Soil-geosynthetic Interaction Research Articles

Application of material point method in modeling soil-geosynthetics interactions-a literature survey

The material point method (MPM) has garnered significant attention in recent years owing to its advantages in solving soil–water-structure interaction problems involving large deformations in geotechnical engineering. The MPM combines the benefits of point-based and mesh-based approaches (finite element method) with both Eulerian computational mesh and continuum descriptions of materials. The successful integration of MPM in simulated landslides, internal erosion, and excavation has been frequently reported. However, solving the soil–geosynthetic interaction problem with the MPM has not been explored, although such problems often entail large deformations. The goal of this study is to collate studies on the simulation of geosynthetics and their interactions with soil using MPM. This paper first discusses the basics of MPM and the formation of thin membrane materials using MPM. It also includes limited applications of MPM in simulating soil–geosynthetic interactions. The applications demonstrate that the MPM is particularly effective in resolving large deformation problems associated with geosynthetics, including problems of landfill settlement, reinforced-slope stability, and geocontainer dropping.

Evaluation of the behaviour of pipes buried in geosynthetic reinforced ground

This paper aims to present an evaluation of the soil-geosynthetic interaction used for protection and reinforcement of a buried flexible pipe inserted in non-reinforced soils and soils reinforced with geosynthetics subjected to the application of a surface overload. Therefore, tests were carried out in models (scale 1:3.5) and numerical simulations using PLAXIS 2D program. In laboratory tests, a 200 mm diameter PVC pipe was used buried in a non-cohesive soil and different types of geosynthetics. The reinforcement used were four geogrids and a woven geotextile, with tensile stiffness varying between 34 kN/m2 and 2000 kN/m2. The results of the experimental tests showed beneficial effects regarding the insertion of the reinforcement in the massif with buried pipe. The efficiency of reinforcement depends on the type of arrangement considered and its physical and mechanical properties. Overall, the numerical simulations showed that the predicted results reproduced the patterns of the experimental results, but some significant deviations between predictions and measurements were observed.

Effects of cyclic loading on soil-geogrid interaction characteristics

The benefits of geosynthetic-reinforced soil systems over conventional earth-retaining structures are now well established. These reinforced systems are often subjected not only to static loads, but also to seismic and/or traffic loads, in which case the effects of repeated loading on soil-geosynthetic interaction characteristics should be properly considered. This study investigates the behaviour of a geogrid typically used for soil reinforcement under cyclic pullout loading through load-controlled laboratory pullout tests. To examine the influence of cyclic loading amplitude, number of cycles and static pullout force acting on the geogrid at the onset of cyclic loading, distinct loading patterns are considered. A well-graded residual soil from granite is used as backfill material. A comparison between the cyclic and monotonic pullout response of the reinforcement is then established in order to identify any potential strength loss attributed to cyclic loading. The experimental results show that the ultimate pullout resistance of the geogrid embedded in medium dense residual soil from granite may be adversely affected by cyclic loading. The cumulative cyclic displacements of the reinforcement are more pronounced during the initial loading cycles, but tend to stabilize with the increasing number of cycles when the soil is densely compacted. In the presence of dense soil, the cyclic strains of the geogrid specimen are particularly significant at the front section and almost negligible towards the back end.

Guest Editorial for the Special Issue on “Soil–Geosynthetic Interaction”

Guest Editorial for the Special Issue on “Soil–Geosynthetic Interaction”

Soil–Geosynthetic Interaction Under Triaxial Conditions: Shear Strength Increase and Influence of the Specimen Dimensions

Geosynthetics are extensible reinforcements used to enhance the engineering performance of a soil. The transfer of stresses from the soil to the reinforcement is achieved through soil–geosynthetic interaction. The proposals from the literature to assess the shear strength of the reinforced soil under triaxial conditions use three different approaches. These involve analysing the reinforced soil: (i) as a homogeneous composite material (Approach A), (ii) as two different materials (Approach B), or (iii) as soil having the same fundamental shear strength, with the effect of the reinforcement represented as an additional lateral or confining stress (Approach C). In this paper, triaxial tests data of a soil reinforced with a geosynthetic, and specimens with different dimensions (diameters 70 and 150 mm) were used to assess changes in shear strength and to carry out a statistical analysis. The increases in shear strength of the reinforced soil and of the soil–geosynthetic interface were analysed using equations from the literature. The difference between the triaxial results obtained from specimens of different sizes was assessed using an Analysis of Covariance (ANCOVA). When the joint term of the regression model was not statistically significant, the characterisation from different specimen sizes was used to generate soil failure envelopes. Thus, a new methodology to obtain a failure envelope with different specimen sizes is presented.

