Design Of Geosynthetic-reinforced Soil Structures Research Articles

A new apparatus for the study of pullout behaviour of soil-geosynthetic interfaces under sustained load over time

At present, the design of geosynthetic-reinforced soil structures is executed with reference to the tensile strength of the reinforcement obtained from in-air short-term tensile tests, decreasing this value by means of several factors. Among these, the creep effect resulting from in-air tensile creep tests reduces tensile strength the most. Consequently, this procedure does not take into account the effects of soil confinement and interaction on the tensile response of the reinforcements. This paper illustrates a new large-scale pullout prototype apparatus, with the capacity to investigate the behaviour of a geosynthetic reinforcement embedded in a compacted soil and subject to a tensile load kept constant over time. The apparatus allows the verification of how the soil can modify the prediction of the long-term behaviour of geosynthetics. Results in terms of confined tensile strains were analysed, and the comparison of those values with the strains obtained by in-air tensile creep tests has led to the conclusion that the creep reduction factor might be conservative. Moreover, the confined tensile strains were related to the apparent coefficients of friction to propose a new procedure capable of determining the design interaction parameter under long-term pullout load as a function of the allowable reinforcement strains.

Particle shape effects on the cyclic shear behaviour of the soil–geogrid interface

The accurate determination of the interface shear strength is essential in the design of geosynthetic-reinforced soil structures. The particle geometries of three types of soil materials and a spherical granular medium are imaged and quantified using binary image-based methods and described in terms of regularity. Cyclic direct shear tests are conducted to investigate the effects of particle regularity on the interface shear strength, stress–displacement relationship, shear stiffness, and damping ratio. The results reveal that the interface shear strength and deformation strongly depend on particle regularity. The vertical displacement ratio is found to increase with particle regularity under the same cycle number. The interface stiffness is observed to increase with the cycle number for particle regularities of 0.453, 0.565, and 0.672 but decreases with the cycle number for a particle regularity of 0.971. For a given regularity, the trend of damping ratio with the increasing cycle number is contrary to the that of shear stiffness. Finally, it is observed that the cyclic friction angle decreases with increasing particle regularity, the relationship of which is determined using linear regression. Thus, the systematic quantification of particle shape characteristics can lead to a better understanding of soil–geogrid interface behaviour.

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.

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.

The influence of a cyclic loading history on soil-geogrid interaction under pullout condition

The knowledge of soil-geosynthetic interface behaviour is a key point in the design of geosynthetic-reinforced soil structures. The pullout ultimate limit state can be reproduced conveniently by means of pullout tests performed with large-size laboratory apparatuses, which allow studying the interaction mechanisms that develop in the anchorage zone. During the service life of geosynthetic-reinforced soil structures, reinforcements may be subjected to long-term cyclic vehicular loads or short-term seismic loads in addition to dead loadings, such as the structure's self-weight and other sustained loads. In order to study the influence of a cyclic loading history (a sinusoidal function with fixed amplitude A, number of cycles N and frequency f) on the post-cyclic peak pullout resistance, the writers carried out a series of multi-stage pullout tests on a high density polyethylene extruded uniaxial geogrid embedded in a compacted granular soil for different vertical effective stress σ′v values. Moreover, the stability of the soil-geosynthetic interface from a point of view linked to the cyclic loading application has also been investigated. Test results showed that the design pullout resistance parameters are affected by the applied cyclic loading history for specific combined conditions (A, N and σ′v) and it should be taken into account for designing geosynthetic reinforced soil structures.

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.

Limit state design framework for geosynthetic-reinforced soil structures

Conventional design of geosynthetic-reinforced soil structures is divided into two categories, walls and slopes, based on the batter of its facing system. Internal stability, characterized as sufficient reinforcement anchoring and strength, is performed using earth pressure-based design criteria for reinforced walls while reinforced slopes are founded on limit equilibrium (LE) slope stability analyses. LE analyses are also used to assess the global or compound stability of both types of structures, accounting for the geometry of the reinforced, retained and foundation soils. The application of LE-based methods typically results in determination of a slip surface corresponding to the lowest attained Safety Factor (SF), known as the Factor of Safety (Fs); however, it yields little information about reinforcement loading or connection load. In this study, use of the analyzed spatial distribution of SF known as a Safety Map, is modified to attain a prescribed constant Fs at any location in the reinforced soil mass. This modified framework, implemented through an iterative, top-down procedure of LE slope stability analyses originating from the crest of a reinforced structure and exiting at progressively lower elevations on the facing, enables the determination of a Tension Map that illustrates the required distribution of reinforcement tension to attain a prescribed limit state of equilibrium. This tension map is directly constrained by a pullout capacity envelope at both the rear and front of each reinforcement layer, providing a unified, LE-based approach towards assessing an optimal selection of mutually dependent strength and layout of the reinforcement. To illustrate the utility of the Limit State framework, a series of instructive examples are presented. The results demonstrate the effects of facing elements, closely-spaced reinforcements, secondary reinforcement layers, and is compared to conventional design approaches.

