Geosynthetic-reinforced Soil Research Articles

Closure to “Numerical Investigation of Geosynthetic-Reinforced Soil Bridge Abutments under Static Loading” by Yewei Zheng and Patrick J. Fox

Closure to “Numerical Investigation of Geosynthetic-Reinforced Soil Bridge Abutments under Static Loading” by Yewei Zheng and Patrick J. Fox

FEM Analysis of Back-to-Back Geosynthetic-Reinforced Soil Retaining Walls

Reinforced soil retaining walls with two opposite sides (back-to-back) are commonly used for embankments approaching bridges. This type of walls has complex geometry. The distance between two opposing walls is a key parameter used for determining the analysis methods in the federal highways administration guidelines. The objective of this research is to study under static conditions, the effect of the distance between the two back-to-back geosynthetic-reinforced soil retaining walls of embankment approaching bridges on the internal and external stability. Finite element method incorporated in the Plaxis software and analytical methods were used for this research. Parametric studies were carried out by varying the distance between side walls, to investigate their effects on the critical failure surface, the lateral earth pressure behind the reinforced zone, the wall displacement and on the required tensile strength of reinforcement.

Discussion of Xiao C. et al., “Experimental study on performance of geosynthetic-reinforced soil model walls on rigid foundations subjected to static footing loading,” Geotextiles and Geomembranes (2016), Vol. 44, Issue 1, February 2016, p. 81–94

Discussion of Xiao C. et al., “Experimental study on performance of geosynthetic-reinforced soil model walls on rigid foundations subjected to static footing loading,” Geotextiles and Geomembranes (2016), Vol. 44, Issue 1, February 2016, p. 81–94

Lateral bearing capacity and failure mode of geosynthetic-reinforced soil barriers subject to lateral loadings

In addition to self-weight and vertical surcharge, geosynthetic-reinforced soil (GRS) structures have recently been used as barriers to resist lateral forces from natural disasters, such as floods, tsunamis, rockfalls, debris flows, and avalanches. The stability of such structures subject to lateral loading is often evaluated using conventional external stability analyses with the assumption that the reinforced soil mass is a rigid body. However, this assumption contradicts the flexible nature of reinforced soil. In this study, finite element (FE) models of back-to-back GRS walls were developed to investigate the performance of GRS barriers subject to lateral loading. The FE analysis results indicated that the failure mode and lateral bearing capacity of GRS barriers depend largely on the aspect ratio (L/H: ratio of wall width to wall height). When 0.5 < L/H < 1.0, the GRS barriers would fail internally because of internal sliding along the soil–reinforcement interface at the loading side and the active soil failure at the opposite side. When 1.0 < L/H < 3.0, bottom sliding failure would occur along the foundation–reinforcement interface. When L/H > 3.0, passive soil failure would occur within the GRS barriers at the side subject to the lateral force. The ultimate lateral bearing capacity of GRS barriers increased with an increase in L/H: the ultimate lateral capacity factor NL was 1.4–20.1 times Ka for L/H = 0.5–3.0. In addition to the effect of L/H, the numerical results indicated that the backfill friction angle ϕ, unit weight γ, and reinforcement vertical spacing Sv considerably affected the lateral bearing capacity of GRS barriers. A hypothetical case study of a GRS barrier against a tsunami force is provided, and a viable method using vertical preloaded soil anchors for improving wall lateral capacity is analyzed and discussed.

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.

Design and construction of the first GRS integrated bridge with FHR facings in Europe