Advances in the Use of Geosynthetics for Stabilization of Unbound Aggregate Layers

The governing mechanism associated with the stabilization of unbound aggregate layers in pavements is lateral restraint. Reproducing this mechanism in the laboratory maybe challenging because, while the original loading source is cyclic (traffic), lateral restraint develops through interlocking and interfacial friction between the geosynthetic and the aggregate to restrain the development of permanent lateral strains. Considering the relevance of lateral restraint in the quantification of the benefits of geosynthetics embedded within (or adjacent to) unbound aggregate layers, this study focuses on two experimental approaches to quantify this mechanism. The first experimental approach aims at defining a design parameter, identified as the Stiffness of the Soil-geosynthetic Composite (KSGC), which is obtained from Soil-Geosynthetic interaction (SGI) tests and is practical for use in specifications and design. The second experimental approach that quantifies the lateral restraint mechanism involves one-third scale accelerated pavement tests (APTs), which were performed on pavement test sections stabilized with various geosynthetics, diverse in terms of geometry and materials. The rutting from these sections was compared to that in the non-stabilized (control) section to evaluate the Traffic Benefit Ratio (TBR) at failure rut depth for each geosynthetic. The TBR obtained showed a strong linear correlation to the KSGC of the corresponding geosynthetic determined by SGI tests. Overall, the KSGC parameter was found to represent a suitable indicator of the performance of pavements with unbound aggregate layers stabilized using geosynthetics.

Evaluation of required stiffness and strength of cellular geosynthetics

Cellular geosynthetics have been widely used in engineering practices for their enhanced confining effects. However, limited research has been reported on methods of evaluating the required stiffness and strength of cellular geosynthetics based on the strength of geosynthetic-encased soil in engineering practices. In this study, an analytical approach was formulated to estimate the required stiffness and strength of cellular geosynthetics based on the equivalent strength of geosynthetic–soil composites considering soil nonlinearity, soil plasticity, soil dilatancy, and soil–geosynthetic interaction. Large-scale triaxial compression tests on geosynthetic-encased sand were performed to validate the effectiveness of the method. Parametric studies were conducted using the validated method to investigate the effects of nonlinear elastic parameters, the peak strength of the infill soil, and the aperture size of cellular geosynthetics on the required stiffness and strength.

Analytical solution for calculation of pull out force-deformation of geosynthetics reinforcing unsaturated soils

Performing tests on geosynthetic-reinforced soils under pullout conditions gives a comprehensive understanding of soil-geosynthetic interaction. However, they may be expensive and time-consuming. Therefore, the development of theoretical solutions may be beneficial to recognize characteristics of geosynthetic-reinforced soils under pullout conditions. This paper proposes an analytical solution for the determination of pullout force-deformation variation of planar geosynthetics reinforcing unsaturated cohesive-frictional soils. The solution predictions were validated with experimental results, demonstrating the capability of the presented solution. The solution can extend limited results obtained from experiments to a wider range of conditions. It has been demonstrated that under 20 kPa overburden stress, with decreasing the soil suction from 1153 to 312 kPa, corresponding to increasing the moisture content from 16% to 20%, the peak pullout force decreases by about 28%. The results also illustrate that by increasing the value of matric suction, the amount of U* (relative soil-reinforcement displacement corresponding to the total mobilization of friction) decreases.

Use of geosynthetics to mitigate problems associated with expansive clay subgrades

Geosynthetics have recently been used for base course stabilization of roadways subjected to environmental loads associated with the presence of expansive clay subgrades. Repeated cycles of wet and dry seasons have often led to significant, non-uniform moisture changes within clay subgrades, resulting in differential settlements between the roadway edges and its centerline and, ultimately, in environmental longitudinal cracks. This paper quantifies the field performance of different sites in order to assess the effectiveness of using geosynthetics to stabilize the base course of roadways constructed on expansive clay subgrades. This includes evaluation of five full-scale field projects that had been subjected to actual traffic and environmental loads. The long-term performance of geosynthetic-stabilized and control sections was evaluated by quantifying the development and extent of longitudinal cracks and the degradation of the base course stiffness. Collectively, the performance evaluation of the multiple geosynthetic-stabilized and control sections in the five case studies demonstrates that geosynthetics can effectively mitigate roadway problems associated with expansive clay subgrades. In addition, field performance data also indicates that unconfined stiffness and tensile strength may not be sufficient for proper geosynthetic selection, pointing to the need for selecting them using properties that also quantify the soil-geosynthetic interaction.

