Length Of Stone Columns Research Articles

Experimental and Numerical Study of Floating Stone Columns Using Unit Cell Model Test

Stone columns serve as a remedial measure to alleviate settlements and expedite the consolidation process, particularly in soft soils where excessive settlement and slow consolidation are the common problems. This paper presents the experimental and numerical investigation into the effects of area replacement ratio and stone column length on the consolidation settlement of floating stone columns using finite element software i.e. PLAXIS 3D. The model setup adopted unit cell concept resembling an infinite column grid pattern with uniform load. The study examined two distinct area replacement ratios (15% and 33%) and five varying lengths of stone columns (80 mm, 160 mm, 240 mm, 320 mm, and 400 mm). The findings indicated that as column length and area replacement ratio increase, settlement decreases, together with the improvement in consolidation rate. Generally, the numerical results agreed with the experimental results albeit slightly faster rate of consolidation obtained.

Numerical investigation of the settlement behavior of hybrid system of floating stone columns and granular mattress in soft clay soil

The composite system of stone columns reinforced soil overlaid by granular mattress (GM) has been given more attention as a hybrid improvement technique. This work presents 3D finite element analyses using PLAXIS 3D software for a composite system of floating stone columns group constructed in a deep soft clay deposit in Port-Said in Egypt. The limited area of loading was investigated in conjunction with the system key parameters, i.e., length, diameter, and the GM main properties. Results validated the effectiveness of the GM in settlement reduction and system behavior enhancement. The GM thickness results in considerable settlement reduction up to value of 1.5d where d is the stone column diameter. The thickness increase can be an economical substitution instead of increasing stone column length where a new factor named “Mattress improvement factor: Ƞgm” is defined to represent the occurring improvement due to GM existence. Results also show notable reduction of settlement with increasing the length to a limiting value of L = 1.8B where B is the footing width.

Influence of Stone Columns on Seismic Response of Buildings Considering the Effects of Liquefaction

In this paper, the behaviour of a soil-foundation system supported on a stone column-reinforced liquefiable soil strata is investigated through finite element analysis. The numerical analyses are performed on a five story reinforced concrete moment resisting building supported on a raft foundation. The influence of stone column slenderness ratio on liquefaction mitigation is studied by varying the length of stone columns at a constant area replacement ratio. The results are obtained based on the excess pore pressure, free-field soil settlement, foundation settlement, acceleration response, superstructure's inter-story drift, and lateral story displacement for each ground motion. The results showed that the liquefaction of free-field soil had a major impact on the foundation settlement and building lateral deformation. With the inclusion of stone columns, excess pore pressure ratio in the free-field region reduced considerably, which had immediate effects on the building's lateral deformation.

Optimization of the Ultimate Bearing Capacity of Reinforced Soft Soils through the Concept of the Critical Length of Stone Columns

The objective of this paper is to develop an analytical equation based on the concept of the critical-length of columns in order to optimize the ultimate bearing-capacity of soft soils, supporting a strip footing and reinforced by a group of floating stone columns. Optimization procedure was performed on three-dimensional numerical models simulated on the Flac3D computer code, for various soft-soils with different undrained-cohesions (Cu=15–35kPa), reinforced by columns of varying lengths (L) and area replacement ratio (As=10-40%), considering different footing widths B. Obtained results indicate that the optimal bearing-capacity ratio (Ultimate bearing-capacity of reinforced soil/unreinforced soil) is reached for the column critical-length ratio (Lc/B) and increase with increase of the later ratio, depending on As and Cu. Analysis of results also showed that the optimal values of the bearing-capacity ratio in the reinforced soils remain bounded between the lower and higher values (1.28-2.32), respectively for minimal and maximal values of the critical-length ratio (1.1) and (4.4). Based on these results, a useful analytical equation is proposed by the authors, for the expression of the critical-length; thus ensuring an optimal pre-dimensioning of the stone columns. The proposed equation was compared with the data available in the literature and showed good agreement. Doi: 10.28991/cej-2021-03091737 Full Text: PDF

Effect of Stone Columns on Strength and Consolidation Characteristics of Black Cotton Soil

