods that applied to reduce the settlement and increase the stability of structures. This paper focuses on the non-uniform diameters of the stone column and aimed at providing some practical insights into the design by reducing the constructing material of the stone column with increasing the bearing capacity of the soft soil. In this study, two dimensional (2D) Finite Element analyses through PLAXIS 2D were performed to investigate the bearing capacity of uniform and non-uniform stone columns in soft soil. A Mohr-Coulomb constitutive soil model was adopted in the numerical analyses. The non-uniform stone columns modelling by creating two different diameters and lengths. The upper diameter d 1 was always larger than the lower half diameter d 2 with investigated ratio d 2 : d 1 of 1:2, 1:4 and 1:5. The columns were 10 m long but varied in diameter length with nine different length ratios (l 1 : l 2). By comparing between uniform and non-uniform shapes, one can determine the optimum column's design. The highest bearing capacity obtained by the non-uniform stone column was one with the d 2 :d 1 ratio equals to 1:5. The most economical shape for the stone column (the least volumes used to construct stone columns) was achieved at a ratio of d 2 :d 1 = 1:2 and a length ratio of l 1 :l 2 = 3:7.

2023, Sustainable Civil Infrastructures

Mumbai marine clay is problematic in nature for substructures and it needs to be strengthening before making it available for any construction activity. Out of many available geotechnical solutions, stone columns are quite handy in significantly improving the bearing capacity of marine clay, which also supplements by dissipating excess pore water pressure and reducing the settlements of clay beds under external loads. For the investigation on behavior of stone columns, marine clay was collected from the Uran site near Mumbai, India. Slurry consolidated clay beds were prepared and gravity loading mechanism was adopted to consolidate the clay beds in the laboratory. Laboratory tests were performed to ascertain the uniformity and repeatability of the clay beds by slurry consolidation. Augers of 50, 75 and 100 mm diameter were used to bore the holes in the clay bed and aggregates were used for stone column preparation by replacement method. The static displacement controlled tests were performed on the stone column reinforced clay bed. In the present study, the loading is applied on stone column alone, as it leads to the ultimate axial capacity of stone column improved ground. Clay bed reinforced with stone column exhibits improved load-settlement response, compared to unreinforced clay beds. The improvement was very significant at higher settlements. The failure load of reinforced clay bed is around 6 times more than the unreinforced clay bed. In order to further understand the behaviour of stone column reinforced ground, numerical modeling of stone column reinforced clay bed is conducted using FLAC 3D. Using the validated numerical model of the stone column, the effect of variation of the diameter of stone column on its performance was studied. From the numerical analysis and physical tests conducted in laboratory, it is observed that the stone columns with smaller diameter when subjected to vertical loading carried higher bearing pressures when compared to that of the larger diameter stone columns, which may be due to the greater confining and larger bulge formed at a depth of 2-3 times the diameter of stone columns.

2023

The foundations at the Huey P. Long Bridge in New Orleans presented special challenges for construction due to the requirements for very deep drilled shafts in the Mississippi River beneath the existing bridge structure. Drilled shafts were constructed to depths of approximately 200ft in alluvial soils using a rotator system with full-length segmental casing, and base grouted to improve axial resistance. This paper describes the design and construction of the work platform to provide access to the restricted-access site beneath the existing bridge, the approach used to construct the drilled shafts, and the results of a load test program to verify the axial resistance.

2022, SN Applied Sciences

Lime column (LC) is an effective method for ground improvement which causes reduction in settlement and enhancement in bearing capacity of soft soils, subsequently resulting in economical foundations. This study investigates the effects of LC on soft soil with the help of laboratory tests. Firstly, basic laboratory tests are performed on natural soil followed by load deformation tests on untreated and LC treated soils. LC effects on natural soil are studied by changing various parameters like shear strength of soil (54 kPa, 32 kPa, 14 kPa), slenderness ratio (4, 5.5, and 7) and spacing to diameter ratio (2, 3). Experimental results show that LC significantly increases load carrying capacity and stiffness for soils having higher shear strength. For soil samples of 54 kPa and 32 kPa, the load carrying capacity increases by 48 and 21% respectively whereas 75% increase in average stiffness (β = 1.75) is observed. However, LC are found to be ineffective at very low shear strength of 14 kPa. In case of group LC, increasing column spacing from 2 to 3D (D: diameter of LC) significantly decreases load carrying capacity of treated soil. It is also observed that increase in LC slenderness ratio causes a decrease in ultimate load carrying capacity. Article highlights • Lime columns can be used to increase load carrying capacity of soft soils, especially in soils having relatively higher shear strength. • In case of group columns, the spacing should not be more than two times the diameter of the column. • The length of the lime columns should not be more than four times the diameter for better results.