Abstract The bridge across the Pavlovski potok stream in the village of Žerovinci in northeast Slovenia is a part of the investment into the modernisation of the Pragersko–Hodos railway line, one of the biggest investments in the infrastructure in Slovenia at the moment. Its design was accompanied by very short deadlines and a deep layer of soft foundation soil. For this reason the reinforced concrete abutments of a nearby railway bridge were founded on 24 m deep piled foundations. Short deadlines and a limited budget forced the authors of this paper to find an alternative solution. Deep piled foundations were replaced by shallow foundations made of compacted fill material reinforced with geosynthetics. The bridge was completed by the end of 2014. From the results of previous laboratory tests that were obtained within the scope of the EU co-funded research project “Research voucher”, the basic characteristics of the building materials for the geosynthetic reinforced soil (GRS) bridge abutments, as well as the deformation properties of the typical reinforced soil were obtained. These data were used for the design of the abutments. The staged construction procedure of full height rigid (FHR) facings for the GRS retaining walls (RW) was used for the construction of this GRS integrated bridge. Partial pre-stressing of geogrids and consequently the increased stiffness of the reinforced soil was achieved by following the staged construction procedure. The bridge system consisted of a cast-in-situ RC slab, which was placed on top of the GRS immediately behind the FHR facings, i.e. the bridge was constructed as a simply-supported slab supported by a pair of GRS abutments, without the use of bearings. Thus the described bridge across the Pavlovski potok combines two approaches for GRS integrated bridge design, one of which has been used in Japan (Tatsuoka et al., 2009), with full structural integration of the deck onto the pair of FHR facings, other being proposed by the FHWA (Adams et al., 2010), without full integration of the deck onto the GRS RWs. A system for the monitoring of structure performance was established in order to ensure optimization of this kind of structure. Data for this GRS integrated bridge, which were obtained during the construction works were compared to similar data obtained from the construction of a nearby railway bridge with reinforced concrete abutments. The comparison provides a good basis for future decisions when choosing the type of bridge abutments.

FE simulation of rate-dependent behaviours of polymer geosynthetic reinforcements for an estimation of mobilized tensile force in a reinforced soil

Polymer geosynthetic reinforcements exhibit obvious viscous behaviours depending on the loading strain rate. These behaviours can be accurately described using a nonlinear three-component constitutive model. After implementing this model into the finite element software ABAQUS, it can be widely used for the analysis of geosynthetic-reinforced soil problems. The algorithm for the numerical integration of this elasto-viscoplastic model is explained in detail. Simulations of both in-isolation geosynthetic reinforcement and geosynthetic-reinforced soil specimen are performed to analyse the viscous behaviours of polymer geosynthetics under complex loading histories. The corresponding user subroutines are provided in the supplementary data for potential readers.

A 12-year long-term study on the external deformation behavior of Geosynthetic Reinforced Soil (GRS) walls

Geosynthetics reinforced soil (GRS) walls constructed on weak grounds may change in both the horizontal earth pressure and deformation on wall facing. However, only few studies were done in the literature to measure and analyze the horizontal external deformation behavior of GRS walls constructed on soft grounds for a long period of time. The present study describes the external deformation behavior of GRS walls observed for 12-year long-term performance. The horizontal deformation of the geosynthetics-wrapped-facing GRS walls shows a passive behavior along one third of the wall height, from top going downwards, and active behavior for the rest of the wall height. Even if the geogrid and nonwoven geotextiles are exposed directly to sunlight and rainfalls in a span of 12 years, they have functioned well as wall facing. Therefore, the geosynthetic reinforcement material is strong enough to resist ultraviolet rays.

A numerical investigation into the behavior of geosynthetic-reinforced soil segmental retaining walls

Geosynthetic-reinforced soil segmental retaining walls are now an economical solution. The design of these structures is carried out according to the Association of State Highway and Transportation Officials or the Federal Highway Administration design guidelines, which are based on the limit equilibrium approach. Many experimental, analytical, and numerical studies have identified several sources of conservatism in these design guidelines. This article focuses on a numerical study undertaken using the finite difference code Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D), to investigate the influence of facing batter angle, soil/facing interface friction angle, and backslope angle on the behavior of geosynthetic-reinforced soil segmental retaining walls. The numerical results show that the design guidelines lead to overly conservative design where the soil/facing interface is rough and/or the backslope is present.