One fifth of the land mass in India is covered by black cotton soil (BCS). Due to low permeability, poor strength and high compressibility, they pose huge engineering problems. In the present study, the influence of stone column on the consolidation and strength behaviour of black cotton soil is meticulously studied. Laboratory tests are carried out on both end bearing and floating type stone columns, having diameters (d) 50 mm, 60 mm and 70 mm with l/d ratios 3, 4 and 5 (l being the length of stone column), installed in black cotton soil. The load-settlement and consolidation behaviour of unreinforced and reinforced black cotton soil are compared. It has been found that for d = 60 mm and l/d = 3, the load carrying capacity for reinforced BCS increases by 23% and 20% for end bearing and floating type stone columns, respectively. Moreover the void ratio (e), coefficient of compressibility (av), coefficient of volume change (mv), coefficient of consolidation (Cv), coefficient of permeability (k) and compression index (Cc) of black cotton soils are significantly affected by the l/d ratio and diameter of the stone column. The present study reflects the efficiency of stone columns as suitable ground improvement technique for BCS.

Experimental and Numerical Investigation of Load Carrying Capacity of Vertically and Horizontally Reinforced Floating Stone Column Group

The present work investigates the load carrying capacity of vertically and horizontally reinforced floating stone column group through model testing. Two group arrangements using three and four stone columns for both unreinforced and reinforced conditions are testing under compressive loading. The geotextile reinforcement is provided throughout the stone column length (300 mm) in vertical direction and in form of circular discs at spacing of 30 mm along column length for reinforcing horizontally. The results of load-settlement behavior and failure mechanism of both unreinforced and reinforced group is studied. The testing results indicated reduced lateral bulging and higher stone column load capacity for horizontally reinforced columns in comparison to vertically reinforced. For validation of experimental results, numerical modeling using finite element (FE) code Plaxis 2D has also been conducted. The FE results depict lower load capacity as compared to model testing for settlement of 30 mm. However, the failure mode for both model tests and FE analysis was marked by bulging near the top of stone column and punching. Furthermore, load capacity is also check using empirical relations as given by IS code and previous literature. It is observed that experimental and analytical results are found in good agreement.

Deformation and Stability Analysis of Embankment over Stone Column-Strengthened Soft Ground

Compacted granular columns are commonly used to support embankments over soft soils. Using a reinforcement layer under the embankment causes the total stress to be further transferred to the column rather than soft soil, thus reducing total deformations of the subsoil. In this paper two dimensional (2D) numerical analysis was used to study the influence of stone columns and basal geosynthetic on deformations and stability of an embankment over soft deposit by means of Plaxis 2D finite element code. Unit cell to plane strain conversion approach was applied to transform columns into equivalent walls thus allowing to simulate a full embankment over a group of columns. Comprehensive parametric analysis was then performed to investigate the role of different critical parameters on embankment behavior. Results showed that the use of stone columns yielded the total deformations of the subsoil to significantly reduce, while its influence was less remarkable as a high stiffness geogrid was placed under the embankment. It was also found that the stone column length was the most influential parameter on the embankment total deformations, so that increasing columns length from 0.25Hs to 0.75Hs reduced the vertical and horizontal deformations by about two and five times, respectively. In addition, the use of a high stiffness basal geogrid caused the stability of the embankment to remarkably improve as the value of safety factor at the end of construction increased from 1.25 to about 1.9.

SEISMIC PERFORMANCE OF TANK FOUNDATIONS IMPROVED WITH STONE COLUMNS

Natural soil in engineering practices is not always considered to bear loads of above structures. In such a way, it is necessary to improve the quality of the soil before any constructions. Tanks are the structures that apply significant load to the beneath soil and they are usually constructed with circular foundations in shape. Seismic loads can apply irremediable damages to the structures and sometimes tanks are the highly important infrastructures during earthquakes. One of the most techniques that has been recently widely used on soft deposits and loose fine-grained soils is stone columns or singular piles. The stone columns increase the strength of loose soils and also decrease settlements induced by applying the loads. In this study, a linear numerical model of the structure-foundation-soil-stone column was simulated using the ABAQUS. Results show that with the increase of the length of stone columns a decrease in settlement occurred, while the increase of length more than a specific threshold had no significant effect to decrease the uplift and settlement.

