Studies on the Behavior of Single and Group of Geosynthetic Encased Stone Columns

The stone columns (or granular piles) are increasingly being used as ground reinforcement elements for supporting a wide variety of structures including buildings and flexible structures. The stone columns derive their load capacity from the confinement offered by the surrounding soil. In very soft soils this lateral confinement may not be adequate and the formation of the stone column itself may be doubtful. Wrapping the individual stone columns with suitable geosynthetic is one of the ideal forms of improving the performance of stone columns. This type of encasement by geosynthetic makes the stone columns stiffer and stronger. In addition, encasement prevents the lateral squeezing of stones in to the surrounding clay soil and vice versa, preserves drainage function of the stone column and frictional properties of the aggregates. In spite of many advantages, the behavior and the mechanism of the geosynthetic encased stone columns is not thoroughly understood. This paper investigates the qualitative and quantitative improvement of individual load capacity of stone column by encasement through laboratory model tests conducted on stone columns installed in clay bed prepared in controlled condition in a large scale testing tank. The load tests were performed on single as well as group of stone columns with and without encasement. Tests were performed with different geosynthetics for the encasement of stone column. The results from the load tests indicated a clear improvement in the load capacity of the stone column due to encasement. The increase in the axial load capacity depends very much upon the modulus of the encasement and the diameter of the stone column. The increase in the stress concentration on the stone columns due to encasement was also measured in the tests. The results from the tests were used to develop the design guidelines for the design of geosynthetic encasement for the given load and settlement.     

A Theoretical Solution for Consolidation Rates of Stone Column‐Reinforced Foundations Accounting for Smear and Well Resistance Effects

Theoretical and experimental studies have proven that stone columns can be used for accelerating the consolidation rate of soft soil by providing a drainage path and reducing stresses in the soil. In constructing stone columns in fine‐grained soils, however, soil zones at the interface between the columns and their surrounding soil can become smeared and the fine‐grained soil particles can also be mixed into aggregates in the columns. The smear and well resistance due to aggregates contaminated with the fine‐grained soil particles reduce the effectiveness of stone columns in dissipating excess pore water pressures. A theoretical solution is developed in this article for computing the consolidation rates of stone column reinforced foundations accounting for smear and well resistance effects. In the derivations, stone columns and soft soil are both considered deforming one‐dimensionally and the stone columns having a higher drained elastic modulus than the surrounding soft soil. A modified coefficient of consolidation is introduced to account for the effect of the stone column‐soil modular ratio or stress concentration ratio. A parametric study investigates the influences of six important factors on the rate of consolidation. These influence factors include the diameter ratio of the influence zone to the stone column, the permeability of the stone column, the stress concentration ratio, the size of the smeared zone, the permeability of the smeared zone, and the thickness of the soft soil. To assist geotechnical engineers in utilizing the new solution for the design of stone column reinforced foundations, an illustrative design example is presented at the end of this article.     

Investigation of Load-Transfer Mechanisms in Geotechnical Earth Structures with Thin Fill Platforms Reinforced by Rigid Inclusions

The reinforcement of soft soils by rigid inclusions is a practical and economical technique for wide-span buildings and the foundations of embankments. This method consists of placing a granular layer at the top of the network of piles to reduce vertical load on the supporting soil and vertical settlement of the upper structure. The study focuses on the modeling of load-transfer mechanisms occurring in the reinforced structure located over the network of piles with a coupling between the finite-element method (geosynthetic sheets) and discrete element method (granular layer; concrete slab in some cases). The importance of granular layer thickness to increase load-transfer intensity and to reduce vertical settlement was observed. However, without a basal geosynthetic sheet, the compressibility of soft soil has a great influence on the mechanisms. A method predicting the intensity of load transfers was proposed, based on Carlsson’s solution. The main parameters concerned are the geometry of the work and the peak and residual friction angles of the granular layer.     

Performance of Geosynthetic-Encased Stone Columns in Embankment Construction: Numerical Investigation

This paper presents the results of a numerical investigation into the performance of geosynthetic-encased stone columns (GESCs) installed in soft ground for embankment construction. A three-dimensional finite-element model was employed to carry out a parametric study on a number of governing factors such as the consistency of soft ground, the geosynthetic encasement length and stiffness, the embankment fill height, and the area replacement ratio. The results indicate among other things that additional confinement provided by the geosynthetic encasement increases the stiffness of the stone column and reduces the degree of embankment load transferred to the soft ground, thereby decreasing the overall settlement. It is also shown that the geosynthetic encasement has a greater impact for cases with larger stone column spacing and/or weaker soil. Also revealed is that unlike isolated column loading conditions, full encasement may be necessary to ensure maximum settlement reduction when implementing GESCs under an embankment loading condition. Practical implications of the findings are discussed in detail.     