2022

This paper presents two set of 2D finite element analyses that study on the settlement ratio of floating stone column for small and large column groups. The settlements of floating stone columns are difficult to be predicted especially for large column groups. Unit cell model was used in this study for large column group assuming infinite column grid. The influence of key parameters such as column length, area replacement ratio, loading intensity and post installation lateral earth pressure on the settlement ratio were investigated. Based on the results, a simplified solution was proposed to calculate the settlement ratio for floating stone columns. On the other hand, the settlement performance of floating stone column in small group was examined for different area replacement ratio and column length. Design chart developed from this study was validated by a case study where a reasonable agreement was obtained.

2022, ICWRDEP 2021

2022, The Open Civil Engineering Journal

Background: In weak clay soil, a proper ground improvement technique using a stone column can be limited by the absence of sufficient lateral confining pressure. Stone columns should be strengthened to provide the minimum required lateral confining pressure. Objective: The aim of this study is to find out the significant improvement of the composite stone columns compared to the conventional stone columns by comparing the bearing capacity enhancement and the treated soil stiffness for both models. Composite stone columns with a solid concrete part at the top-head not only enhance the bearing capacity of the stone columns but also decrease the bulging failure and increase the surrounding soil stiffness. Methods: The 2D finite element analyses were carried out to simulate an experimental study conducted by Ambily and Gandhi on conventional stone columns. ABAQUS software program with the Mohr-Coulomb criterion for soft clay soil and stones was used in the simulation. First, a prelimina...

2022

Soft Clay is often known to be a problematic soils in construction because of its low strength and high compressibility characteristic. Before construction begins, ground improvement needs to be done to improve the soil bearing capacity so that the superstructure can be placed on top of it and reduce settlement and consolidation. In this study, the soft clay is going to be reinforced with crushed brick column which is part of construction waste produced during construction stage to achieve sustainable construction. This study was aimed to investigate the effectiveness of single and group crushed brick column in improving the shear strength by using laboratory scale model. Kaolin is being used as soil sample while crushed brick as the reinforced column. Few laboratory tests are conducted to determine the properties of kaolin clay and crushed brick. Unconfined Compression Test (UCT) also used to test the shear strength of the reinforced kaolin samples The improvement of shear strength of single crushed brick column with area replacement ratio of 4.00% (10mm column diameter) are 3.34%, 4.60% and 1.07% at sample penetration ratio, Hc/Hs of 0.6, 0.8 and 1.0 respectively while for area replacement of 10.24% (16 mm column diameter) are 7.56%, %, 16.37% and 4.97% at the same penetration ratio.. The result shows that the improvement shear strength for height penetration ratio of 1.0 is 13.33% which was the highest, while 0.6 and 0.8 are 12.31% and 9.79% respectively for group crushed brick columns with diameter 10mm. The improvement of shear strength is highest for height penetration ratio of 1.0 with 16.10% improvement which was slightly higher than 0.6 and 0.8 which were 13.49% and 11.35% respectively in sample reinforced with grouped columns with diameter 16mm. It can be concluded that the shear strength of soft clay could be improved by installation of single and group crushed brick column.

2022

Kochi is underlain by deep soft marine clay deposit. These clay layers are highly compressive and of very low shear strength. In order to develop infrastructure like Ports, Transshipments terminals, highways in such type of soil, it is necessary to provide suitable foundation technique. Ground improvement using vibro replacement is the one of the suited technique to increase the shear strength of soils and to reduce the compressibility. This paper presents case history of application of vibro replacement columns to support Jetty facilities at ICTT (International Container Transshipment Terminal) at Vallarapadam port and to support the connecting 4 lane highway to ICTT. The various aspects of sub soil conditions, design, construction methodology, quality control and test results are discussed in this paper.