Experimental and numerical modelling of group of geosynthetic-encased stone columns

The advantage of utilizing stone columns in very low bearing capacity of soil has been demonstrated as an effective strategy to improve the load bearing characteristics of soils. For stone columns, the load bearing capacity primarily relies upon the circumferential confinement given by the local delicate soils. In this research article, several tests were performed on group of 3 and 4 stone columns having diameter of 40 mm and length of stone columns of 300 mm. Stone columns were tested for both unreinforced and geotextile reinforced stone columns, i.e. encased stone columns and horizontally reinforced stone columns. Reinforced stone columns have been considered to explore the load settlement behaviour and failure mechanism. The principle target of this exploration is to compare the adequacy of encased stoned columns and horizontally reinforced stone columns. Results show that horizontal reinforced stone columns provide better ground improvement in comparison with encased stone columns. Utilizing of geotextile helps reduce failure due to the lateral bulging. Finite element modelling (FEM) has also been performed with Plaxis 2D and 3D software. The FEM results show that the load bearing capacity of reinforced stone columns is better than unreinforced stone columns both depicting failure due to bulging. The validation of experimental and FEM results are found in good agreement within permissible range of variation.

Seismic Performance of Footings on Stone Columns Treated Dry Sand Beds

An extensive experimental study has been carried out to study the seismic performance of footings on stone columns treated dry sand beds. Physical models have been prepared in a laminar shear box of size 1 m3 on the shaking table which has a single degree of freedom motion for representing horizontal shaking action under seismic vibrations. Effect of the configuration of stone columns, aspect ratio of footing, relative density of sand bed, frequency and peak ground acceleration (PGA) of cyclic input motion on vertical settlement of footing has been studied. Optimum length of stone columns has been found to lie between 13–16 times the diameter. Direction of shaking perpendicular to length of footing has been found to be more critical. Effect of aspect ratio has been found to be more pronounced at larger PGA. Influence of the frequency on response of footings has also been quantified.

Improvement of Soft Soil by Stone Column Treated with Bentonite

Soft clayey soils cover wide Iraqi areas specially the regions close to rivers and the southern part of this country Heavy weight structures like: highways, dams, multiple story buildings are suffering unacceptable settlement, when constructing on soft soils. The high contamination of water in such soils decrease the effective stress and reduce bearing capacity. The need was appeared to improve such problematic soil by the use of new technique of stone column treated with different percentages of natural bentonite by a series of field tests using full scale concrete footing constructed on soft soil in addition to a laboratory model to investigate settlement with time at constant stress. The soil that used in this study is natural clayey soil, brought from a location south of Diyala governorate, from a farm area. The study includes also: The effect of stone column diameter treated with bentonite on the behavior of footing constructing on soft clayey soil, The effect of stone column length on the behavior of footing on such soils. Results of field and laboratory model tests reviled that the treated model by stone column mixed with 40% bentonite is the ideal one, which reduces the settlement by 55%. In other hand problems of uneven settlements appear when using 60% bentonite as a mix proportion. The Ideal slenderness ratio (Ds/Ls<25%). The effective depth of stone column treated with bentonite is (1/3H).

Assessment of Group of Stone Column Under Sesmic Condition

The study of group of stone column is concerned with evaluation and comparison of the behaviour of group of encased stone column and ordinary stone column for static and seismic load condition. Stone column system with specified geometry is modelled and analysed in FEM software program MIDAS GTS 3D. Analysis is categorised into four cases. Pseudostatic method is used for seismic load calculation. The objective of research is to analyse the performance of stone column by considering the effect of various parameters such as diameter of stone column, static and seismic load condition in cohesive clay and silty clay. Effect of encasement is also observed for static loading condition and seismic loading condition. Result shows that encased stone column reduces the settlement and length of stone column.