Effects of Moisture on the Mechanical Behavior of Sand–Geogrid: An Experimental Investigation

Soil–geosynthetic interaction is a key factor for designing and analyzing reinforced soil structures. In this investigation, the effect of different moisture content on the behavior of soil–geogrid interfaces is evaluated using direct shear and pullout tests. The results are presented and discussed in terms of direct shear and interface shear behavior, the frictional resistance of interfaces, efficiency factor, pullout resistance, pullout behavior, and apparent friction coefficient. The results revealed that a decrease in moisture content resulted in an increase in the both interface friction angle and adhesion of specimens. Moreover, the interface friction angle of sand–geogrid specimens in pullout tests was higher than that of direct shear tests. According to interface shear results, the efficiency factors ranged from 0.87 to 0.99. Based on pullout results, the apparent friction coefficient was found to be in the range of 0.77–1.72.

Soil-geosynthetic interaction in inclined-plane shear and geosynthetic short-term tensile response – Influence of installation damage

This paper contributes to better understanding how installation damage affects the soil-geosynthetic interaction in inclined plane shear movement and the tensile response of geosynthetics. Installation damage was induced in the field and simulated in laboratory adapting a standardised procedure. Three geosynthetics were studied: one geotextile and two geogrids. The installation damage affected the soil-geosynthetic interface strength and the sliding mechanisms observed. The results showed that installation damage often caused an increase in the skin friction available associated to increased surface roughness, not always apparent from visual inspections. For most samples, the friction mobilised in solid area of the geogrids increased after installation damage, which depended on the geogrid. The reduction factors for installation damage for the inclined plane shear strength and the tensile strength of the geosynthetics were compared. Although the interface shear strength did not change significantly for any of the geosynthetics studied, the tensile strength of the geotextile was severely reduced. The structure of the geosynthetics had a higher impact on their tensile response after damage than on the soil-geosynthetic interface in inclined plane shear. The mechanical damage induced in laboratory underestimated the reduction factor for the interface strength in inclined shear plane movement relative to field installation damage. The results show that the interface shear strength was not significantly influenced by installation damage and that the standardised laboratory test used to induce mechanical damage is able to represent satisfactorily the effect of the installation damage on the inclined plane shear response of soil-geosynthetic interfaces.

Novel experimental techniques to assess the time-dependent deformations of geosynthetics under soil confinement

A new experimental approach to assess the impact of soil confinement on the long-term behavior of geosynthetics is presented in this paper. The experimental technique described herein includes a novel laboratory apparatus and the use of different types of tests that allow generation of experimental data suitable for evaluation of the time-dependent behavior of geosynthetics under soil confinement. The soil-geosynthetic interaction equipment involves a rigid box capable of accommodating a cubic soil mass under plane strain conditions. A geosynthetic specimen placed horizontally at the mid-height of the soil mass is subjected to sustained vertical pressures that, in turn, induce reinforcement axial loads applied from the soil to the geosynthetic. Unlike previously reported studies on geosynthetic behavior under soil confinement, the equipment was found to be particularly versatile. With minor setup modifications, not only interaction tests but also in-isolation geosynthetic stress relaxation tests and soil-only tests under a constant strain rate can be conducted using the same device. Also, the time histories of the reinforcement loads and corresponding strains are generated throughout the test. Results from typical tests conducted using sand and a polypropylene woven geotextile are presented to illustrate the proposed experimental approach. The testing procedure was found to provide adequate measurements during tests, including good repeatability of test results. The soil–geosynthetic interaction tests were found to lead to increasing geotextile strains with time and decreasing reinforcement tension with time. The test results highlighted the importance of measuring not only the time history of displacements but also that of reinforcement loads during testing. The approach of using different types of tests to analyze the soil–geosynthetic interaction behavior is an innovation that provides relevant insight into the impact of soil confinement on the time-dependent deformations of geosynthetics.