2022, Journal of Geotechnical and Geoenvironmental Engineering

The inclusion of granular columns in soft clay deposits leads to improvements in bearing capacity and overall stiffness, along with a reduction in consolidation settlement. Many laboratory investigations have focused on aspects of bearing capacity, but published data on settlement performance are limited. This paper reports on some interesting findings obtained from a laboratory model study with respect to these issues. In this investigation, 300 mm diameter by 400 mm long samples of soft kaolin clay were reinforced with single or multiple granular columns of various lengths using the displacement and replacement installation methods. The experimental findings revealed that, for the same area replacement ratio, limited settlement reduction was achieved for single long floating columns and end-bearing column groups. Marginal improvements in settlement performance were also achieved for columns installed by the displacement method. No settlement reduction was achieved for short single floating columns, whereas short floating granular column groups produced increased settlements. These observations were verified using contact pressure measurements between the footing and column/surrounding clay.

2022, Journal of Geotechnical and Geoenvironmental Engineering

Deep excavations may cause settlement and damage to adjacent buildings, even if they are found on piles. The corresponding pile deformations are determined by axial and lateral effects. This paper describes an analytical model relating axial pile deformation to the vertical soil displacement resulting from the deep excavation and also suggests ways to determine the pile response to lateral displacements. The axial pile-soil interaction is clearly different for end-bearing and friction piles. Common generalizations that end-bearing piles settle the same as the soil settlement at the base level and friction piles with the ground surface settlement present lower and upper bounds, which are only valid for certain idealized cases. The settlement of piles with a large component of shaft friction is determined mainly by the actual load on the pile relative to the pile ultimate capacity. The lateral pile response is governed mainly by the relative stiffness of the pile to the soil. The proposed model was validated with measurements of the North South Line project in Amsterdam.

2022, International Journal of Geomechanics

Ground improvement via stone column has gained popularity around the globe as an effective ground improvement technique to improve the load bearing capacity of soft soils. End-bearing stone columns are commonly used but occasionally floating stone columns are adopted as it has been proven to achieve the necessary load carry capacity and serviceability requirements. However, stone column parameters are often estimated without much basis for simulation and design purpose due to lack of practical research and real-life case studies in this area. Hence, in order to develop fundamental understanding on the load-settlement responses of the floating stone columns in soft clays, detailed back-analysis was performed in this paper. Throughout the back-analysis, all the numerical models were validated by comparing the analysis results with the field measurements. This study highlights (i) the existence of some critical interpreted parameters, namely soil cohesion, c', friction angle, ' and dilation angle, ' as well as (ii) soil-structure interaction mechanism that influences the load-settlement responses of the floating stone columns. The successful characterization of the stone column materials and the modeling technique of stone columns in finite element analyses have shown great potential in assessing the real-life performance of floating stone columns in soft clays.

2022, Acta Geotechnica

Numerous approaches exist for the prediction of the settlement improvement offered by the vibro-replacement technique in weak or marginal soil deposits. The majority of the settlement prediction methods are based on the unit cell assumption, with a small number based on plane strain or homogenization techniques. In this paper, a comprehensive review and assessment of the more popular settlement prediction methods is carried out with a view to establishing which method(s) is/are in best agreement with finite element predictions from a series of PLAXIS 2D axisymmetric analyses on an end-bearing column. The Hardening Soil Model in PLAXIS 2D has been used to model the behaviour of both the granular column material and the treated soft clay soil. This study has shown that purely elastic settlement prediction methods overestimate the settlement improvement for large modular ratios while the methods based on elastic-plastic theory are in better agreement with finite element predictions at higher modular ratios. In addition, a parameter sensitivity study has been carried out to establish the influence of a range of different design parameters on predictions obtained using a selection of elastic-plastic methods.

2021, SN Applied Sciences

2021, Geotechnical and Geological Engineering

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.

2021

Experimental investigation was carried out to ascertain the behaviour of stone column under load in soft clay bed. The group effect of the stone column was also studied in this investigate as the stone column reinforced bed was prepared in group. Based on the experimental results pressure settlement response of the stone column reinforced clay was studied. The experimental data was further used for regression analysis to fit the equation for bearing capacity of the improved soft clay bed. Both the linear and nonlinear regression was carried out and the best suited regression model was presented here in this paper. It is observed that the non linear regression model is best fitted for the stone column reinforced clay bed.