Settlement Behavior of Soft Subgrade Reinforced by Geogrid‐Encased Stone Column and Geocell‐Embedded Sand Cushion: A Numerical Analysis

The geosynthetic‐encased vertical column and geosynthetic‐embedded horizontal cushion are recognized as the effective methods to reduce the settlement of the soft subgrade. This paper investigated the settlement behavior of a soft subgrade reinforced by geogrid‐encased stone column and geocell‐embedded sand cushion using the finite element analysis method (Plaxis 2D). The simulating settlement was in good agreement with the field monitoring data, indicating the reasonability of the designed model and adopted parameters. After that, the factors, geocell layer in sand cushion, encasement length around stone column, and standing time between embankment filling stages, were employed to study their influences on the subgrade settlement. The results showed that the embedment of geocell reduced construction settlement, postconstruction settlement, and differential settlement is attributed to the increase in stiffness of sand cushion and therefore the uniform distribution of additional stress on subgrade surface. When the encasement length of stone column increased from 1D (one time the column diameter) to 8D (full encasement), the settlement in construction stage and postconstruction stage decreased by 32.2% and 35.1%, respectively, which is benefited from the increase in the compression modulus of the column. The maximum lateral deformation occurred at the position of about 2D from the top of the stone column, and it decreased more significantly when the encasement length increased from 1D to 4D than that from 4D to 8D. The encasement length up to 4D is found to be adequate in reducing the subgrade settlement and the column lateral deformation based on the consideration of performance and economy. The extension of the filling interval increased the construction settlement caused by soil consolidation, while it decreased the postconstruction settlement.

Influence of Stone Column Length on the Settlement of Soft Clayey Layer

Currently, geotechnical engineers use stone columns to strengthen weak soils. The usage of stone columns to strengthen soft soil has proven successful, increasing their bearing capacity, reducing the settlement and decreasing the consolidation time. The present study was carried out to investigate the impact of the length of stone columns on the behaviour of soft soils and determine the effective length of stone columns, within which they have the greatest impact on the process of settlement of the soft clay layer. The analysis was performed utilizing a three-dimensional limited distinction numerical model FLAC3D. The results of the study confirmed that with the increase in the length of the stone columns, the settlement of the clay layer decreases. This decrease in settlement is significant to a depth corresponding to the relative length of the piles (L/d) = 10, after that the decrease in settlement practically stops.

Modelling stone columns under a soil–cement bed using an artificial neural network

The bulging effect of stone columns under a vertical load can be restricted by placing a stiff soil–cement compacted layer over the stone columns. The technique also improves the load-carrying capacity of the stone columns considerably. A series of laboratory experiments was conducted to obtain the load-carrying capacity of stone columns by varying parameters such as the thickness of the soil–cement bed, spacing between stone columns, length of stone columns and settlement due to loading to determine the bearing capacity. To avoid a complex interaction between the input variables, an artificial neural network model was adopted to predict the load-carrying capacity. The prediction efficiency of the model was found to be superior to that of a multi-variable regression model. Finally, a neural interpretation diagram was developed, from which the relative effect of an individual input parameter could be visualised.

Investigation of the Effect of Dimensional Characteristics of Stone Column on Load-Bearing Capacity and Consolidation Time

One of the best methods for rehabilitating loos and soft soils is the application of stone columns. This method enhances the soil properties by increasing its load-bearing capacity, decreasing the soil subsidence, and accelerating the consolidation rate. In the present paper, numerical analysis of a stone column of 10 m in length into a clayey soil using ABAQUS software is presented. The stone column was modelled based on the concept of unit cell, i.e. a single stone column with the surrounding soil. In this respect, material of the stone column was modelled using the elastoplastic behavioural model of Mohr-Coulomb, while Cam Clay behavioural model was used for the surrounding clayey soil. Furthermore, throughout the analyses performed in this study, effects of different parameters (e.g. applied load on rigid foundation, and the stone column length and diameter) on the subsidence and consolidation time of the rigid foundation were examined. The results indicated that, construction of a stone column into clayey soil decreases the subsidence and consolidation time of the soil considerably. In additions, increases in length and diameter of the stone column were found to significantly contribute to reduced subsidence and consolidation time of soil.

