Pile-supported geogrid-reinforced embankments are an exciting new foundation system that is utilized when sites are limited by a soft soil or clay. In this system, an embankment is supported by a bridging layer, consisting of granular fill and one or multiple layers of geogrid reinforcement. The bridging layer transfers the load to piles that have been driven into the soft soil or clay. The load from the embankment induces large deformations in the geogrid reinforcement, causing tensile forces in the ribs of the geogrid. Many of the current methods used to design geogrid reinforcement for this system simplify the approach by assuming that the reinforcement has a parabolic deformed shape. The purpose of this thesis is to thoroughly examine the behavior of the geogrid in a pile-supported embankment system, in an effort to determine the accuracy of the parabolic deformed shape, and identify the most important parameters that affect reinforcement design. The geogrid was analyzed using a three-dimensional model that included a cable net to represent the geogrid and linear springs to represent the soil underneath the geogrid. A larger pressure was applied to the geogrid regions that are directly above the pile caps so that arching effects could be considered, and the stiffness of the springs on top of the pile were stiffer to account for the thin layer of soil between the geogrid and the pile cap. A Mathematica algorithm was used to solve this model using the minimization of energy method. The results were compared to another model of this system that used a membrane to represent the geosynthetic reinforcement. Additionally, the maximum strain was compared to the strain obtained from a geosynthetic reinforcement design formula. A parametric study was performed using the Mathematica algorithm by varying the pile width, embankment pressure applied to the soil, embankment pressure applied to the pile, stiffness of the soil, stiffness of the soil on top of the pile, stiffness of the geogrid, geogrid orientation, rotational stiffness of the geogrid, and the layers of geogrid reinforcement. / Master of Science

Column-supported, geosynthetic-reinforced embankments provide effective geotechnical foundations for applications in areas of weak subgrade soils. The system consists of a soil bridging layer with one or more embedded layers of geosynthetic reinforcement supported by driven or deep mixed columnar piles. The geosynthetic promotes load transfer within the bridging layer to the columns, allowing for larger column spacings and varied alignments. This technique is generally used when differential settlements of the embankment or adjacent structures are a concern and to minimize construction time. Recent increase in the popularity of this composite system has generated the need to further investigate its behavior and soil-structure interaction. Current models of geosynthetics are oversimplified and do not represent the true three-dimensional nature of the material. Such simplifications include treating the geosynthetic as a one-dimensional cable as well as neglecting stress concentrations and pile orientations. In this thesis, a complete three-dimensional analysis of the geosynthetic is performed. The geosynthetic was modeled as a thin flexible plate in a single square unit cell of the embankment. The principle of minimum potential energy was then applied, utilizing central finite difference equations. Energy components from vertical loading, soil and column support, as well as bending and membrane stiffness of the geosynthetic are considered. Three pile orentation types were implemented: square piles, circular piles, and square piles rotated 45° to the edges of the unit cell. Each of the pile orientations was analyzed using two distinct parameter sets that are investigated in previously published and ongoing research. Vertical and in-plane deflections, stress resultants, and strains were determined and compared to other geosynthetic models and design guides. Results of each parameter set and pile orientation were also compared to provide design recommendations for geosynthetic-reinforced column-supported embankments. / Master of Science

A geotechnical composite foundation system that has become increasingly popular over the years is a column-supported, geosynthetic-reinforced embankment. This system consists of strong columns or piles placed in soft clay, a bridging layer of sand or sand and gravel, and one or more layers of geosynthetic reinforcement. It is often used in soft ground situations where there is a need for faster construction and/or where there are adjacent structures that would be affected by settlement caused by the new embankment. The geosynthetic reinforcement is placed in the bridging layer to help transfer the load to the columns and decrease the total and differential settlements. Current methods of analysis for this material are extremely simplified, and do not thoroughly model the behavior of the system. Therefore, a more comprehensive analysis needs to be conducted that will better predict the true effect of the geosynthetic layer or layers. In this thesis, one geosynthetic layer was considered. Models were developed using two different computer programs: Mathematica and ABAQUS. In Mathematica, the Rayleigh-Ritz method was used to approximate the deflections and tensile forces in the membrane. This method considered the geosynthetic reinforcement as a plate and minimized the total energy of the system. In ABAQUS, a finite element modeling program, the membrane was analyzed as a shell, and results were compared with some results from Mathematica. A parametric study was completed in Mathematica to determine the effects of different parameters. The parameters varied involved the geogrid properties (Poisson's ratio, modulus of elasticity, and thickness), the vertical load, the soil stiffness above the piles, the soil stiffness between the piles, the size of the piles, and the distance between the piles. / Master of Science