Required reinforcement stiffness for vertical geosynthetic-reinforced-soil walls at strength limit state

In the design of geosynthetic-reinforced-soil (GRS) retaining walls for strength limit state, it is usually assumed that the soil strength is fully mobilised, but generally design guidelines do not have the specification on the required long-term reinforcement stiffness that may mobilise the design soil strength under the design load. In this study, a validated analytical method for reinforcement loads of vertical GRS walls was employed to investigate the required reinforcement stiffness at the strength limit state. The analysis was based on the compatible soil-reinforcement deformation at the potential failure surface. The study found that design of the strength limit state of GRS retaining walls should adequately take into account reinforcement stiffness, without which the targeted state cannot be obtained. The mobilised lateral soil strain and the required local reinforcement stiffness J/Sv to mobilise the lateral soil strain are both larger under higher vertical soil stress. The required local reinforcement stiffness increases with an increase in the soil stiffness, a decrease in the soil dilatancy and a decrease in the peak friction angle. A series of design tables was provided, which could be used to select proper geosynthetic reinforcements for vertical GRS walls at the strength limit state.

Deformations of geosynthetic reinforced soil under bridge service loads

This paper evaluates the results of 13 large scale Geosynthetic Reinforced Soil (GRS) column load tests, also known as performance tests (PT) or mini-pier tests, to study the effect of tensile strength (Tf), vertical reinforcement spacing (Sv), facing elements, and backfill properties on the deformations of GRS at 200 kPa, typical bridge bearing pressures, and also at 400 kPa. The results indicate that GRS performs well under service conditions. A semi-empirical expression is proposed for prescribed bearing pressures to limit vertical strain to 0.5% of the abutment height. In addition, recommendations for estimating lateral deformation for GRS bridge abutments are also provided. At 200 kPa surcharge for this series of tests, vertical settlements ranged from 8.3 to 33.9 mm (or from 0.4% to 1.7% axial strain); lateral deformations ranged from 3.0 mm to 10.1 mm (or 0.6%–2.0% lateral strain); and reinforcement strain ranged from less than 1% during construction to less than 3% during loading. The lateral deformation results indicate that the maximum displacement occurs in the top third region of the wall face. Comparing the vertical and lateral displacement data shows that most GRS models experienced negligible positive volume changes up to about 1% under typical bridge service loads.

Research and construction of geosynthetic-reinforced soil integral bridges

Geosynthetic-reinforced soil (GRS) integral bridge was developed to overcome several inherent serious problems with conventional type bridges comprising a simple-supported girder (or multiple girders) supported via bearings typically by RC abutments retaining unreinforced backfill (and a pier or piers for multiple girders). The problems include: (a) relatively high construction and maintenance costs with relatively long construction time resulting from the use of bearings and massive abutment structures usually supported by piles; (b) bumps immediately behind the abutments; and (c) a relatively low stability of the girders supported by roller bearings and the approach embankment against seismic and tsunami loads. For a GRS integral bridge, a pair of GRS walls (and an intermediate pier or piers if necessary for a long span) are first constructed. After the deformation of the supporting ground and the backfill of the GRS walls has taken place sufficiently, steel-reinforced full-height-rigid (FHR) facings are constructed by casting-in-place concrete on the wall face wrapped-around with the geogrid reinforcement. Finally a continuous girder is constructed with both ends integrated to the top of the FHR facings. The girder is also connected to the top of an intermediate pier, or piers, if constructed. The background and history of the development of GRS integral bridge is described. The first four case histories, one completed in 2012 for a new high-speed train line and the other three completed in 2014 to restore a railway damaged by a great tsunami of the 2011 Great East Japan Earthquake, are reported.

Evaluation of the effect of toe restraint on GRS walls

The effect of toe restraint on the behavior of geosynthetic-reinforced soil (GRS) walls was evaluated using the results of three large-scale walls constructed at the COPPE/UFRJ Geotechnical Laboratory. The walls were similar, with the exception of the toe restriction. The walls were well-instrumented in order to monitor the values of the reinforcement load, toe load, horizontal facing displacement, horizontal stress at the back of the block facing and vertical displacement on the top of the walls. Results clearly showed the influence of the toe restriction on the walls’ behavior. A higher toe restraint led to a greater toe load and lower maximum reinforcement load, as well as lower horizontal facing displacement and vertical displacement. Furthermore, the measured values of the reinforcement load were compared to the values predicted by design methods found in the literature, in order to verify the prediction accuracies of those methods. Results are presented and some limitations of those methods are discussed.