Numerical Analysis of Geosynthetic-Reinforced and Pile-Supported Earth Platforms over Soft Soil

Geotechnical engineers face several challenges when designing structures over soft soils. These include potential bearing failure, intolerable settlement, large lateral pressures and movement, and global or local instability. Geosynthetic-reinforced and pile-supported earth platforms provide an economic and effective solution for embankments, retaining walls, and storage tanks, etc. constructed on soft soils; especially when rapid construction and/or strict deformation of the structure are required. The inclusion of geosynthetic(s) in the fill enhances the efficiency of load transfer, minimizes yielding of the soil above the pile head, and potentially reduces total and differential settlements. A numerical study has been conducted to investigate pile-soil-geosynthetic(s) interactions by considering three major influence factors: the height of the fill, the tensile stiffness of geosynthetic, and the elastic modulus of pile material. While current methods have not fully addressed important effects of the geosynthetic stiffness and pile modulus on the soil arching ratio, numerical results suggested that the stress concentration ratio and the maximum tension in geosynthetic increase with the height of the embankment fill, the tensile stiffness of geosynthetic, and the elastic modulus of the pile material. The distribution of tension force in the geosynthetic reinforcement indicated that the maximum tension occurs near the edge of the pile.     

Implementation of a Critical State Two-Surface Model to Evaluate the Response of Geosynthetic Reinforced Pavements

A finite-element model was developed using ABAQUS software package to investigate the effect of placing geosynthetic reinforcement within the base course layer on the response of a flexible pavement structure. A critical state two-surface constitutive model was first modified to represent the behavior of base course materials under the unsaturated field conditions. The modified model was then implemented into ABAQUS through a user defined subroutine, UMAT. The implemented model was validated using the results of laboratory triaxial tests. Finite-element analyses were then conducted on different unreinforced and geosynthetic reinforced flexible pavement sections. The results of this study demonstrated the ability of the modified critical state two-surface constitutive model to predict, with good accuracy, the response of the considered base course material at its optimum field conditions when subjected to cyclic as well as static loads. The results of the finite-element analyses showed that the geosynthetic reinforcement reduced the lateral strains within the base course and subgrade layers. Furthermore, the inclusion of the geosynthetic layer resulted in a significant reduction in the vertical and shear strains at the top of the subgrade layer. The improvement of the geosynthetic layer was found to be more pronounced in the development of the plastic strains rather than the resilient strains. The reinforcement benefits were enhanced as its elastic modulus increased.     

Simplified Method for Consolidation Rate of Stone Column Reinforced Foundations

Field observations and numerical studies demonstrated that stone columns could accelerate the rate of consolidation of soft clays. A simplified method for computing the rate of consolidation is presented in this paper by assuming that stone columns; (1) are free draining; (2) have higher drained elastic modulus than soft clay; and (3) are deformed 1D. The formats of the final solutions in vertical and radial flows are similar to those of the Terzaghi 1D solution and the Barron solution for drain wells in fine-grained soils, respectively. Modified coefficients of consolidation are introduced to account for effects of the stone column-soil modular ratio. The new solutions demonstrate stress transfer from the soil to stone columns and dissipation of excess pore water pressures due to drainage and vertical stress reduction during the consolidation. Comparisons between the results from this simplified method and the numerical study by Balaam and Booker in 1981 exhibit reasonable agreement, when the stress concentration ratio is in the practical range (2–6). The discrepancies in the results from these two methods are discussed. This paper also includes design charts and a design example.     

Consolidation of a Double-Layered Compressible Foundation Partially Penetrated by Deep Mixed Columns

Deep mixed columns often penetrate partially into the soft soil as floating columns due to the depth of the end-bearing layer. Partially penetrated soft soil by columns and the underlying compressible soft soil create a double-layered compressible foundation. So far, no reasonable solution is available to estimate the consolidation of such a double-layered foundation. This paper proposes an analytical solution for consolidation of a double-layered compressible foundation partially penetrated by deep mixed columns considering one-side or two-side vertical drainage The Laplace transform method was used to solve the consolidation equation for the double-layered system while Stehfest’s algorithm was used to solve the inverse Laplace transform for time-dependent loading. A consolidation algorithm was used to calculate the time-settlement relationship of an embankment constructed upon the double-layered foundation partially penetrated by deep mixed columns. The calculated settlements were compared well with field measurements.     