Column-supported embankments with or without basal geosynthetic reinforcement can be used in soft ground conditions to reduce settlement by transferring the embankment load to the columns through stress redistribution above and below the foundation subgrade level. Column-supported embankments are typically used to accelerate construction and/or protect adjacent facilities from additional settlement. The column elements consist of driven piles or formed-in-place columns that are installed in an array to support a bridging layer or load transfer platform. The bridging layer is constructed to enhance load transfer using several feet of compacted sand or sand and gravel that may include one or more layers of high-strength geotextile or geogrid reinforcement. Mobilization of the mechanisms of load transfer in a column-supported embankment requires some amount of differential settlement between the columns and the embankment as well as between the columns and the foundation soil. When the embankment height is low relative to the clear spacing between columns, there is the risk of poor ride quality due to the reflection of the differential foundation settlement at the surface of the embankment. The minimum embankment height where differential surface settlement does not occur for a particular width and spacing of column is the critical height. The conventional approach is to express critical height as a fixed ratio of the clear span between adjacent columns; however, there is no consensus on what ratio to use and whether a single ratio is applicable to all realistic column arrangements. The primary objective of this research is to improve the understanding of how column-supported embankments deform in response to differential foundation settlement. A bench-scale experimental apparatus was constructed and the equipment, materials, instrumentation, and test procedures are described. The apparatus was able to precisely measure the deformation occurring at the sample surface in response to differential settlement at the base of the sample. Critical heights were determined for five combinations of column diameter and spacing representing a wide range of possible column arrangements. In addition, tests were performed using four different column diameters in a single column configuration with ability to measure the load acting on the column and apply a surcharge pressure to the sample. In total, 183 bench-scale tests were performed over a range of sample heights, sample densities, and reinforcement stiffnesses. Three-dimensional numerical analyses were conducted to model the experiments. The critical heights calculated using the numerical model agreed with the experimental results. The results of the laboratory tests and numerical analyses indicate that critical height depends on the width and spacing of the columns and is not significantly influenced by the density of the embankment fill or the presence of reinforcement. A new method to estimate critical height was developed and validated against extensive case histories as well as experimental studies and numerical analyses performed by others. / Ph. D.

Column-supported geosynthetic-reinforced embankments have great potential for application in soft ground conditions when there is a need to accelerate construction and/or protect adjacent facilities from the settlement that would otherwise be induced by the new embankment load. The columns in column-supported embankments can be driven piles, vibro-concrete columns, deep-mixing-method columns, stone columns, or any other suitable type of column. A bridging layer consisting of several feet of sand or sand and gravel is also used to help transfer the embankment load to the columns. Geosynthetic reinforcement is often employed in bridging layers to enhance load transfer to the columns and increase the spacing between columns. Several methods have been developed to calculate the load on the geosynthetic reinforcement, but the calculated loads differ by over an order of magnitude in some cases, and there is not agreement on which method is correct. In this research, a new method was developed for calculating the load on the geosynthetic reinforcement. The new method employs one of the existing mechanistically-based approaches, and combines it with consideration of the stiffnesses of the embankment, geosynthetic, column, and subgrade soil. The new method was verified against the results of a large numerical parameter study, for which the numerical procedures themselves were verified against closed-form solutions for membranes, pilot-scale experiments, and instrumented field case histories. The results of the numerical analyses and the new calculation procedure indicate that the net vertical load on the portion of the geosynthetic reinforcement between columns increases with increasing clear spacing between columns and increasing geosynthetic stiffness. The net vertical load on the geosynthetic decreases with increasing stiffness and strength of the foundation and embankment soils and with increasing elevation of the geosynthetic above the top of the columns or pile caps. A key finding of the research is that, if the subgrade support is good, geosynthetic reinforcement does not have a significant effect on system performance. The new calculation procedure is implemented in an easy-to-use spreadsheet, and recommendations for designing geosynthetic-reinforced bridging layers are provided. / Ph. D.

