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Experimental and theoretical studies on deformation characteristics of Geosynthetic-Reinforced Soil (GRS) abutments induced by vertical loads
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Geosynthetic-reinforced retaining (GRR) walls have been increasingly built with limited fill space, which has posed challenges for designing such walls with satisfactory performance, especially under surface loading, such as bridge and building foundations. This study performed a series of laboratory model tests to investigate the performance of GRR walls constructed with limited fill space under strip footing loading. The devised experimental program consisted of constructing nine model tests with different retained medium distance, geosynthetic-reinforced fill width, and reinforcement layout. In each model test, a load was applied on the top of the wall through a 200 mm wide rigid plate to simulate a strip footing. Marks on facing units and the loading plate were installed to measure wall deformations and footing settlements, respectively. The test results show that limiting the size of the retained medium and the reinforced fill affected the internal stability of the wall, the wall deformation, and the footing settlement. Reduction of the reinforced fill width from 0.5H to 0.3H (H is the wall height) resulted in excessive wall deformation and footing settlement, and even sudden failure of the model wall. On the other hand, the test results reveal that connecting geosynthetic reinforcement to the stable retained medium resulted in substantial reduction in the deformation of the wall facing and the settlement of the footing. The test results also demonstrate that bending geosynthetic reinforcement upward along the back of the reinforced fill enhanced internal stability and resulted in considerable reduction in the lateral deformation of the wall facing and increase in the bearing capacity of the footing.
The centrifuge and finite element methods are used to study the behavior of geosynthetic-reinforced soil retaining walls having a concrete block facing. A well instrumented PWRI Wall was used for simulation, with a special interest on the construction sequences. The backfill soil, geogrid reinforcements, and facing blocks were simulated using Nevada sand, reinforcement produced using 3D printing technique, and aluminum blocks, respectively. The results of single-stage and multi-stage constructions were compared. It was found that the facing deformations and tensile strains in the reinforcement layers can be simulated more closely using staged constructions compared to single-stage construction. The results were further reinforced using a finite element procedure. The study also looked into the effects of having loose soil behind the vicinity of the facing blocks. Soils with reduced stiffness and strength would increase the lateral facing displacements and tensile strains in the reinforcement layers.
Geosynthetic-reinforced retaining (GRR) walls have been increasingly used to support roadways and bridge abutments in highway projects. In recent years, advances have been made in construction and design of GRR walls for highway applications. For example, piles have been installed inside GRR walls to support bridge abutments and sound barrier walls. Geosynthetic layers at closer spacing are used in GRR walls to form a composite mass to support an integrated bridge system. This system is referred to as a geosynthetic-reinforced soil (GRS)-integrated bridge systems (IBS) or GRS-IBS. In addition, short geosynthetic layers have been used as secondary reinforcement in a GRR wall to form a hybrid GRR wall (HGRR wall) and reduce tension in primary reinforcement and facing deflections. These new technologies have improved performance of GRR walls and created more economic solutions; however, they have also created more complicated problems for analysis and design. This paper reviews recent studies on these new GRR wall systems, summarizes key results and findings including but not limited to vertical and lateral earth pressures, wall facing deflections, and strains in geosynthetic layers, discusses design aspects, and presents field applications for these new GRR wall systems.
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–Hodoš 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.
Reinforced soil has become a common means of retaining soil and supporting spread footings due to ease of construction and cost-efficiency. However, the behavior of these structures under service state conditions is inherently complex and dependent on the selection of design factors. Factors that govern design include earth pressures, settlements and reinforcement strains, but are not easily attained from closed form solutions. This study aims to provide insight into the complex behavior of reinforced soil walls surcharged by a discrete footing in context of these design factors, facilitating the development of a numerical model. A nonlinear elastic-plastic constitutive soil material model was calibrated to experimental plane strain test data, while the performance of a wall model was compared against full-scale laboratory testing of an identical geosynthetic-reinforced wall from prior literature, demonstrating satisfactory agreement. This agreement enabled a parametric study of the same wall model with a discrete spread footing evaluating the effects of reinforcement type and vertical spacing, footing location, footing dimensions, and toe restraint on lateral earth pressure distributions, wall deformations and reinforcement strains. The results of the study demonstrated that simplified consideration of only reinforcement type on service-state performance of reinforced walls neglects the effects of closely spaced reinforcements on behavior. Use of closely spaced reinforcements created increased earth pressures, but also reduced lateral displacement, footing settlements and reinforcement strains. Furthermore, it enabled achievement of equivalent service state criteria for footings located closer to the wall facing, potentially enabling increased right-of-way or reduced deck length when supporting a bridge superstructure.
