Geosynthetic-reinforced Soil Research Articles

Passive Force–Displacement Behaviour of GRS Bridge Abutments

Many bridge abutments suffered severe damages due to pounding of superstructure elements of the bridge during seismic excitation. Collision of the girder is resisted by passive pressures mobilised in the backfill soils. Seismic load on the bridge structure causes the bridge abutments to undergo lateral translation (Δ) and rotation. The present study focuses on the evaluation of passive force (Pp) developed in the reinforced backfills of the geosynthetic-reinforced soil (GRS) bridge abutments. The GRS abutments of nine configurations with three different geogrid spacing and three different geogrid lengths are modelled using finite element (FE) approach under lateral push. Hypoplastic soil constitutive model with inter-granular strain concept is used to model the soil behaviour. User material subroutine, VUMAT is developed to simulate the soil behaviour in Abaqus. The maximum passive resistance (Pp,ult) increases by 12% in the GRS abutments with closer geogrid spacing.

Required unfactored geosynthetic strength of three-dimensional reinforced soil structures comprised of cohesive backfills

Conservative design of Geosynthetic-reinforced soil structures (GRSSs) is commonly limited to two-dimensional (2D) conditions, ignoring the influence of possible cohesion in backfill material. However, the actual stability of GRSSs is directly influenced by the presence of cohesion – true or apparent – in backfill as well as three-dimensional (3D) effects. In this study, a 3D rational failure mechanism based on the kinematic approach of limit analysis is adopted to assess the stability of GRSSs comprised of cohesive backfills. Within this study, the influence of 3D effects, varying pore water pressures, varying backfill cohesion, and a range of slopes on long-term stability are illustrated in a series of convenient design charts. The results of 3D stability analyses for geosynthetic reinforced walls constructed with cohesive backfills are compared with the results obtained from design guidelines. As expected, when GRSSs are well-drained and relatively narrow in width - or when increasing levels of cohesion are present in the backfill - more stable conditions are realized. For practical scenarios, however, it is critical that cohesive soils should be utilized as backfill with great caution and reliable drainage conditions. Nonetheless, the presented solutions are directly useful towards the assessment of failures of real GRSSs, as they may be constructed with marginal fills that exhibit cohesion, accumulate pore water pressure and often exhibit failure conditions that are three-dimensional in nature.

Numerical evaluation of the effect of differential settlement on the performance of GRS-IBS

This paper presents the numerical results of the performance of the Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS) due to differential settlement underneath the reinforced soil foundation (RSF), underneath the reinforced zone including the RSF, and underneath the retained soil. The performance was evaluated in terms of reinforcement strain, lateral facing deformation, bump at the approach roadway – bridge slab intersection, and lateral facing pressure. Four different differential settlement values of 50, 100, 150, and 200 mm were selected for this study under three different service loading conditions corresponding to bridge spans of 24.4, 30.5, and 36.6 m. Simulations were conducted using the two-dimensional (2D) PLAXIS 2016 finite element (FE) program. The hardening soil model was used to simulate the backfill material behavior. The interface between the backfill material and the reinforcement was simulated using the Mohr-Coulomb model. The reinforcement and facing block were simulated using the linear elastic model. The results of FE analyses indicate that the differential settlement under the RSF has a high impact on both the strain distribution along the reinforcement and the lateral facing displacement.

Reliability-based design optimization of geosynthetic-reinforced soil walls

Structural design optimization usually aims to extract more function from less material. This can result in more failure modes designed against their limits, thus reducing safety. The compromise between cost reduction and safety is particularly relevant for geotechnical structures, as they are subject to significant scatter and uncertainties in their properties. In this context, this paper presents an application of Reliability-Based Design Optimization (RBDO) for the problem of geosynthetic-reinforced soil retaining walls. Design variables include the mean strength of the geosynthetic, spacing between reinforcements, and the length of the geosynthetic. Random variables include soil angle of friction, soil unit weight, the strength of the geosynthetic and the friction angle between the reinforcement and soil mass. The solution procedure proposed herein combines the ant colony optimization (ACO) algorithm with the First Order Reliability Method (FORM) algorithm, which results in nested optimization loops. Results show that significant savings can be achieved in structural design without hindering structural safety.