2021

A series of model footing load tests were carried out on floating stone columns reinforced clay bed to understand the load deformation behavior. All the experiments were carried out on stone columns in groups to understand the group effects and interactions. The parameters varied in this experimental investigation were length and spacing of the stone column without changing the undrained shear strength of the clay. The undrained strength of clay bed was kept constant as 5kPa in all the tests. In this paper the results obtained from experimental investigations were analysed in terms of load carrying capacity, deformation behaviour of stone column and the interaction amongst the stone columns. It was observed that the bearing capacity of the stone column reinforced clay bed can withstand a load more than 4 times that of the soft clay bed alone. The optimum length and spacing of the stone column were obtained from experimental results and strong interaction has been observed for too small spacing of stone column.

2021, Acta Geotechnica

2021, IOP Conference Series: Earth and Environmental Science

Pumice and scoria are pyroclastic rocks that can be found where volcanic mountains appear. In Indonesia, pumice and scoria are found in large deposits in the area of Mount Kelud at Kali Putih River in Blitar Regency, Province of East Java. Thus, it is important to evaluate their potential uses as alternative backfill materials. Standard Proctor compaction tests were conducted to determine the maximum dry density (MDD) and optimum moisture content (OMC) to evaluate their compaction properties. Constant head permeability tests were also conducted to determine the coefficients of permeability. Direct shear tests were also applied to investigate the shear strength characteristics of these materials. The results showed that the MDD values of scoria are slightly higher than pumice. The OMC values of pumice and scoria are 23% and 15.5%, respectively. The internal friction angle of pumice and scoria are higher than natural sand, which are approximately 55-64º and 46-59º respectively. Pumice and scoria with different relative densities had coefficients of permeability that ranged from 0.0003 to 0.002 cm/s and 0.001 to 0.002 cm/s respectively. Due to their mechanical properties, it can be concluded that these materials have potential as alternative backfill materials.

2021, Applications of Computational Mechanics in Geotechnical Engineering V

This paper presents the numerical simulation using an elastoplastic analysis of the bearing capacity of shallow foundations. The problem involves axisymmetric conditions on reinforced soil using finite element method (FEM). The foundation soil is modeled as a nonassociative elastoplastic Mohr-Coulomb material. The reinforcement is modeled as a linear elastic material. The ultimate bearing capacity obtained in this study is compared to solutions obtained using limit equilibrium and limit analysis. A parametric study was conducted for different configurations of reinforcement for a special case of frictionless foundation soil. The numerical results show good agreement with analytical results indicating the suitability of the numerical model used in this study and implemented into the code ANLOG-Non-Linear Analysis of Geotechnical Problems.

2021

In geotechnical engineering, the analysis of small footing founded on limited number of stone columns is normally treated as 3D problem. However, the 3D analysis requires extensive computational effort and time compared to 2D analysis. Therefore, in this study, a series of numerical analyses are performed to investigate the reliability of the 2D axisymmetric concentric ring model to be used in problems where stone columns are adopted as ground improvement method to support a small foundation. In this approach, a group of stone columns was modeled as a series of concentric rings of the same area replacement ratio as actual columns. The results of the 3D finite element model for stone column reinforced foundation provide the basis for this numerical comparison. End bearing column system was tested first followed by floating column system. The deformation characteristic of the footing under different geometry configuration and area ratio demonstrate the feasibility of the concentric ri...

2021, IOP Conf. Series: Earth and Environmental Science

The emphasis of current work is on assessing the settlement improvement ratio, which is described as the ratio between the settled soils treated with a stone column and the settlement of the non-treated soil (Sr = Streated/Suntreated). The research was conducted using a 300 mm diameter and 300 mm high stone-column container testing model. On 14 modeled stone columns made only from crush stones and using various backfill content, model tests including static axial compression tests were performed. The substance used in the stone backfill column had been changed by sand or lime or cement percentages. The shear strength prepared by the containers varied between 5.5 kPa and 13.5 kPa. Results show that the settlement ratio values, Sr achieved with crush stone, crushed stone +50% sand, crushed stone +5% dry lime, crushed stones +10% dry lime, crushed stone +2.5% cement +5.0% crushed stone +5.0% cement, respectively, was 0.23, 0.12, 0.16, 0.15 and 0.09. In other words, there is a drop in the settlement from 77% to 91%.