Particle size effects on coarse soil-geogrid interface response in cyclic and post-cyclic direct shear tests

The interface shear behavior of soil against geosynthetic is important in the design of geosynthetic-reinforced soil structures. This study involves the use of a series of Monotonic Direct Shear (MDS) tests, Cyclic Direct Shear (CDS) tests and Post-cyclic Direct Shear (PCDS) tests to investigate the coarse silica soil-geogrid interface monotonic and cyclic shear behavior. In particular, the influence of cyclic shear on the interface monotonic behavior is analyzed in detail. The effect of soil particle size on interface cyclic and post-cyclic shear behavior is examined and discussed. The results indicate that in cyclic direct shear, the interfaces exhibit cyclic hardening, the interface damping ratio decreased with increasing cycle number and soil relative density, and an interface with a larger particle size has a higher contraction value. In the monotonic condition, the shear stress–displacement curves exhibit post-peak strain softening behavior, and the apparent adhesion and friction angle at the interface both increased with increasing soil particle size. Cyclic shear increased the apparent adhesion at the interface and intensified the soil dilatancy.

Peak Versus Residual Shear Strength in Geosynthetic-Reinforced Soil Design

Current design guidelines for geosynthetic-reinforced soil structures disagree over the shear strength parameters that should be selected to characterize the backfill material. Most geosynthetic reinforcing materials are classified as extensible inclusions for almost all practical applications. The extensible nature of geosynthetic reinforcements has led to the recommendation by several agencies and reinforced soil designers toward the use of the residual shear strength instead of the peak shear strength for design. However, common practice in the US has been the use of the peak shear strength. The main purpose of this paper is to provide experimental evidence regarding selection of either peak or residual shear strength to characterize the backfill material for the design of geosynthetic-reinforced soil structures. Specifically, experimental results from reduced-scale models tested in a geotechnical centrifuge indicate that the stability of geosynthetic-reinforced slopes is governed by the peak soil shear strength.

Effects of vertical acceleration on seismic design of geosynthetic-reinforced soil structures

The stability and permanent displacement of geosynthetic-reinforced soil structures, under the influence of combined horizontal and vertical seismic accelerations, are investigated. The required length and tensile reinforcement of the geosynthetic are determined by considering different possible modes of failure, including tieback/compound failure, direct sliding and pullout. The analysis is conducted by extending the methodology proposed earlier by the authors. While vertical acceleration in the downward direction results in an increased tensile reinforcement by the geosynthetic, acceleration in an upward direction requires a large geosynthetic length to resist direct sliding and tie-back failure. The vertical acceleration has a greater effect on the required tensile reinforcement of the geosynthetic in stabilizing a steep slope when compared to a flatter slope. When the horizontal seismic coefficient exceeds 0·2, the effect of vertical acceleration on the required geosynthetic length becomes significant, and is recommended to be included in design. The proposed methodology of design is verified with the performance of a geosynthedc-reinforced soil retaining wall during the Hanshin earthquake. Effects of seismic coefficients on permanent displacement are illustrated through several selected major earthquake records. L'article examine la stabilité et le déplacement permanent de structures de sol renforcées de matériaux géosynthétiques sous l'influence combinée d'accélérations horizontales et verticales. On détermine la longueur et l'armature de traction nécessaires des matériaux géosynthétiques en considérant différents modes de rupture possibles, y compris la rupture composée/sous tension, le glissement direct et l'arrachement. L'analyse repose sur une extension de la méthodologie proposée antérieurement par les auteurs. Alors que l'accélération verticale dans la direction gravitationnelle se traduit en un plus grand renforcement de traction des matériaux géosynthétiques, l'accélération vers le haut exige une plus grande longueur de matériaux géosynthétiques pour résister à la rupture par glissement direct et à la rupture sous tension. L'armature de traction que doivent offrir les matériaux géosynthétiques pour stabiliser une pente dépend davantage de l'accélération verticale dans le cas de pentes raides que dans celui de pentes plus douces. Si le coefficient sismique horizontal dépasse 0,2, l'effet de l'accélération verticale sur la longueur de matériau géosynthé tique nécessaire devient important, et il est recommand6 d'en tenir compte dans la conception. La méthodologie proposée de conception d'ouvrages pouvant résister au déplacement permanent est vérifiée à partir de la performance d'un mur de souténement en sol renforcé de matériaux géosynthétiques pendant le tremblement de terre de Hanshin. L'article présente une sélection de comptes rendus de grands tremblements de terre pour illustrer les effets des coefficients sismiques sur le déplacement permanent.

Performance of Geosynthetic-Reinforced Soil Structures at Limit Equilibrium State

Potential failure mechanisms representing various limit equilibrium states of geosynthetic-reinforced soil walls/slopes are employed to study the behavior of reinforced soil walls/slopes. Parametric analyses were conducted to investigate the effects of varying the values of pore water pressure, soil strength, geosynthetic strength, soil-geosynthetic interaction coefficients, vertical spacing of geosynthetic, surcharge load, facing slope, and backslope. In the analyses, geosynthetic-reinforced soil walls/slopes are assumed to have competent foundations. Seepage forces and other dynamic forces such as earthquake forces have not been taken into account. Similarly, the stabilizing effects of facing rigidity and strength are neglected. The results are presented in a nondimensional graphical form. Methods of optimizing the design of geosynthetic-reinforced soil structures are suggested.