A Generic Design Protocol for Geosynthetic Reinforced Soil Foundation

This technical note presents a generic step-by-step design protocol based on consideration of bearing capacity of reinforced soil foundation (RSF) on a level ground. The technical note begins with a synopsis of notable studies on bearing capacity of geosynthetic reinforced soil (GRS) and a brief description of an analytical model for load-carrying capacity of closely-spaced soil-geosynthetic composites. The generic RSF design protocol was developed based on findings of the notable studies and the analytical model. Specifically, the design protocol determines (i) reinforcement configuration of RSF (including reinforcement depths, spacing, and width), (ii) required reinforcement stiffness and strength for selection of reinforcement in RSF, and (iii) the increase in ultimate bearing capacity of RSF if the width of RSF is increased to fulfill design requirements A design example for RSF of a GRS bridge abutment is also given to illustrate how the design protocol can be applied.

Measurement of shear strength and interface parameters by multi-stage large-scale direct/interface shear and pull-out tests

It is essential to measure the shear strength of soils and interface parameters between soils and geosynthetics for the safe design and stability analysis of geosynthetic-reinforced soil structures. These parameters recommended for engineering projects are normally measured by laboratory single-stage direct/interface shear and pull-out tests. The conventional single-stage tests are carried out on at least three representative specimens under three different normal stresses. However, a large quantity of specimens is required for large-scale tests, with tedious sample preparation procedures, so that large-scale single-stage testing becomes very labour intensive, time consuming and expensive. Given that the multi-stage testing method is able to measure the shear strength parameters by testing only one representative specimen, this paper investigates the feasibility, reliability and applicability of the multi-stage testing method in large-scale direct/interface shear and pull-out tests. Two compacted soils and a geogrid were tested using both single-stage and multi-stage tests. It was found that the shear strengths obtained from the multi-stage tests were slightly lower that those obtained from the single-stage tests, and the inferred apparent cohesion and friction angle matched closely. In addition, the limitations of the multi-stage testing method were highlighted. The measured direct shear strength of the soils, the interface shear strength and pull-out shear strength between the soils and the geogrid are also compared and discussed in this paper.

Numerical simulation of the deformation response of geosynthetic reinforced soil mini-piers

This paper presents a numerical investigation of the deformation response of geosynthetic reinforced soil (GRS) mini-piers under service load conditions. The backfill soil was characterized using a nonlinear elasto-plastic constitutive model that incorporates a hyperbolic stress-strain relationship and the Mohr-Coulomb failure criterion. The geotextile reinforcement was characterized using linearly elastic elements with orthotropic stiffness. Various interfaces were included to simulate the interaction between different components. The three-dimensional numerical model was validated using experimental data from GRS mini-pier loading tests, including average settlements and maximum lateral facing displacements. Simulation results from a parametric study indicate that backfill soil friction angle, backfill soil cohesion, reinforcement vertical spacing, and reinforcement stiffness have the most significant effects on settlements and lateral facing displacements for GRS mini-piers under service load conditions.

Prediction of creep behaviour from load relaxation behaviour of polymer geogrids

Evaluation of creep behaviour of polymer geosynthetic reinforcement is important for predicting the long-term performance of geosynthetic-reinforced soil (GRS) structures and determining the creep reduction factor of geosynthetic reinforcement for the design lifetime of a GRS structure. The inherent link between the creep behaviour and the stress relaxation behaviour was confirmed by performing a comprehensive series of sustained loading (SL) tests and stress relaxation (SR) tests and numerical simulations of SL and SR tests on several geogrid types. In the framework of the non-linear three-component (NTC) model, the creep strain in SL is entirely irreversible, while the tensile load decrement in SR is associated with the developed positive irreversible strain with zero total strain rate. Measured systematic relationships between the elapsed times necessary to reach the same irreversible strain rate in SL and SR tests and those between the irreversible strain increments necessary to reach the same irreversible strain rate in SL and SR tests are presented and simulated by the NTC model. It is shown that, based on these relationships, the time history of creep strain can be predicted from given results of SR tests performed for a much shorter period. This method is validated by successful simulations of experimental results.