2021, Acta Geotechnica

2020, International Journal of Academic Engineering Research (IJAER)

Sabkha soils are widely distributed throughout Saudi Arabia, especially along the coastal areas. Typical problems encountered in foundations and structures built over Sabkha beds. The use of stone columns as a technique of soil improvement is frequently implemented in Sabkha soils. Besides, their use in Sabkha soils has been found to provide moderate increases in load carrying capacity accompanied by significant reduction in settlement. In this paper a case study chosen from the Eastern Saudi Arabia is studied in detail to investigate the performance behavior of ground improved with stone columns. The stone columns were constructed using the wet method. Comparison between Pre-and Post-Cone Penetration Test CPT was conducted in this study to discuss different methods that are used to assess the improvement of the soil layers upon installation of stone columns.

2018

The current rate of development in the construction industry has given rise to higher demands for land. In pursuit of satisfying the needs of property developers, engineers have resorted to new ground improvement technologies to eventually implement construction on lands which were previously regarded as unfeasible or uneconomical for this purpose. This study therefore aimed at investigating the potential use of singular stone columns in improving a soft wet clay of South African origin. Bench scale tests were conducted to evaluate the effect of the moisture content of the base soil and the column diameter. The results indicated a clear improvement in the vertical applied stress as well as in the settlement of the clay, when subjected to a compressive load. These findings were ultimately used to generate information regarding the stress concentration ratio and the settlement reduction ratio.

2018, Geotechnical Engineering Journal of the SEAGS & AGSSEA Vol. 49 No. 1 March 2018 ISSN 0046-5828

Soft clay deposits are globally widespread and often coincide with strategic transport links and growing urban developments. These soft deposits are often waterlogged and are composed of clay with varying degrees of silt, sand and organic matter. These soils have low undrained shear strength and high compressibility, contributing to construction problems in relation to stability and settlement. Granular columns, also referred to as flexible piles, are one of the techniques widely considered in the industry for improving soft deposits for low-moderate structural loading. The purpose of this article is to highlight some of the key investigations carried out in the topic of granular columns at Queen's University Belfast, the UK. The investigations focused on several aspects: (a) the interaction between columns and surrounding clay (b) containment of columns in geo-grid for enhanced strength performance (c) settlement performance under single or multiple column configuration (d) stress distribution under the footing and along the column (e) assessment of consolidation and creep settlement under constant loading and (f) granular columns for anchoring purposes and therefore stabilization of slopes. Overall observations are: settlement improvement factors were moderate under isolated loading, but granular columns are very effective in providing pull-out capacity in the form of anchors.

2017

Stone columns have proved to be the most suited technique for improving the bearing capacity of weak or soft soils. Being cost effective and environmental friendly, stone columns are used worldwide for supporting flexible structures such as embankments, oil storage tanks etc. which can tolerate some settlements. However, when the stone columns in very soft soils are loaded; stone columns undergo excessive settlement due to low lateral confinement provided by the soft soil, leading to the failure of the structure. The excessive settlements may be avoided or at least reduced to a great extent, when the columns are encased with a suitable geosynthetics. In the present study, the effect of reinforcement on bearing capacity of composite soil has been investigated by conducting laboratory plate bearing tests on model floating and end-bearing single stone columns of 30 mm diameter and 300 mm length. The columns were reinforced by providing geosynthetic encasement over varying column length...

2017

As in piles, the efficiency of a group (E g) of stone columns is defined as the ratio between the capacity of the group to the capacity of each stone column in the group multiplied by single stone column capacity. In this paper, the group efficiency of 24 model stone columns installed in soft clay is considered. These groups consist of 2, 3 and 4 columns. The tests were conducted on stone columns with length to diameter ratio (L/D) of 6 and 8. A laboratory setup was manufactured in which two proving rings were used to measure the total load applied to the soil-stone column system and the individual load carried directly by the stone column. The foundation steel plates have 220 mm diameter and 5 mm thickness. These plates contain 1, 2, 3 and 4 holes, respectively. The spacing between all holes equals twice the stone column diameter (D), center to center. The stone column capacity is taken as the load corresponding to a settlement equals to 50% of the diameter of stone column. The results illustrated that the group efficiency decreases with increasing the number of stone columns, also the stone columns with L/D of (8) provided higher efficiency than those with L/D of (6).