When an embankment is to be constructed over ground that is too soft or compressible to adequately support the embankment, columns of strong material can be placed in the soft ground to provide the necessary support by transferring the embankment load to a firm stratum. This technology is known as column-supported embankments (CSEs). A geosynthetic-reinforced load transfer platform (LTP) or bridging layer may be constructed immediately above the columns to help transfer the load from the embankment to the columns. There are two principal reasons to use CSEs: 1) accelerated construction compared to more conventional construction methods such as prefabricated vertical drains (PVDs) or staged construction, and 2) protection of adjacent facilities from distress, such as settlement of existing pavements when a roadway is being widened. One of the most significant obstacles limiting the use of CSEs is the lack of a standard design procedure which has been properly validated. This report and the testing described herein were undertaken to help resolve some of the uncertainty regarding CSE design procedures in light of the advantages of the CSE technology and potential for significant contributions to the Strategic Highway Research Program, which include accelerated construction and long-lived facilities. Twelve design/analysis procedures are described in this report, and ratings are assigned based on information available in the literature. A test facility was constructed and the facility, instrumentation, materials, equipment, and test procedures are described. A total of 5 CSE tests were conducted with 2 ft diameter columns in a square array. The first test had a column center-to-center spacing of 10 ft and the remaining four tests had center-to-center spacings of 6 ft. The Adapted Terzaghi Method of determining the vertical stress on the geosynthetic reinforcement and the Parabolic Method of determining the tension in the geosynthetic reinforcement provide the best agreement with the test results. The tests also illustrate the importance of soft soil support in CSE performance and behavior. A generalized formulation of the Adapted Terzaghi Method for any column/unit cell geometry and two layers of embankment fill is presented, and two new formulations of the Parabolic Method for triangular arrangements is described. A recommended design procedure is presented which includes use of the GeogridBridge Excel workbook described by Filz and Smith (2006, 2007), which was adapted for both square and triangular column arrangements. GeogridBridge uses the Adapted Terzaghi Method and the Parabolic Method in a load-displacement compatibility design approach. For completeness, recommended quality control and quality assurance procedures are also provided, and a new guide specification is presented. / Ph. D.

Column-supported embankments (CSE) enable accelerated construction on soft soils, high performance, and protection of adjacent facilities. The foundation columns transfer embankment and service loading to a competent stratum at depth such that loading on the soft soil can be reduced. This has the beneficial effects of reducing settlement and lateral displacement, and improving stability. Selection of column type depends on the design load, cost, constructability, etc., although unreinforced concrete columns are commonly used. A load transfer platform (LTP) is often included at the embankment base. This is a layer of coarse-grained fill that may include one or more layers of geosynthetic reinforcement. The LTP improves vertical load transfer to columns by mobilizing the shear strength of the LTP fill and the membrane effect of the geosynthetic. The geosynthetic reinforcement also responds in tension to lateral spreading. Herein, lateral spreading is defined as the lateral displacements occurring in response to lateral earth pressures in the embankment and foundation. Excessive lateral spreading can lead to bending failure of the concrete columns, tensile failure of the geosynthetic reinforcement, and instability of the system. Design procedures recommend inclusion of geosynthetic reinforcement to mitigate lateral spreading, with assumptions for the lateral thrust distribution, failure mode, and calculation of geosynthetic tensile capacity. The necessity and sufficiency of these assumptions have not been fully validated. In addition, unreinforced concrete columns have low tensile strength and can fail in bending, but recommendations for calculating column bending moments are not available. This research examines the limitations in CSE lateral spreading design with the goal of advancing fundamental understanding of lateral spreading mechanics. The research was performed using three-dimensional finite difference analyses. Limiting conditions for lateral spreading analysis were identified using case history records, and an undrained-dissipated approach was validated for the numerical analysis of limiting conditions (i.e., undrained end-of-construction and long-term excess pore pressure dissipated). The numerical model was calibrated using a well-documented case history. Additional analyses of the case history were performed to examine the lateral earth pressures in the foundation, column bending moments, and geosynthetic contribution to resisting lateral spreading. A parametric study was conducted to examine the lateral thrust distribution in 128 CSE scenarios. A refined substructure model was adopted for analyzing peak geosynthetic tensions and strains. Lastly, failure analyses were performed to examine the effect of different CSE design parameters on embankment failure height, failure mode, and deformations. The research produced qualitative and quantitative information about the following: (1) the percent thrust resistance provided by the geosynthetic as a function of its stiffness; (2) the geosynthetic contribution to ultimate and serviceability limit states; (3) the change in lateral thrust distribution throughout the embankment system before and after dissipation of excess pore water pressures; (4) the column-soil interactions involved in embankment failure; and (5) identification of two failure modes in the undrained condition. Design guidance based on these findings is provided. / Doctor of Philosophy / Column-supported embankments (CSEs) have been designated by the Federal Highway Administration as a critical technology for new highway alignment projects and widening of existing highways. CSEs enable accelerated construction and high performance in weak soils, which are factors critical to project success. In a CSE, columns are installed in the weak soil, followed by rapid construction of the soil embankment that provides the necessary elevation and foundation for the roadway. The columns transfer most of the embankment and traffic loading to a competent soil stratum at depth. Concrete without steel reinforcement is commonly used to construct the columns, although material selection depends on cost, constructability, expected load, etc. Layers of geosynthetic reinforcement can also be included at the embankment base. The geosynthetics help to transfer loads to the columns and resist excessive movement that could lead to instability. The entire embankment system should be designed for safety and economy. This research was motivated by uncertainties in design to mitigate lateral spreading. Lateral spreading refers to lateral displacements occurring in response to lateral earth pressures in the embankment and foundation. Excessive lateral spreading can lead to failure of the columns, geosynthetic reinforcement, and the entire embankment system. This research aims to advance fundamental understanding of lateral spreading in CSEs and to re-evaluate current design assumptions. Corresponding design guidance is provided.