Analyzing the deformation and failure of geosynthetic-encased granular soil in the triaxial stress condition

The equivalent strength and stiffness of geosynthetic-encased soil are two important parameters for analysis of cellular geosynthetic-reinforced foundations and earth structures. However, limited analytical approaches exist for the estimation of these parameters, and this limitation hinders their applications in geotechnical engineering practice. In this study, an analytical method is proposed for the prediction of the stress-strain response of geosynthetic-encased soil by employing the soil response in the triaxial stress condition and the theory of thin cylinders. This method has the advantages of theoretical rigorousness and convenience for use and can consider soil nonlinearity, soil dilatancy, soil plasticity and soil-geosynthetic interaction. Different types of yield criteria for soils can be readily incorporated into the proposed method. The proposed method is validated against the results of triaxial compression tests on geosynthetic-encapsulated sand. The good agreement between the predicted stress-strain curves and the measured curves demonstrates the effectiveness of the method. In addition, design tables and parametric studies are provided by employing the formulated analytical method for application purposes.

Interaction Between a Lateritic Soil and a Non-woven Geotextile in Different Moisture Conditions

This work evaluates the increase in stiffness provided by the variation of moisture content (matrix suction) and the grain size structure at the interface between a cohesive tropical soil (lateritic clay) and a nonwoven geotextile. For this purpose, monotonic pullout tests with soil suction monitoring were performed on small-sized equipment under three scenarios (O Optimum, D Dry and DP Dry Post-Compaction) and two vertical pressures (14 and 28 kPa). The Dry Post-Compaction tests were conducted with the same matrix suction of the Dry tests. The monotonic pullout tests evaluated the soil-geosynthetic interaction under constant displacement to calculate the apparent confined stiffness of the geosynthetic (Jc). The Wide Width Tensile tests were conducted to assess the unconfined stiffness of the nonwoven geotextile (Jn). The Dry scenario, compacted with higher compaction energy than the others and, consequently, altering its grain size structure, presented the best performance. This indicates that other parameters besides the suction may be influencing the interaction between the soil and the non-woven geotextile. Even under small vertical pressures, the apparent confined stiffness improves considerably compared to the unconfined stiffness. This indicates that the use of the unconfined stiffness obtained by index tests may be a conservative measure in paving projects.

Effect of soil-reinforcement interaction coefficient on reinforcement tension distribution of reinforced slopes

This paper examines the effect of the mobilized reinforcement tension within reinforced soil slope at a different level of soil-geosynthetic interaction. The mobilized reinforcement tension is assumed, in most design methods for the internal stability of reinforced slopes, to be equal to mobilized soil forces computed using a limit equilibrium method. However, comparison with the reinforcement tension force measured in the field has shown that this approach is conservative. This paper examines the effects of the soil-reinforcement interaction coefficient on the tensile redistribution of geosynthetics. The modified process of Bishop Method of slope stability analysis is used to locate the critical slip surface and to calculate the mobilized reinforcement tensile force. The reinforcement forces obtained from field data and on centrifuge model test results for a reinforced slope problem are used to examine the relationship between mobilized reinforcement tensile force and mobilized soil shear strength.

Pullout Behavior of Different Geosynthetics—Influence of Soil Density and Moisture Content

Geosynthetics have increasingly been used as reinforcement in permanent earth structures, such as road and railway embankments, steep slopes, retaining walls and bridge abutments. The understanding of soil-geosynthetic interaction is of primary importance for the safe design of geosynthetic-reinforced soil structures, such as those included in transportation infrastructure projects. In this study, the pullout behaviour of three different geosynthetics (geogrid, geocomposite reinforcement and geotextile) embedded in a locally available granite residual soil is assessed through a series of large-scale pullout tests involving different soil moisture and density conditions. Test results show that soil density is a key factor that affects the reinforcement pullout resistance and the failure mode at the interface, regardless of geosynthetic type or soil moisture content. The soil moisture condition may considerably influence the pullout response of the geosynthetics, particularly when the soil is in medium dense state. The geogrid exhibited higher peak pullout resistance than the remaining geosynthetics, which is associated with the significant contribution of the passive resistance mobilised against the geogrid transverse members to the overall pullout capacity of the reinforcement.