2016

Two analytical closed-form elastic-rigid-plastic solution methods to predict the rigid foundation behaviour on stone-column reinforced soil are compared, both of which takes into account the stone material yield within the host soil according to stress-dilatancy theory. These are the mechanistic methods developed by the author (2004, 2005) [that presently incorporates the dilatancy angle as per Schanz and Vermeer (1996)] and that developed by Pulko and Majes (2006), that takes into account the column yield as per Rowe dilatancy theory. Since optimal design of stone column necessitates optimum stress concentration on column, which is generally composed of dense gravel with/or without sand, the vertical stress on granular column is often close to its peak strength and the material dilates. To arrive at closed-form analytic solution to this complex soil-stone column interaction problem, some common assumptions like axi-symmetric soil-stone column 'unit-cell', elastic host soil and rigid-plastic Mohr-Coulomb stone column material are combined with equilibrium and kinematic conditions. The results reflect the beneficial effect of dilatancy within the optimal range or techno-economic domain of area-ratio. The settlement predictions are compared with other analytical methods available in literature as well as with measured settlement reduction of stone-column reinforced ground at different subsoil conditions around the globe reported in literature.

2016

The stone columns improve the performance of foundations on soft and loose soil due to the ability of composite ground to sustain increased structural loads under reduced settlements. The interaction between the two basic elements: the ambient subsoil and installed column, present a complexity of behaviour, both in terms of applied stresses and resulting strains. Moreover, as the same is provided in-group in a regular array beneath foundations, the performance of an individual column is likely to be influenced by the presence of neighbouring columns. The present paper addresses this very issue. Analysis of reported field hydro-test data of large diameter oil storage tanks in soft ground in reference to some existing theories led to the development of the stone-column ‘group efficiency factor’. In order that such factor is of general use, it was felt necessary to examine the applicability of the factor to different sizes of groups under varied subsoil conditions. With this in view, some model test results reported in literature of single and group of columns have been analysed and in the process an explicit relationship regarding the stone-column ‘group-effect’ could be established.

2016

The stone columns improve the performance of foundations on soft and loose soil due to the ability of composite ground to sustain increased structural loads under reduced settlements. The interaction between the two basic elements: the ambient subsoil and installed column, present a complexity of behaviour, both in terms of applied stresses and resulting strains. Moreover, as the same is provided in-group in a regular array beneath foundations, the performance of an individual column is likely to be influenced by the presence of neighbouring columns. The present paper addresses this very issue. Analysis of reported field hydro-test data of large diameter oil storage tanks in soft ground in reference to some existing theories led to the development of the stone-column 'group efficiency factor'. In order that such factor is of general use, it was felt necessary to examine the applicability of the factor to different sizes of groups under varied subsoil conditions. With this in view, some model test results reported in literature of single and group of columns have been analysed and in the process an explicit relationship regarding the stone-column 'group-effect' could be established.

2016

–In finite element analysis, mesh size is a critical issue. It closely relates to the accuracy, computing time and efforts required for meshing of finite element models, which determines their complexity level. This paper presents study of the effects of mesh size on accuracy of numerical analysis results. Based on these results the guidelines for choosing the appropriate mesh strategy in finite element modelling are provided. The static and buckling analysis is carried out to know the effects of mesh sizes by using Femap and NX-Nastran. The model under study is of a structure made up of steel plate. Index Terms— Finite element analysis, mesh size, static analysis, buckling analysis. I. INTRODUCTION In finite element analysis (FEA), the accuracy of the FEA results and required computing time are determined by the finite element size (mesh density). According to FEA theory, the FE models with fine mesh (small element size) yields highly accurate results but may take longer computing time. On the other hand, those FE models with coarse mesh (large element size) may lead to less accurate results but smaller computing time. Also, small element size will increase the FE model's complexity which is only used when high accuracy is required. Large element size, however, will reduce the FE model's size and is extensively used in simplified models in order to provide a quick and rough estimation of designs. Due to its importance, in generating FEA models, the foremost problem is to choose appropriate elements size so that the created models will yield accurate FEA results while save as much computing time as possible. The objective of this paper is to present guidelines for choosing optimal element size for different types of finite element analyses. In order to achieve that goal, in this study, a series of static, and buckling analyses were performed on a structure model made up of plates to reveal the effects of the element size on the accuracy of the FEA results. The solver NX-Nastran and Femap pre and post-possessor, used for modeling and analyses involved in this work.

2016

2015

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Tunnel Engineering – investigation, planning, design, documentation and construction supervision of tunnels for roads, rail, power supply, water supply and sewerage systems.