Le sous-sol français est traversé par un nombre considérable de cavités souterraines naturelles ou anthropiques : après mine, carrières, karsts, tunnels et ouvrages civils abandonnés, etc. Ces cavités sont à l’origine de différents risques de mouvements de terrains tels que les effondrements localisés (fontis) et les affaissements qui peuvent être graves de conséquence pour les biens et les personnes. Pour réduire ce risque, un renforcement par géosynthétique des remblais sur cavités potentielles peut être mis en œuvre. C’est dans ce cadre que s’inscrit cette thèse menée au sein de l’INERIS (projet de recherche EREVAN - Evaluation et Réduction de la Vulnérabilité des biens exposés aux Aléas Naturels et miniers), en partenariat avec le laboratoire 3SR. L’un des objectifs de ces travaux est notamment de mieux appréhender, suite à l’ouverture d’une cavité sous-jacente, le comportement et les mécanismes d’effondrement des remblais renforcés par géosynthétique, en particulier cohésifs, afin d’en optimiser le dimensionnement.Dans le cadre de cette thèse, différentes expérimentations sur des modèles physiques de laboratoire et en vraie grandeur ont été réalisées. Les résultats obtenus en laboratoire ont permis de préciser le rôle mécanique des renforcements géosynthétiques dans le cas d’effondrement localisé sous un remblai granulaire ou/et cohésif, une importante base de données expérimentales a ainsi été constituée. Une expérimentation en vraie grandeur a permis de valider l’intérêt au plan technique, économique et environnemental de la technique de renforcement par géosynthétique des zones sujettes à des risques fontis.La contribution particulière de ce travail réside dans l’utilisation de modèles physiques et de techniques de mesures originales développés pour simuler l’apparition d’une cavité et suivre de manière quantitative les mécanismes induits notamment dans le cas des remblais cohésifs. En particulier, une évaluation précise des mécanismes de transferts de charge et de l’interaction sol – renforcement géosynthétique due à un effondrement localisé a été rendue possible par le développement et la validation d’une technique de traitements des résultats par photogrammétrie. L’intensité de la charge transmise par le sol sur le renforcement géosynthétique, la géométrie de sa répartition, ainsi que son évolution sous l’effet d’une surcharge éventuelle en surface ont ainsi été plus spécifiquement étudiées.Les résultats expérimentaux ont été comparés avec des formulations analytiques issues de méthodes de dimensionnement existant dans la littérature. Cette comparaison nous a permis de mieux cerner les domaines de validité des méthodes de dimensionnement analytiques actuelles du renforcement géosynthétique que ce soit pour le cas d’un remblai granulaire ou cohésif et dans certains cas de formuler certaines recommandations. / The French underground is occupied by a considerable number of natural or anthropogenic underground cavities: former mining areas, quarries, karsts, tunnels and abandoned civil structures, etc. These cavities are the source of various risks of ground movements such as sinkholes and subsidence which can have a large impact on the safety people and structures or infrastructures. In order to reduce this risk, a reinforcement of the embankments by geosynthetic in the zones of potential cavities can be implemented. In this context, the thesis has been funded and managed by INERIS (research project EREVAN - Evaluation and Reduction of the Vulnerability of the properties exposed to the natural and mining Hazards), in partnership with 3SR laboratory. One of the objectives of this research is in particular to better understand, further to the opening of an underlying cavity, the behavior and the mechanisms of collapse of reinforced embankment, especially in the case of cohesive soil, in order to optimize its design.As a part of this work, various experiments on physical models in laboratory and on site have been realized. The results obtained in laboratory allowed to determine the behavior of the geosynthetic reinforcement following the collapse of a granular or/and cohesive embankment over a cavity, an important experimental database has thus been established. Full scale experiment allowed to validate the technical, economic and environmental benefits of geosynthetic reinforcement of zones subject to sinkhole.The particular contribution of this work is in the use of original physical models and measurement techniques used to simulate the occurrence of a sinkhole and follow in an accurate quantitative way the involved mechanisms, notably in the case of a cohesive backfill. In particular, a specific evaluation of load transfer mechanisms and soil - geosynthetic reinforcement interaction due to sinkhole has been enabled by the development and the validation of an image processing technique. The intensity of the load transmitted by the ground onto the geosynthetic reinforcement, the geometry of its distribution, as well as its evolution due to possible overburden load have been specifically investigated.Experimental results have been compared with analytical formulations resulting from existing design methods in the literature. This comparison allowed us to better define the domains of validity of the current analytical methods for design of geosynthetic reinforcement whether for granular or cohesive backfill, and in certain cases to formulate some recommendations.