Impact of temperature on the pullout of reinforcing geotextiles from unsaturated silt

This study investigates the thermal soil-geosynthetic interaction mechanisms of reinforcing geotextiles confined in compacted silt that may be encountered when using mechanically-stabilized earth (MSE) walls as geothermal heat sinks. A thermo-mechanical geosynthetic pullout device was used that incorporates standard components for geosynthetic pullout or creep testing but also heating elements at the top and bottom of the soil box to apply boundary temperatures and dielectric sensors embedded in the soil layer to monitor distributions in temperature and volumetric water content. Two test series were performed: the first involves monotonic pullout of woven polypropylene geotextiles after reaching steady-state conditions under different boundary temperatures without a seating load, and the second involves monotonic pullout of woven polyethylene-terephthalate geotextiles after reaching steady-state conditions under different boundary temperatures with a seating pullout load. The results indicate that the pullout resistance of both geotextiles decreased with increasing temperature. Although heating led to drying of the unsaturated silt layers as expected, measurements from the second test series indicate accumulation of water at the silt-geotextile interface. An effective stress analysis considering thermal softening of soils indicates that the increase in effective saturation at the silt-geotextile interface was the cause of the decrease in pullout resistance with heating.

HDPE geogrid-residual soil interaction under monotonic and cyclic pullout loading

The understanding of soil-geosynthetic interaction under cyclic loading conditions is essential for the safe design of geosynthetic-reinforced soil structures subjected to repeated loads, such as those induced by road and railway traffic and earthquakes. This paper describes a series of large-scale monotonic and multistage pullout tests carried out to investigate the behaviour of an HDPE uniaxial geogrid embedded in a locally available granite residual soil under monotonic and cyclic pullout loading. The effects of the pullout load level at the start of the cyclic stage, cyclic load frequency and amplitude, number of cycles and soil density on the load-strain-displacement response of the reinforcement are evaluated and discussed. Test results show that the cumulative displacements measured along the length of the geogrid during cyclic loading increased significantly with the pre-cyclic pullout load level and the load amplitude. In contrast, the cumulative cyclic displacements were found to decrease with increasing frequency and soil density. In medium dense soil conditions, the geogrid post-cyclic pullout resistance decreased by up to 20%, with respect to the value obtained in the comparable monotonic test. However, for dense soil, the effect of cyclic loading on the peak pullout forces recorded during the tests was almost negligible.

Global and local sand–geosynthetic interface behaviour

The influence of overall particle shape, mean particle size, initial relative density and effective normal stress on the global and local soil–geosynthetic interaction is investigated here through an extensive series of direct shear interface tests between six sands in contact with one multifilament woven geotextile, one non-woven geotextile and one geomembrane. Analysis of the soil–geosynthetic data highlights mobilisation of higher peak friction angles with increases in initial relative density and angularity of sand particles and decrease in the effective normal stress. The average frictional efficiencies of the sand–geosynthetic interfaces are 0·95 and 0·79, respectively, for the sand–geotextile and sand–geomembrane interfaces. It is shown that the interface slip increases with the initial relative density of sand and regularity of particles, and decreases with increasing the mean particle size and effective normal stress. Analysis of the local particle movements by means of digital particle image velocimetry revealed that the average thickness of the shear zone for the sand–geosynthetic interfaces tested ranges from 3·8 to 11·9 (with an average of 7·0) times the mean particle size of the contact sand. The experimental data indicate that thicknesses of the shear and dilation zones are almost equal for the sand–geosynthetics interfaces tested.

Investigation of the Soil-Geosynthetic Interaction Using Direct Shear Testing and FEM Method

Verification of the design of the reinforced earth retaining structures is usually based on the global and internal stability check. Beside the analytical solutions, numerical modelling can simulate the real behaviour of the structure including mutual interaction of the particular elements. Additionally, stress-strain relation is involved in the final verification together with the required stability of the system. Parameters such as reinforcement stiffness or interaction ability with the soil allow to calculate more realistic axial forces acting in the reinforcing elements. Analytical solutions don’t take into account the influence of the deformation of the soil mass on the overall and internal stability. Finite Element Method can describe both of these mechanisms which take place simultaneously. Estimation of the interaction parameters is crucial to determine the forces with sufficient accuracy, especially when 2nd limit state is critical for the structure design. The paper is aimed at the laboratory testing and numerical modelling of the interaction of the geosynthetic reinforcement and the soil during the sliding described by the interaction coefficient. A direct shear test was adopted to investigate the actual value of the interaction coefficient which is related to the certain displacement of the reinforcement in the soil. There is a recommendation not to use the interface element at the soil-reinforcement contact when the grid shaped reinforcing elements such as geogrids are used for the numerical simulations. This approach was approved by the study presented in this paper. Reinforcement with the grid structure and rigid joints provides higher level of interaction with the soil environment so reduction of the shear strength at this contact is unrealistic.