Geosynthetic-Reinforced Soil (GRS) mass, comprising soil and layers of geosynthetic reinforcement, is not a uniform mass. To examine the behavior of a GRS mass by a laboratory test, a sufficiently large-size specimen of soil and reinforcement is needed to produce a representative soil-geosynthetic composite. This paper presents a generic test, referred to as the Soil-Geosynthetic Composite (SGC) test, for investigating stress-deformation behavior of soil-geosynthetic composites in a plane strain condition. The specimen dimensions, 2.0 m high and 1.4 m wide in a plane strain configuration, were determined by the finite element method of analysis. The configuration, specimen dimensions, test conditions, and procedure of the SGC test are described. In addition, the results of a SGC test with nine sheets of reinforcement, as well as those of an unreinforced soil test conducted in otherwise identical conditions, are presented. In the test, the soil mass was subject to a prescribed value of confining pressure, applied by vacuum through latex membrane covering the entire surface area of the mass in an air-tight condition. Vertical loads were applied on the top surface of the soil mass until a failure condition was reached. The behaviors of the soil masses, including vertical displacements, lateral movement, and strains in the geosynthetic reinforcement, were carefully monitored. The measured data allow the behavior of reinforced and unreinforced soils to be compared directly, provide a better understanding of soil-geosynthetic composite behavior, and serve as the basis for verification of numerical models to investigate the performance of GRS structures. J. Ross Publishing, Inc.
In current design methods for reinforced soil walls, it has been tacitly assumed that reinforcement strength and reinforcement spacing play an equal role. This fundamental design assumption has led to the use of larger reinforcement spacing (0.3-1.0 m) in conjunction with stronger reinforcement to reduce construction time. Recent studies, however, have clearly indicated that the role of reinforcement spacing is much more significant than that of reinforcement strength. With closely spaced (reinforcement spacing <= 0.3 m) reinforcement, the beneficial effects of geosynthetic inclusion is significantly enhanced, and the load-deformation behavior can be characterized as that of a composite material. A reinforced soil mass with closely spaced geosynthetic reinforcement is referred to as geosynthetic-reinforced soil (GRS). In this study, an analytical model is developed for predicting the ultimate load-carrying capacity and required reinforcement strength of a GRS mass. The model was developed based on a semiempirical equation that reflects the relative roles of reinforcement spacing and reinforcement strength in a GRS mass. Using measured data from field-scale experiments available to date, it is shown that the analytical model is capable of predicting the ultimate load-carrying capacity and required reinforcement strength of a GRS mass with good accuracy. (C) 2013 American Society of Civil Engineers.
Some forty years ago, when geotechnical centrifuge modelling had been rediscovered and was being developed once more after the early work of Phillips (1869), only a few studies were devoted to the questions and concerns about scaling laws and similitude conditions. During the first decades, it was relatively easy for researchers to keep themselves informed about the main outcomes of these studies and to take them into account when designing new centrifuge model tests. This is obviously not true today following the welcome growth in terms of the large number of centrifuge facilities now in operation around the world. It is increasingly difficult, but yet absolutely essential, to know about the relevant developments concerning studies into the scaling laws and, furthermore, into the limits of the domains of the use of centrifuge modelling. On the other hand, new media offers a significant opportunity to provide this resource to the physical modelling community. New topics are investigated by many researchers as they become more inventive in the ways in which geotechnical centrifuge modelling is applied to solve pressing problems within geotechnical engineering, and across other disciplines too. Innovative work presenting comparisons between centrifuge model tests and true scale tests are providing original data on the validity of the scaling factors. During the TC2 meeting at St John's (Canada) in July 2002, the first author, J. Garnier (LCPC), suggested making an inventory of the scaling laws and similitude questions relating to centrifuge modelling. The aim of this catalogue is to present the questions already solved (with inclusion of the references of the papers where the results have been presented) and the unsolved problems (on which research should continue). The first draft of this catalogue is now available and it is hoped that it will become a useful tool for scientists and researchers involved in centrifuge modelling. Of course, this catalogue will be regularly updated, every four years during the International Conferences on Physical Modelling in Geotechnics. The latest version of the catalogue is available on the TC2 website (www.tc2.