Numerical study on maximum reinforcement tensile forces in geosynthetic reinforced soil bridge abutments

This paper presents a numerical study of maximum reinforcement tensile forces for geosynthetic reinforced soil (GRS) bridge abutments. The backfill soil was characterized using a nonlinear elasto-plastic constitutive model that incorporates a hyperbolic stress-strain relationship with strain softening behavior and the Mohr-Coulomb failure criterion. The geogrid reinforcement was characterized using a hyperbolic load-strain-time constitutive model. The GRS bridge abutments were numerically constructed in stages, including soil compaction effects, and then loaded in stages to the service load condition (i.e., applied vertical stress = 200 kPa) and finally to the failure condition (i.e., vertical strain = 5%). A parametric study was conducted to investigate the effects of geogrid reinforcement, backfill soil, and abutment geometry on reinforcement tensile forces at the service load condition and failure condition. Results indicate that reinforcement vertical spacing and backfill soil friction angle have the most significant effects on magnitudes of maximum tensile forces at the service load condition. The locus of maximum tensile forces at the failure condition was found to be Y-shaped. Geogrid reinforcement parameters have little effect on the Y-shaped locus of the maximum tensile forces when no secondary reinforcement layers are included, backfill soil shear strength parameters have moderate effects, and abutment geometry parameters have significant effects.

Experimental evaluation of the effect of compaction near facing on the behavior of GRS walls

Experimental studies have been carried out to evaluate the effect of the compaction condition at the back of block facing on the behavior of geosynthetic reinforced soil (GRS) walls. Three GRS walls with 1.2 m high were constructed at the COPPE/UFRJ Geotechnical Laboratory. The walls were well-instrumented in order to monitor the values of the reinforcement load, toe horizontal load, horizontal facing displacement, horizontal stress at the back of the block facing, and vertical displacement on the top of the walls. The behavior of the walls has been investigated at the end of construction and during the surcharge application (post-construction). At the end of the loading, the toes of the walls were gradually released to also verify the influence of the different toe restraints. The results clearly show the effect and call attention to the importance of the compaction conditions near the facing on the behavior of GRS walls.

Investigation of load transfer mechanisms in granular platforms reinforced by geosynthetics above cavities

Geosynthetic-reinforced soils constitute an interesting solution for bridging cavities. Many methods have been developed to analyze the stability of soil-geosynthetic-cavity systems, but none of them is able to take into account all the complexities of these mechanisms. Many researchers have assumed mechanisms developed in the reinforced granular platform when cavities appear, such as load transfer and expansion of materials. However, they are not fully understood because many factors can influence the design, such as the cavity opening processes, the type, and the density of the soil.In this study, a new laboratory apparatus is developed to simulate two different cavity opening procedures (trapdoor and progressive opening) for different geometric configurations. A series of tests is conducted for three granular soils with two different geosynthetic sheets. By measuring the shape of the surface soil settlement and the geosynthetic deflection, the expansion coefficient is calculated. A novel tactile pressure sensor is used to observe the load transfer during the cavity opening. The experimental data are analyzed and the influence of the experimental conditions (geometric and soil properties and the opening procedure) are also discussed. Correspondingly, elicited findings can be used to propose recommendations to improve the existing design methods.

Relationship between Arias intensity and the responses of reinforced soil retaining walls subjected to near-field ground motions

A validated dynamic Finite Element procedure was employed to investigate the seismic responses of geosynthetic-reinforced soil retaining walls subjected to various near-field ground motions. The study is focused on the relationship between the ground motion parameters and the reinforcement loads and residual facing displacement. Twenty-five near-field motions from two earthquakes were scaled to a peak acceleration of 0.4 g and employed as the excitation motions. It was found that the maximum reinforcement load in the retaining wall is determined by the vibration characteristic of the earth structure as well as the intensity of the input motion, and the Arias intensity at the top of the reinforced soil zone is well correlated with the maximum reinforcement load. The residual lateral facing displacement is also affected by these two factors, but a very good correlation with the Arias intensity cannot be established.