Role of suction stress on service state behavior of geosynthetic-reinforced soil structures

Abstract Several design guidelines adopt limit state methods (e.g., earth pressure or limit equilibrium slope stability methods) to estimate the reinforcement loads for geosynthetic-reinforced soil structures (GRSSs). Field measurements usually reveal lower tensile loads in the reinforcements than that predicted by conventional design methods. Consequently, limit state methods have been criticized for being conservative or inaccurate. However, these lower-than-expected loads are primarily due to redundancy in design, attributed to several factors such as toe resistance, soil volumetric dilation, underestimation of soil shear strength, and the effect of suction stress. While these factors commonly contribute to the performance of the GRSSs, they are not accounted for in design procedures. This disregard is accredited to complexities and uncertainties associated with reliable quantification of these factors during the life span of the structure. By properly quantifying the role of suction stress, this study aims to quantitatively explain a part of the discrepancy which exists between the tensile loads in the reinforcement, which are estimated by working-condition design methods versus those by limit state design methods. A suction stress-based formulation is presented for calculating the active earth pressures coefficient ( K a ) of unsaturated backfill under unsaturated steady flow conditions. The formulation is derived by implementing an analytical solution of one-dimensional steady flow into a limit equilibrium-based effective stress analysis. The proposed formulation is used in conjunction with the earth pressure method to illustrate the effect of suction stress on predicted reinforcement loads. Two backfills, referred to as marginal and high quality backfill, along with three representative annual rain events are examined. The results are compared with two classical earth pressure methods as well as two empirical methods. The results show that the suction stress contribution can lead to significantly lower reinforcement loads than that predicted by classic earth pressure methods for backfill types. Empirical methods lead to lower loads in some cases and higher predicted loads in others. The proposed formulation provides a mechanics-based approach to explain a part of the discrepancy which exists between the measured tensile loads in the reinforcement in the field versus those by limit state design methods.

Geosynthetic-reinforced soil structures with concave facing profile

This paper presents a new method to determine the optimal profile of facing elements in geosynthetic-reinforced soil structures. Flexibility of some facing systems and advances in construction technology allow construction of reinforced soil structures with a non-planar cross section. In this study, the facing profile of a concave geosynthetic-reinforced soil structure (referred to as CGRSS) is idealized by a circular arc defined by a single variable, the Mid-Chord Offset (MCO). For a given setback and elevation change, the optimal facing profile is determined by seeking the MCO which, for a given margin of safety, yields the least tensile load in the reinforcement layers. The proposed procedure for finding the optimal facing profile is incorporated into a limit equilibrium-based log spiral formulation to determine the required tensile strength of the reinforcement. Results are presented in a set of charts showing the required unfactored tensile strength, MCO, and mode of failure for various friction angles, batter angles, and seismic coefficients. It is shown that CGRSSs can decrease the required tensile strength of the reinforcement by up to 30% under static and pseudo-static conditions. This observation justifies employing concave facing profiles in practice.

Behavior of Geosynthetic Reinforced Soil Foundation Systems Supported on Stiff Clay Subgrade

A series of laboratory model tests was performed to investigate the behavior of geosynthetic reinforced stiff clay foundation systems under circular loading. The footing consisted of a rigid circular steel plate with a diameter of 150 mm. Five different series of tests were performed in both homogeneous (clay or sand) and layered configurations. The tests used planar geogrid and three-dimensional geocell reinforcements. Test results indicate that both types of reinforcements substantially improve the performance of the stiff clay foundation bed. A maximum threefold improvement was observed in bearing pressure, depending on the reinforcement type. However, geocell was found to be the most advantageous soil reinforcement technique, giving maximum performance improvement.

Numerical Investigation of Geosynthetic-Reinforced Soil Bridge Abutments under Static Loading

AbstractThis paper presents a numerical investigation of the performance of geosynthetic-reinforced soil (GRS) bridge abutments under static loading conditions. Simulations were conducted using a finite-difference program to model the Founders/Meadows GRS bridge abutment during construction and service. Simulated results are in good agreement with field measurements, including displacements, lateral and vertical earth pressures, and tensile strains and forces in reinforcement. The simulations also indicate that horizontal restraint from the bridge structure has a significant influence on abutment deflections. A parametric study was then conducted to investigate the performance of a single-span full bridge system with two GRS abutments, including effects of bridge contact friction coefficient, backfill soil relative compaction, backfill soil cohesion, reinforcement spacing, reinforcement length, reinforcement stiffness, and bridge load. Results indicate that backfill soil relative compaction, reinforcement...