This paper presents an experimental study on reduced-scale model tests of geosynthetic reinforced soil (GRS) bridge abutments with modular block facing, full-height panel facing, and geosynthetic wrapped facing to investigate the influence of facing conditions on the load bearing behavior. The GRS abutment models were constructed using sand backfill and geogrid reinforcement. Test results indicate that footing settlements and facing displacements under the same applied vertical stress generally increase from full-height panel facing abutment, to modular block facing abutment, to geosynthetic wrapped facing abutment. Measured incremental vertical and lateral soil stresses for the two GRS abutments with flexible facing are generally similar, while the GRS abutment with rigid facing has larger stresses. For the GRS abutments with flexible facing, maximum reinforcement tensile strain in each layer typically occurs under the footing for the upper reinforcement layers and near the facing connections for the lower layers. For the full-height panel facing abutment, maximum reinforcement tensile strains generally occur near the facing connections.
This paper presents a case study of the first geosynthetic reinforced soil-integrated bridge system (GRS-IBS) with with full-height rigid facings in China. Open graded gravel and biaxial geogrid were used for the GRS-IBS. Steel frame and 3D vegetation net were used as temporary facing support during construction of the GRS abutments. Full-height rigid facings were cast in place on strip foundations. Field monitoring results of vertical stress distribution for different construction stages and loading conditions are presented and discussed. For both bridge dead load and truck loads, measured incremental vertical stresses under the beam seat increase significantly with increasing elevation, especially for higher applied vertical stress. The calculated incremental vertical soil stresses using the Boussinesq solution are in reasonable agreement with the measured values, while the 2:1 stress distribution method overestimates the incremental stresses in the lower section of the abutment. The transferred vertical stresses from bridge load application for the GRS abutment with full-height rigid facing are larger than those for the GRS abutment with modular block facing near the top of the abutment, but are smaller near the bottom.
The influence of backfill type and material properties on the performance of field-scale geosynthetic reinforced soil (GRS) abutment models is investigated. Two alternative types of backfill as recommended in the Federal Highway Administration (FHWA) guidelines (called open-graded and well-graded) were used to build two field-scale model abutments and compare their load-bearing performance under a loading beam. Results are presented and discussed relative to the loading beam settlement, facing deformation and reinforcement strains. The well-graded backfill was found to result in smaller beam settlements and facing lateral deformations, especially at smaller loads that were comparable to service load levels. However, it was significantly faster and easier to compact the open-graded aggregate to the unit weight recommended in the guidelines. Nevertheless, performances of both abutment models were found to be satisfactory relative to the limiting requirements on the beam settlement and facing deformations at service load levels.
The paper reports the construction and surcharge load-testing of three (3) large-scale (~2.50 m-tall) GRS bridge abutment models in an outdoor test station to investigate the influences that the facing type and reinforcement spacing could have on their load-bearing performance. The facing types examined included cored Concrete Masonry Units (CMU) in Model #1 and much larger solid concrete blocks in Models #2 and #3. Reinforcement spacing in the first two models was 0.20 m, whereas it was increased to 0.30 m in the third model. Results show that using large facing blocks in GRS abutments could lead to significant improvements in their load-deformation performance relative to those with the CMU facing alternative. This improvement was observed even in the case of model with increased reinforcement spacing. Therefore, use of larger facing blocks could also help reduce the cost of GRS abutments by reducing the need for tighter reinforcement.
The paper presents analytical investigation of bearing capacity of geosynthetic reinforced soil (GRS) bridge abutments with vertical or nearly vertical facing batter angle. A limit analysis (LA) method is proposed using a y-shaped failure mechanism to determine the bearing capacity and the corresponding failure surface geometry within GRS bridge abutments. The proposed method is validated against results from numerical simulations and limit-equilibrium methods. Parametric analyses are subsequently carried out to examine the effects of backfill strength, reinforcement properties, and abutment geometry on the bearing capacity and failure surface geometry of GRS bridge abutments.
This paper focuses on investigation of the effects of the presence of a bearing bed and reinforced soil foundation (RSF), the vertical reinforcement spacing and reinforcement stiffness, and subgrade compressibility on design components, including the lateral displacement of facing, maximum tension in reinforcement (Tmax), connection strength (To), and differential settlement in reinforced soil structures reinforced with woven geotextile. The investigation was conducted through numerical simulation of reinforced soil structures using the finite element program Plaxis 2D under plain strain condition. The results from the numerical analyses showed that the vertical reinforcement spacing has more effect on the design components than the reinforcement stiffness. The effect of slab load in widely spaced structures showed that the Tmax decreased linearly with depth. However, for closely spaced structures with a bearing bed, the reduction in Tmax with depth was bilinear. The existence of To in closely spaced structures was evident. The observed values of To were very close to values of Tmax. Inclusions of the bearing bed and RSF were found to have beneficial effects and can be implemented in design and construction of widely spaced reinforced soil structures to improve the performance.