Consolidation of Soft Clay Foundations Reinforced by Stone Columns under Time-Dependent Loadings

This paper presents an analytical solution for the consolidation of soft soil foundations reinforced by stone columns under time-dependent loadings. The differential equations of the foundations reinforced by stone columns are obtained including smear and well resistance under arbitrary applied loadings. The closed-form solutions of pore pressure and the overall average degree of consolidations are obtained for some common types of loadings, such as step loading, ramp loading, and cyclic trapezoidal loading. By solving the equations using a semianalytical method, the comparisons agree very well with the existing analytical solutions, which verify the correctness and accuracy of the proposed methods. Using the solutions obtained, some selected charts are presented and the relevant consolidation behavior is investigated and discussed.     

Coupled Mechanical and Hydraulic Modeling of Geosynthetic-Reinforced Column-Supported Embankments

Geosynthetic-reinforced column-supported (GRCS) embankments have increasingly been used in the recent years for accelerated construction. Numerical analyses have been conducted to improve understanding and knowledge of this complicated embankment system. However, most studies so far have been focused on its short-term or long-term behavior by assuming an undrained or drained condition, which does not consider water flow in saturated soft soil (i.e., consolidation). As a result, very limited attention has been paid to a settlement-time relationship especially postconstruction settlement, which is critical to performance of pavements on embankments or connection between approach embankments and bridge abutments. To investigate the time-dependent behavior, coupled two-dimensional mechanical and hydraulic numerical modeling was conducted in this study to analyze a well-instrumented geotextile-reinforced deep mixed column-supported embankment in Hertsby, Finland. In the mechanical modeling, soils and DM columns were modeled as elastic-plastic materials and a geotextile layer was modeled using cable elements. In the hydraulic modeling, water flow was modeled to simulate generation and dissipation of excess pore water pressures during and after the construction of the embankment. The numerical results with or without modeling water flow were compared with the field data. In addition, parametric studies were conducted to further examine the effects of geosynthetic stiffness, column modulus, and average staged construction rate on the postconstruction settlement and the tension in the geosynthetic reinforcement.     

Reinforced Stone Columns in Weak Deposits: Laboratory Model Study

This paper investigates the performance of stone columns in a weak deposit such as peat. It evaluates the effects of reinforcing stone columns by jacketing with a tubular wire mesh and bridging reinforcement with a metal rod and a concrete plug. A series of plate loading tests was conducted on isolated stone columns installed in a soil bed consisting of a peat layer sandwiched between two layers of sand. The load–displacement characteristics of footings supported by stone columns were investigated by applying load to a circular plate supported on: (a) untreated soil; (b) soil treated with stone columns; and (c) soil treated with stone columns reinforced with the above reinforcing techniques. The work has shown that the settlement characteristics of the soil can be improved by installing stone columns and that a significant enhancement in the load–settlement response is achieved when the columns are reinforced by the various methods.     

Investigation into the Effect of Bonding on FRP-Wrapped Cylindrical Concrete Columns

In this paper the failure of a three layer system comprising a concrete column, an intermediate epoxy layer, and fiber-reinforced polymer (FRP) confinement is investigated. We perform a series of numerical experiments to investigate how the failure loads and ultimate strains of axially loaded plain cement concrete (PCC) and reinforced cement concrete (RCC) columns change with the type of the bond between concrete and epoxy and between epoxy and FRP. Three types of interfacial behavior are considered: rigid, cohesive compliant, and unbonded contact. An idealized spring model for the resultant confinement stiffness is used to explain the effect the nature of the bond has on the results. It is found that the type of bond has a significant effect on the ultimate strength of PCC columns. The results also indicate that the presence of longitudinal and hoop steel reinforcement allows use of comparatively less stiff FRP sheets as confinement material for RCC columns.     