Prediction of the Effect of Using Stone Column in Clayey Soil on the Behavior of Circular Footing by ANN Model

Shallow foundations are usually used for structures with light to moderate loads where the soil underneath can carry them. In some cases, soil strength and/or other properties are not adequate and require improvement using one of the ground improvement techniques. Stone column is one of the common improvement techniques in which a column of stone is installed vertically in clayey soils. Stone columns are usually used to increase soil strength and to accelerate soil consolidation by acting as vertical drains. Many researches have been done to estimate the behavior of the improved soil. However, none of them considered the effect of stone column geometry on the behavior of the circular footing. In this research, finite element models have been conducted to evaluate the behavior of a circular footing with different stone column configurations. Moreover, an Artificial Neural Network (ANN) model has been generated for predicting these effects. The results showed a reduction in the bending moment, the settlement, and the vertical stresses with the increment of the stone column length, while both the horizontal stress and the shear force were increased. ANN model showed a good relationship between the predicted and the calculated results.

Bearing Capacity of Group of Stone Columns with Granular Blankets

In this research, the bearing capacity of group of floating stone columns with granular blankets was studied using laboratory tests. To investigate the effect of geosynthetic reinforcement on the bearing capacity of these stone columns, geotextiles and geogrids were used for the reinforcement of the stone columns and blankets, respectively. In this paper, certain large body laboratory tests were performed on stone columns with a diameter of 60 mm and lengths of 200 and 350 mm. The main objective of this research is to evaluate the bearing capacity of group of stone columns with granular blankets, record the stress concentration ratio in the group of stone columns without blankets, and investigate the load ratio parameter. The results indicate that the simultaneous application of group of stone columns and granular blankets significantly increased the ultimate bearing capacity of soft soils. Using geosynthetics for the reinforcement of granular blankets or stone columns improved the efficiency of these granular blankets or stone columns. As length of stone columns increased or stone columns were encased in geotextiles, the stress concentration ratio and stiffness of such stone columns were increased. Increasing the length of the short stone columns is more efficient than reinforcing them. Bulging was decreased in reinforced and unreinforced stone columns that were placed under the granular blanket.

Bearing capacity of geogrid reinforced sand over encased stone columns in soft clay

Stone columns develop their load carrying capacity from the circumferential confinement provided by the surrounding soils. In very soft soils, the circumferential confinement offered by the surrounding soft soil may not be sufficient to develop the required load carrying capacity. Hence a vertical confinement would yield a better result. The load carrying capacity is further increased with the addition of a sand bed over the stone columns. In the present study, a series of laboratory model tests on an unreinforced sand bed (USB) and a geogrid-reinforced sand bed (GRSB) placed over a group of vertically encased stone columns (VESC) floating in soft clay and their numerical simulations were conducted. Three-dimensional numerical simulations were performed using a finite element package ABAQUS 6.12. In the finite element analysis, geogrid and geotextile were modeled as an elasto-plastic material. As compared to unreinforced clay bed, an 8.45 fold increase in bearing capacity was observed with the provision of a GRSB over VESC. The optimum thickness of USB and GRSB was found to be 0.2 times and 0.15 times the diameter of the footing. A considerable decrease in bulging of columns was also noticed with the provision of a GRSB over VESC. Both the improvement factor and stress concentration ratio of VESC with GRSB showed an increasing trend with an increase in the settlement. It was observed that the optimum length of stone columns and the optimum depth of encasement of the group of floating VESC with GRSB are 6 times and about 3 times the diameter of the column respectively.

Physical and Numerical Modeling of Stone Column Behavior in Loose Sand

Stone column technique has been successfully applied for the foundation improvement. A rigid box with dimensions of 1.5 × 1.5 × 1.2 m equipped with a jacking-pump system was used to loading the stone column reinforced bed. Laboratory tests are carried out on stone columns with 6.3 cm diameter and different patterns surrounded by loose sand. The parameters varied in this experimental and numerical investigation are length, diameter and number of stone columns. The number of stone columns in these tests are 4, 5 and 9 and lengths of stone columns are 30, 40 and 50. A 40 × 40 × 2 cm steel plate is used as the model foundation. In these tests, the variation of bearing capacity ratio (BCR) and settlement reduction factor (SRF) are reported for different length and number of stone columns. The results show that by increasing the number and length of stone columns, BCR value will increase and SRF value will decrease. Finite-element analyses have also been performed using the ABAQUS software. The numerical results from the FEM were first validated with the experimental results and then some parametric analyses were conducted to investigate the effect of stone column diameter.