Life cycle assessment of a geosynthetic-reinforced soil bridge system – A case study

Road infrastructures are a very important component of the world's total transportation network. Investment in its construction and maintenance is significant on a global scale. The paper presents some results from an environmental study of a geosynthetic-reinforced soil integrated bridge system. The Pavlovski potok stream in Slovenia was used as a demonstration case for this study. It is the first GRS bridge system with full-height rigid (FHR) facings in Europe. It was constructed at the end of 2014. The goal of these analyses was to compare two different types of bridges: the new GRS bridge system, which is comprised of a simple girder partially structurally integrated to FHR facings of GRS bridge abutments and a conventional reinforced concrete road bridge. The results of an environmental life cycle assessment (LCA) show that the GRS bridge system has a much lower environmental impact than an equivalent bridge conventionally built with reinforced concrete.

Soil–Geosynthetic Interface Shear in Different Testing Scales

Geosynthetics have been used to improve mechanical performance of roadway layers (e.g., geosynthetic-reinforced asphalt, geosynthetic-stabilized bases) and a wide range of transportation infrastructures (e.g., geosynthetic-reinforced soil walls). A key aspect in understanding soil–geosynthetic interaction mechanisms involved in each application includes characterization of the interface between geosynthetics and adjacent materials. This study evaluates soil–geosynthetic interface shear in various pullout test scales including standard, smaller than standard, and larger than standard scales. Experimental results obtained from tests conducted in each scale were analyzed to determine the soil–geosynthetic interface shear model. An iteration procedure, similar to that used in t–z analysis of pile loading, was developed to simulate incremental geosynthetic movements. Shape and parameters of the interface shear model were changed to minimize the residual error between experimental and simulated data. It was found that mobilization of the interface shear in the small-scale test differs from that in the standard- and large-scale tests. In the standard- and large-scale tests, the ultimate soil–geosynthetic interface shear mobilized at comparatively small displacements, which could be represented by a linear plastic interface shear model. In the small-scale test, however, the interface shear developed in two phases. A portion of the ultimate interface shear mobilized at comparatively small displacements while additional resistance continued to mobilize at extended displacements. Consequently, the development of interface shear resistance in the standard- and large-scale tests was found to depend on progressive increase of the geosynthetic mobilized length, whereas in the small-scale test the interface shear resistance developed by displacement of the entire geosynthetic.

Numerical Simulation of Deformation and Failure Behavior of Geosynthetic Reinforced Soil Bridge Abutments

This paper presents a numerical investigation of the deformation and failure behavior of geosynthetic reinforced soil (GRS) bridge abutments. The backfill soil was characterized using a nonlinear elastoplastic constitutive model that incorporates a hyperbolic stress–strain relationship with strain-softening behavior and the Mohr–Coulomb failure criterion. The geogrid reinforcement was characterized using a hyperbolic load–strain–time model. The abutments were numerically constructed in stages, including soil compaction effects, and then monotonically loaded in stages to failure. Simulation results indicate that a nonlinear reinforcement model is needed to characterize deformation behavior for high applied stress conditions. A parametric study was conducted to investigate the effects of reinforcement, backfill soil, and abutment geometry on abutment deformation and failure behavior. Results indicate that reinforcement vertical spacing, reinforcement stiffness, backfill soil friction angle, and lower GRS wall height are the most significant parameters. The shape of the failure surface is controlled primarily by abutment geometry and can be approximated as bilinear.