This study presents an evaluation of the connection load (To) and stress-strain conditions right behind the facing of a Geosynthetic Reinforced Soil-Integrated Bridge Structure (GRS-IBS) based on field instrumentation data obtained from an abutment constructed in Virginia. The observations from this site are compared against other projects in Delaware and Louisiana. The lateral stress distribution obtained from the field was observed to be lower than the active lateral earth pressure distribution but higher than predicted using the bin pressure method. The results from all sites showed that the reinforcement strains measured in the field were below the maximum geosynthetic strains allowed in the design of GRS-IBS. The distributions of both lateral stresses and reinforcement strains with depth were found to be approximately uniform. The To values for the Virginia structure were obtained based both on reinforcement strain and lateral stress data, which agreed well with each other. All sites indicated the existence of lateral stresses behind the facing, which contributed to the development of To. The normalized To values for all GRS-IBS projects evaluated in this study showed that the theoretical tributary area approach outlined in MSE design can be conservatively adopted to predict To in the design of GRS-IBS.
The Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS), which consists of closely-spaced layers of geosynthetic reinforcement and compacted granular fill material, is a fast, sustainable and cost-effective method for bridge support. The in-service performance of this innovative bridge support system is largely evaluated through the vertical and horizontal deformations of the GRS abutments. This paper presents the development of nonlinear equations for estimating the maximum lateral displacement and settlement of GRS abutments. The parameters that are considered in the prediction equations include abutment geometry (height, facing batter and foundation width), backfill friction angle, reinforcement characteristics (stiffness, spacing, and length), and applied static loads from 50 to 400 kPa. In the development of the prediction equations, a parametric study was first conducted using a validated finite difference numerical model. The results of the parametric study were then used to conduct a regression analysis to develop the equations for estimating the maximum lateral displacement and settlement of GRS abutments under service loads. The equations were validated using three case studies. The developed prediction equations can contribute to a better understanding and enable simple calculations in designing these structures.
This study evaluated the responses of geosynthetic-reinforced soil (GRS) abutments subjected to bridge slab loading under working stress conditions using two-dimensional finite difference numerical software. A parametric study was conducted to investigate the effects of different combinations of reinforcement spacing Sv and reinforcement stiffness J, beam seat width b, and setback distance ab on the responses of the GRS abutments in terms of additional vertical stresses under the beam seat centerline Δσv induced by the bridge slab load, additional lateral earth pressures behind the abutment facing Δσh-facing and under the beam seat centerline Δσh-cetner induced by the bridge slab load, and maximum tension in the reinforcement Tmax. Numerical analyses evaluated trapezoidal and uniform reinforcement layouts and showed that both reinforcement layouts generated similar responses of the GRS abutments. Under the same ratio of J/Sv, different combinations of Sv and J generated similar distributions of Δσv, Δσh-facing and Δσh-center. The maximum of Tmax with depth decreased almost proportionally with the decrease of Sv. Larger b and ab caused lower Δσv, Δσh-facing, Δσh-center, and smaller Tmax in the upper reinforcement layers. The truncated 2 to 1 distribution method, which considers the effects of abutment facing on the Δσv distribution, could reasonably predict the Tmax in the reinforcement.
This study numerically investigates the behaviour of a geosynthetic-reinforced soil (GRS) wall under surcharge loading. Data from a full-scale GRS physical model wall was used to verify the numerical analysis. The modelling was carried out using the two-dimensional finite difference computer program FLAC to verify the post-construction performance of a full-scale GRS segmental wall under surcharge loading. The real value of compaction induced stress (CIS) specified for the vibrating plate compactor used in the physical model wall was employed in the analyses. Two procedures for modelling the CIS found in the literature were used in the analyses: uniform vertical stress applied to the surface of each layer (type I) and uniform vertical stress applied at the top and bottom of each layer (type II). The results clearly showed that the numerical analyses using compaction procedure type II accurately represent the measured values obtained from the full-scale wall under surcharge loading as well as during construction. The numerical analyses considering type I compaction modelling overestimated the measurements during both construction and surcharge application.
This paper presents a case study of a geosynthetic-reinforced soil (GRS) integrated bridge system (IBS) in which the vertical stresses during and after construction were monitored via instrumentation. The purpose of the study was to evaluate the effects of reinforcement spacing, width of the beam seat, and seasonal variations on the vertical stresses measured in the field. The stress distribution observed in the field was also compared to the theoretically estimated stress distribution. The results showed that the bearing bed where the reinforcements are doubled is effective in reducing the applied stresses by about 1.8 to 5.4 times. The width of the beam seat controlled the magnitude of the applied stresses on the GRS abutment and the applied stress was vertically transferred all the way to the foundation level even in wider beam seats. A comparison between field recorded and theoretical stress values showed that the Boussinesq method provides a better estimate of the field vertical stress distribution than the approximate 2:1 method, although the 2:1 method provides more conservative stresses to be considered for design. Results from long-term monitoring indicated that vertical stress distribution in the GRS abutments was not significantly influenced by seasonal variations.