Unified Elastoplastic–Viscoplastic Bounding Surface Model of Geosynthetics and Its Applications to Geosynthetic Reinforced Soil-Retaining Wall Analysis

The static and dynamic behaviors of reinforced soil structures are possibly subjected to the effects of creep or stress relaxation due to the time-dependent behavior of geosynthetic inclusions and backfill. To simulate the time-dependent monotonic and cyclic behavior of geosynthetics, an isothermal constitutive model is formulated within the framework of elastoplasticity–viscoplasticity. The concept of bounding surface plasticity is first utilized to formulate a time-independent cyclic model of geosynthetics. In order to capture the hardening stiffness of some polyester geosynthetics, an exponential bounding curve is used in simulating the primary loading. The time-independent version of the model was extended into an elastoplastic–viscoplastic model using overstress viscoplasticity with reference to available experimental data. The model was evaluated using creep, stress relaxation, monotonic, and cyclic loading test results obtained for different geosynthetics. It was then incorporated into a finite-element code and the static and dynamic behavior of a geosynthetic reinforced soil wall was analyzed. The analyzed results, with and without consideration to the time-dependent behavior of the reinforcements, were compared. It was demonstrated that although the end-of-construction behavior of the reinforced soil wall was less influenced by the time-dependent properties of geogrids, the long-term performance was considerably affected. The seismic response was also affected to some extent by the rate-dependent behavior of geogrids. The effects were more significant for short and/or large vertical spacing reinforcement layout.     

Long-Term Reinforcement Load of Geosynthetic-Reinforced Soil Retaining Walls

As increasing number of geosynthetic-reinforced soil (GRS) retaining walls are built for permanent purpose, and their long-term behaviors have become one of the most critical issues in design. However, there has been very limited study on long-term reinforcement load and its relation to various parameters of GRS walls. A finite-element procedure for the long-term response of geosynthetic-reinforced soil structures with granular backfills was first validated against the long-term model test. Extensive finite-element analyses considering the viscous properties of geosynthetic reinforcements were then carried out to investigate the load distributions in geosynthetic reinforcements of GRS walls under operational condition. Construction sequence was simulated and a creep analysis of 10?years was subsequently conducted on each model wall. The effects of wall parameters, including backfill soil, reinforcement length, reinforcement spacing, reinforcement stiffness, and creep rate of reinforcement were investigated. It is found from the analyses that: (1) the maximum reinforcement load of GRS walls under working stress condition was generally smaller than that estimated using the FHwA design but it is dependent on the global reinforcement stiffness Sglobal; (2) the surface of maximum reinforcement load did not coincide with the Rankine’s surface suggested by FHwA design guidelines for vertical GRS walls and it was affected by the strength of backfill soil, reinforcement length, reinforcement spacing, and reinforcement stiffness; (3) for GRS walls under operational condition, reinforcement loads were closely related to the mobilized stiffness of backfill soil; (4) isochrone curves can be used to interpret the effects of reinforcement stiffness and creep rate on both short-term and long-term performances of GRS walls under operational condition, and with an increase in the reinforcement stiffness, the maximum reinforcement load increased; and (5) the global reinforcement stiffness Sglobal, which is related to the isochrones stiffness of reinforcement as well as reinforcement spacing was related to the total reinforcement load Ttotalmax and with an increase in the global stiffness, the total reinforcement load increased.     

Deformation Patterns of Reinforced Foundation Sand at Failure

While the stability of foundation soils has been written about extensively, the ultimate loads on reinforced soils is a subject studied to a much lesser degree. There is convincing experimental evidence in the literature that metal strips or layers of geosynthetic reinforcement can significantly increase the failure loads on foundation soils. Laboratory tests were performed to investigate the kinematics of the collapse of sand reinforced with a layer of flexible reinforcement. Sequential images of the deformation field under a model footing were digitally recorded. A correlation-based motion detection technique was used to arrive at an incremental displacement field under a strip footing model. Color-coded displacements are presented graphically. The mechanism retains some of the characteristic features of a classical bearing capacity pattern of failure, but the reinforcement modifies that mechanism to some extent. The strips of geotextile used as model reinforcement give rise to the formation of shear bands in a narrow layer adjacent to the geosynthetic. Reinforcement restrains the horizontal displacement of the soil and alters the collapse pattern. The mechanism of deformation identified in the tests will constitute a basis for limit analysis of reinforced foundation soils.     