Earth pressure coefficients for reinforcement loads of vertical geosynthetic-reinforced soil retaining walls under working stress conditions

Based on the nonlinear elastic theory and stress-dilatancy theory, two earth pressure coefficients were proposed to analyze the reinforcement loads at the potential failure surface of vertical geosynthetic-reinforced soil retaining walls under working stress conditions. The earth pressure coefficients take into account the force equilibrium and compatible deformations between soil and reinforcement, and can be obtained by solving two implicit functions by an iterative or graphic method. The effects of backfill compaction and facing restriction are taken into account in the earth pressure coefficients by two additional stress factors, which have been derived analytically using straightforward approaches. To validate the effectiveness of the proposed methods, comparisons were made with the results from large scale tests and numerical simulations. It was demonstrated that the reinforcement loads predicted by the proposed methods were in good agreement with the experimental or numerical results.

Numerical simulation of two-tier geosynthetic-reinforced-soil walls using two-phase approach

In this study, the mechanical behavior of two-tier Geosynthetic-Reinforced Soil walls is investigated numerically by using the concept of two-phase systems. Comparison of the results of this approach with those of discrete numerical models and centrifuge tests indicates that the approach has the ability to consider the interaction between tiers, predict the reinforcement load and wall-face displacement. Furthermore, it is more cost-effective. The limitations of this approach pertain to the prediction of the failure surface and the wall deformation regime. Totally, the two-phase approach can be properly applied in a fast, effective and safe manner.

3D effects of turning corner on stability of geosynthetic-reinforced soil structures

Current design procedures of Geosynthetic-Reinforced Soil Structures (GRSS's) are for walls/slopes with long straight alignments. When two GRSS segments intersect, an abrupt change in the alignment forms a turning corner. Experience indicate potential instability problems occurring at corners. The purpose of this study is to explore the effects of turning corner on the stability of reinforced slopes. Three-dimensional (3D) slope stability analysis, based on limit equilibrium, resulted in the maximum tensile force of reinforcement. Parametric studies required numerous computations considering various geometrical parameters and material properties. The computed results produced efficient practical format of stability charts. For long-term stability of reinforced slopes with turning corner, the influences of pore water pressure and seismic loading are also considered. Turning corner can improve the stability of reinforced slopes by virtue of inclusion of end effects. However, localized increase of pore water pressure or directional seismic amplification may decrease locally thus stability requiring strength of reinforcement larger than in two-dimensional (2D) plane-strain. While using 2D analysis for non-localized conditions may require stronger reinforcement, it also requires shorter reinforcement than in 3D analysis; i.e., 2D analysis may be unconservative in terms of reinforcement length.

Experimental and Numerical Analysis of Geogrid-Reinforced Soil Systems

Geosynthetic-reinforced soil technique has been increasingly used in civil engineering practice over the last two decades. Understanding the response of soil reinforcements to pullout loading is considered to be essential to any successful analysis and design of reinforced soil structures. This paper presents the results of a series of experimental investigation supported by numerical analysis to examine the pullout behavior of geogrid reinforcing element under static loading. The purpose of the study is to numerically simulate the response of geosynthetic reinforcement to static pullout loading. Finite element Plaxis software was used to model the mechanical behavior of the interface between geogrid–soil dissimilar materials. The numerical model was calibrated and validated using the experimental data generated during this investigation. The results of this study demonstrate that numerical 3D model can give good predictions of the pullout behavior of geogrid-reinforced soil systems under static loads.

Numerical simulation of compaction-induced stress for the analysis of RS walls under working conditions

This paper aimed to verify numerical modelling of compaction-induced stress (CIS) for the analysis of geosynthetic-reinforced soil (GRS) walls under working stress conditions. Data from a full-scale well-instrumented GRS wall was used for a numerical analysis. The results from the wall used in this study have already been used for validation in several other numerical modelling studies. Nevertheless, in none of these studies was the real value of CIS specified for the vibrating plate compactor used in the wall employed. In the present study, the real value of CIS is employed. The CIS is modelled using a new procedure presented in this paper in addition to two other procedures found in the literature. The results indicate that when the real value of CIS was simulated using a strip load applied to the top of each backfill layer, the numerical model accurately represented the measurements. The accuracy of the results, however, depends on the width of the strip load used to model the CIS. Nevertheless, as this type of compaction modelling procedure is time consuming, modelling of CIS by applying a distribution load at the top and bottom of each soil layer is suggested as an alternative procedure.