The previous E4 Råneå road embankment was prone to flooding. Risk of flooding in combination with settlements of the road due to the weak underlying sulphide soil was problematic. The Swedish Transportation Administration improved the length section E4 Råneå by rebuilding the road using the method light embankment piling. The centre-to-centre pile distance was chosen to 1.1 m and embankment height 1.8 m. The embankment is reinforced with geosynthetic reinforcements resting on timber piles, which were installed on till stratum. Two geosynthetics were installed, with their strength properties in opposite direction from each other. Their purpose was to stiffen the soil and reduce loading on the weak sulphide subsoil. Field monitoring equipment were placed in the road to measure the behavior before and after consolidation. To validify the results, used instruments in this thesis concerns: pressure cells, extensometers, piezometers and a hydrostatic profile gauge. The Swedish Transport administration wants to evaluate if an increase in piling distance is possible. From the conventional practice of maximum 1.2 m to 1.4 m. It is also interesting if the increased pile distance holds for a taller embankment of 2.5 m. Answering this would aid in increasing the cost-effectiveness of light embankment piling. The performed investigation has been done in the finite element analysis program Plaxis 3D 2021 by simulating half of an embankment with supplementary load model.  To capture field behavior, PLAXIS SoilTest has been used to calibrate the compressive material parameters obtained in oedometer testing. The geosynthetics have been modeled with regards to creep and their stiffness increase on surrounding soil due to interlocking of soil particles. Guaranteeing the reliability of the numerical analysis was made by a comparison of the base model to field monitoring equipment before conducting the parametric study. The base numerical model was reliable in capturing the result of field monitoring equipment. Deviations in pile loads was observed beneath the light trafficked road lane. Conducting the parametric study, the results indicated an increase in pile head loading, total settlements, differential settlements, and deformations in the geosynthetic reinforcement when pile distance and embankment height increased. With a taller embankment of 2.5 m and increased pile distance of 1.4 m numerical simulated pile head loads were in sizes of the design pile strength. Tensile stress in the geosynthetic reinforcement was below long-term design strength. The ratio pile efficacy, that is how effective the structure is at reducing sub soil load has been evaluated in the parametric study at three unit cells. A logarithmic growth is observed when reducing the pile distance at the middle of the road with consistent behavior between embankment height. When studying cells beneath the heavy trafficked lane a linear relationship could be seen instead. This study suggests it is possible to perform the increase in pile distance of 1.4 m for the current embankment height 1.8 m, but needs to be investigated further for the 2.5 m high embankment.

Use of geosynthetics for reinforcing soil beds supporting shallow foundations has gained tremendous popularity in recent times. In this thesis, to study and understand the behaviour of geosynthetics reinforced soil foundations, model load tests are carried out on square footings resting on sand beds reinforced with geosynthetics. The effects of various parameters like type and tensile strength of geosynthetic material, depth of reinforced zone, spacing of reinforcement layers, width of reinforcement and form of reinforcement on the performance of square footings on reinforced sand beds are studied. Results from these tests are analyzed to understand the effect of various parameters in improving the bearing capacity and reducing the settlement of footings. An equation is developed to estimate the ultimate bearing capacity of square footings resting on geosynthetic reinforced sand beds by multiple regression analysis of the experimental data. The model loading tests on reinforced soil foundations are simulated in the numerical model using the computer program FLAC3D (Fast Lagrangian Analysis of Continua in 3D). Finally parametric studies on a full scale reinforced soil foundation are conducted. From the experimental, analytical and numerical investigations carried out in this thesis, some important conclusions are drawn regarding the effective depth of reinforced zone, optimum spacing and quantity of reinforcement layers. Relative efficiency of various forms of reinforcement is discussed. Validity of the regression and numerical models developed is verified through experimental data from present study and also for data from other researchers.