Numerical study on lateral bearing capacity and failure mode of geosynthetic-reinforced soil barriers

In addition to vertical surcharges, geosynthetic-reinforced soil (GRS) structures have recently been used as barriers to resist lateral forces from natural disasters such as floods, tsunamis, rock falls, debris flows, and avalanches. The stability of such structures is often evaluated by conducting conventional external stability analyses with an assumption that the reinforced soil mass is a rigid body. However, this assumption contradicts the flexible nature of reinforced soil. In this study, finite element (FE) models of back-to-back GRS walls were developed to investigate the behavior of GRS barriers subjected to lateral loadings. The FE results indicate that GRS barriers subjected to lateral loadings fail internally. The failure model and the lateral bearing capacity depend on the aspect ratio (L/H: ratio of wall width to wall height) of GRS barriers. When 0.5 3.0, passive soil failure occurs within GRS barriers at the side subjected to the lateral force. As L/H increases, the lateral bearing capacity of GRS barriers increases to approximately three times the active lateral earth pressure at L/H = 0.7 to the passive lateral earth pressure at L/H = 3.0. In addition to the effect of L/H, the internal soil failure predicted by FE analyses suggests that the soil shear strength plays a major role in determining the lateral bearing capacity of GRS barriers. A hypothetical case study of a GRS barrier against a tsunami force is provided and an improved method is discussed.

Effect of sample size and anchorage on the performance of reinforced soil–aggregate systems

Laboratory tests on geosynthetic-reinforced soil–aggregate systems were carried out by many researchers to understand the improvement in bearing resistance of soil–aggregate systems because of geosynthetic reinforcement. California bearing ratio (CBR) test is one among the common laboratory tests adopted for this purpose. This paper presents the influence of sample size and anchorage of reinforcement in CBR test through tests carried out using conventional and modified CBR moulds with and without anchorage of reinforcement. Clay subgrade and aggregate subbase were simulated in these tests with a geosynthetic layer placed at the interface. Modified set-up was double the size of the conventional set-up and has the provision to anchor the reinforcement. Three different types of geosynthetics namely, geotextile (GT), biaxial geogrid (BG), and geonet (GN) were used in the tests. While the size of the mould significantly affected the test results, effect of anchorage of reinforcement on the bearing resistance of reinforced soil–aggregate systems was not significant. Increase in sample size reduced the boundary interference, which otherwise resulted in over prediction of secant modulus of the reinforced systems, the effect being more pronounced for weaker reinforcement materials.

Cyclic and Post-cyclic Shear Behaviour of a Granite Residual Soil – Geogrid Interface

The dynamic frictional properties of the soil-geosynthetic interface play an important role in the design and stability analysis of geosynthetic-reinforced soil structures under repeated loadings, such as those induced by compaction, traffic and earthquakes. This paper describes a laboratory study carried out using a large-scale direct shear test device, aiming to investigate the cyclic and post-cyclic behaviour of an interface between a granite residual soil and a biaxial woven geogrid. In the cyclic direct shear tests, the interface was subjected to 40 cycles of sinusoidal displacement, with semi-amplitude and frequency ranging from 1-10 mm and 0.05-0.5 Hz, respectively. To evaluate the effect of the cyclic loading on the interface shear strength, monotonic direct shear tests were performed immediately following the cyclic tests. The results indicated that the loading frequency has little impact on the interface shear stiffness during the loading cycles. In contrast, the influence of the displacement semi-amplitude on the interface stiffness was found to be significant. The cyclic loading did not lead to the degradation of the post-cyclic interface shear strength. The post-cyclic peak shear strength tended to increase with the semi-amplitude of the shear displacement, which may be associated with an increase in soil density.