2015, Geotextiles and Geomembranes

This paper presents a newly developed design method for non-encased and encased 27 stone columns. The developed analytical closed form solution is based on previous 28 solutions, initially developed for non-encased columns and for non-dilating rigid-plastic 29 column material. In the present method, the initial stresses in the soil/column are taken 30 into account, with the column considered as an elasto-plastic material with constant 31 dilatancy, the soil as an elastic material and the geosynthetic encasement as a linear-32 elastic material. To check the validity of the assumptions and the ability of the method 33 to give reasonable predictions of settlements, stresses and encasement forces, 34 comparative elasto-plastic finite element analyses have been performed. The agreement 35 between the two methods is very good, which was the reason that the new method was 36 used to generate a parametric study in order to investigate various parameters, such as 37 soil/column parameters, replacement ratio, load level and geosynthetic encasement 38 stiffness on the behaviour of the improved ground. The results of this study show the 39 influence of key parameters and provide a basis for the rational predictions of settlement 40 response for various encasement stiffnesses, column arrangements and load levels. The 41 practical use of the method is illustrated through the design chart, which enables 42 preliminary selection of column spacing and encasement stiffness to achieve the desired 43 settlement reduction for the selected set of the soil/column parameters. 44 45

2015

Numerous analytical approaches exist for the prediction of the settlement improvement offered by the installation of granular columns in weak or marginal soil deposits. In this paper, an appraisal of some of the more popular settlement prediction methods applicable to vibro-replacement design is carried out. The settlement improvement factors calculated using the different design methods are compared with both the 'bottom feed' and 'top feed' data from a previously published database of settlement improvement factors for stone columns in soft clays and silts. The calculated improvement factors are plotted as a function of the main variables in vibro-replacement design, namely the area-replacement ratio (a measure of the amount of insitu soil replaced with stone), and the modular ratio, which relates the stiffness of the column to that of the in-situ soil. In addition, finite element analyses have been carried out using PLAXIS 2D for comparison with both the field data and the design method predictions. Load settlement behaviour (primary settlement) has been analysed using the Hardening Soil Model to model the behaviour of both the column material and the treated soil. The majority of the design methods appear to predict similar settlement improvement factors. Purely elastic methods overestimate the settlement improvement factor for large modular ratios while it appears that better matches between finite element and analytical predictions are found at higher modular ratios, as the analytical methods assume a significant bulging mechanism which is more prevalent in soft soils.

2015, Acta Geotechnica

Numerous approaches exist for the prediction of the settlement improvement offered by the vibroreplacement technique in weak or marginal soil deposits. The majority of the settlement prediction methods are based on the unit cell assumption, with a small number based on plane strain or homogenisation techniques. In this paper, a comprehensive review and assessment of the more popular settlement prediction methods is carried out with a view to establishing which method(s) is/are in best agreement with finite element predictions from a series of PLAXIS 2D axisymmetric analyses on an end-bearing column. The Hardening Soil Model in PLAXIS 2D has been used to model the behaviour of both the granular column material and the treated soft clay soil. This study has shown that purely elastic settlement prediction methods overestimate the settlement improvement for large modular ratios, while the methods based on elastic-plastic theory are in better agreement with finite element predictions at higher modular ratios. In addition, a parameter sensitivity study has been carried out to establish the influence of a range of different design parameters on predictions obtained using a selection of elastic-plastic methods.

2015

This study investigated the behaviour of single rammed stone columns. Prior to this assessment, the load – settlement and bulging responses of the columns were evaluated by conducting laboratory tests using a specially designed bench scale tank. The latter was a mild steel cylindrical tank which represented unit cells reinforced with single stone columns. The testing technique enabled the evaluation of the influence of variation of column diameter on stone column deformation responses and also provided data that was compared with the numerical results. In the numerical analysis, Mohr-Coulomb and modified Drucker-Prager models were used in the idealization of the behaviour of the column and soil materials respectively. Results revealed that the bigger the column diameter, the higher the loading capacity of the columns. Increasing the column diameter,D by 1.5, 2 and 3 times its initial size generally led to improved load carrying capacity by approximately 2, 4 and 10 times the initial strength respectively. The radial expansion of the columns was prominent in their upper parts with the highest value being experienced at a depth of about 0.5D from the ground surface.