Simulation of Upward Seepage Flow in a Single Column of Spheres Using Discrete-Element Method with Fluid-Particle Interaction

The interaction between soil particles and pore water makes the behavior of saturated and unsaturated soil complex. In this note, upward seepage flow through a granular material was idealized using a one-column particle model. The motion of the individual particles was numerically simulated using the discrete-element method taking the interaction with the fluid into account. The fluid behavior was simulated by the Navier-Stokes equation using the semi-implicit method for pressure-linked equation. This approach has already been applied in powder engineering applications. However, there are very few studies that have used this approach in geotechnical engineering. This note first describes the qualitative and quantitative validation of this method for hydraulic gradients below the critical one by comparing the results with an analytical solution. Then, the ability of the method to simulate the macroscale behavior due to the interaction between particles and pore water at hydraulic gradients exceeding the critical hydraulic gradient is discussed.     

In Situ Pore Pressure Responses of Native Peat and Soil under Train Load: A Case Study

Settlement and formation of piping holes on surfaces were observed along a rail embankment subject to normal traffic load. Piezometers were installed in the native peat and soil underneath the embankment inside and outside problematic area to measure the pore pressure responses during train traffic. Peculiar pore pressure responses were observed. Cyclic pore pressures were only measured during the first 60–80?s of the 6-min train passage, and thereafter the pressures decayed rapidly to the initial values. The pore pressure changes in the shallow peat layer were lower than those in the deep soil layer. Possible mechanisms causing such peculiar pore pressure responses, surface settlement, and piping holes were explored and identified. It was found that the stiffness contrast between the stiff, upper granular fill and the soft, native peat material could lead to a redistribution of tensile stress in the granular fill layer to the peat layer due to the moving train load. This stress redistribution promotes the propensity of vertical piping in the peat layer.     

Experimental and Theoretical Investigations on Geocell-Supported Embankments

This paper studies the advantages of geocell reinforcement on the performance of earth embankments constructed over weak foundation soil through laboratory model tests and proposes a simple method for the design of geocell-supported embankments. Model embankments were constructed above a layer of geocells formed using geogrids on top of a soft clay bed prepared in a steel test tank. Uniform surcharge pressure was applied on the crest and pressure-deformation behavior of the embankment and strains in the walls of geocells were monitored continuously until the failure was reached. The influence on the behavior of the embankment of various parameters—like tensile stiffness of geocell material, height and length of geocell layer, pocket-size of the cell, pattern of formation of geocells, and type of fill material inside the cells—was studied. Geocell reinforcement was found to be advantageous in increasing the load-bearing capacity and reducing the deformations of the embankments. The experimental results were validated using a general purpose slope-stability program and a design procedure useful for the preliminary design of geocell-supported embankments is illustrated.     

Parametric Studies on the Behavior of Reinforced Soil Retaining Walls under Earthquake Loading

The finite element procedures are extremely useful in gaining insights into the behavior of reinforced soil retaining walls. In this study, a validated finite element procedure was used for conducting a series of parametric studies on the behavior of reinforced soil walls under construction and subject to earthquake loading. The procedure utilized a nonlinear numerical algorithms that incorporated a generalized plasticity soil model and a bounding surface geosynthetic model. The reinforcement layouts, soil properties under monotonic and cyclic loadings, block interaction properties, and earthquake motions were among major variables of investigation. The performance of the wall was presented for the facing deformation and crest surface settlement, lateral earth pressure, tensile force in the reinforcement layers, and acceleration amplification. The effects of soil properties, earthquake motions, and reinforcement layouts are issues of major design concern under earthquake loading. The deformation, reinforcement force, and earth pressure increased drastically under earthquake loading compared to end of construction.     

Analyzing the Reinforcement Loads of Geosynthetic-Reinforced Soil Walls Subject to Seismic Loading during the Service Life

It is more rational to analyze permanent geosynthetic reinforced soil (GRS) walls against seismic loading based on their behavior during service life, but it has seldom been attempted. Calibrated finite-element procedure was used to investigate the reinforcement loads of GRS walls subject to seismic loading during service life, the results of which were compared to those predicted by Federal Highway Administration (FHwA) guideline. Parametric studies were carried out to investigate the effects of various wall parameters and characteristics of earthquake excitations. It is found that due to the isotach behavior of geosynthetics, the reinforcement loads during earthquake that occurs 10 years after construction were similar to those if the earthquake occurs at the end of construction. The FHwA method predicted roughly the maximum reinforcement load but it could not consider strain softening of soil and characteristics of earthquakes. The horizontal locations of maximum reinforcement load in lower reinforcement layers were farther away from the facing units than Rankine’s surface, which is believed to come from the potential compound failure.