A scaled plane-strain shaking table test was conducted in this study to investigate the seismic performance of a Geosynthetic Reinforced Soil-Integrated Bridge System (GRS-IBS) with a full-length bridge beam resting on two GRS abutments at opposite ends subjected to earthquake motions in the longitudinal direction. This study examined the effects of different combinations of reinforcement stiffness J and spacing Sv on the seismic performance of the GRS-IBS. Test results show that reducing the reinforcement spacing was more beneficial to minimize the seismic effect on the GRS abutment as compared to increasing the reinforcement stiffness. The seismic inertial forces acted on the top of two side GRS abutments interacted with each other through the bridge beam, which led to close peak acceleration amplitudes at the locations near the bridge beam. Overall, the GRS-IBS did not experience obvious structure failure and significant displacements during and after shaking. Shaking in the longitudinal direction of the bridge beam increased the vertical stress in the reinforced soil zone. The maximum tensile forces in the upper and lower geogrid layers due to shaking happened under the center of the beam seat and at the abutment facing respectively.
This paper presents experimental results from physical model tests on four half-scale geosynthetic reinforced soil (GRS) bridge abutment specimens constructed using well-graded angular backfill sand, modular facing blocks, and uniaxial geogrid reinforcement to investigate the effects of applied surcharge stress, reinforcement vertical spacing, and reinforcement tensile stiffness for working stress, static loading conditions. Facing displacements increased for the upper section of the walls after the application of surcharge stress and were greater for larger reinforcement vertical spacing and reduced reinforcement tensile stiffness. Bridge seat settlements were proportional to the applied surcharge stress, strongly affected by larger reinforcement vertical spacing, and only slightly affected by reduced reinforcement tensile stiffness. Measured vertical and lateral soil stresses generally were lower than calculated values for static loading conditions. The maximum tensile strain in each reinforcement layer occurred near the facing block connection for lower layers and under the bridge seat for higher layers. A companion paper presents experimental results for the same GRS bridge abutment specimens under dynamic loading conditions.
This study proposes a limit equilibrium approach to estimating the bearing capacity of strip footings placed on geosynthetic-reinforced soil structures (GRSSs). To assess the multiple mechanisms that may govern the ultimate resistance sustained by GRSSs, logarithmic-spiral, two-part wedge, two-sided general shear, one-sided general shear, and failure above the uppermost geosynthetic layer are proposed. Each of these mechanisms is assessed considering geometry, geotechnical properties, and geosynthetic rupture, whereupon an algorithm selects the minimum, critical bearing capacity and associated failure mechanism for design. Additionally, the effects of foundations placed near the transition between reinforced and unreinforced soil are evaluated. Considering these factors, both the failure mechanism and bearing capacity attained from this analysis are compared with rigorous numerical models, demonstrating agreement. The multimechan-ism approach is then extended to assess bearing capacity considering various geometric configurations and material properties. Finally, a set of dimensionless charts are presented for convenient assessment of the ultimate bearing capacity of strip footings placed on GRSSs.
Current design regulations most often require use of limit equilibrium methods for the internal stability analyses of geosynthetic-reinforced soil (GRS) walls. However, the limit-equilibrium based approaches generally over-predict reinforcement loads for GRS walls when comparing with measured data from full-scale instrumented walls under working stress conditions. Wall toe resistance has an important influence on the performance of GRS walls but is ignored in limit equilibrium-based methods of design. This paper reports centrifuge modelling of GRS walls which have different toe restraint conditions but are otherwise identical. The GRS wall models prepared in this study isolate the influence of wall toe resistance on the performance of walls. Based on measured data from four centrifuge wall model tests, a reduction in wall toe resistance (by reducing the interface shear resistance at the base of the wall facing or removing the soil passive resistance in front of the wall toe or both) induces larger maximum facing deformation and reinforcement strain and load. The results also demonstrate that the wall models with typical toe restraint conditions are most likely operated under working stress conditions while those with poor toe restraint conditions may experience (or be close to reach) a state of limit equilibrium.
In this study, both two-dimensional (2D) and three-dimensional (3D) numerical analyses were carried out to evaluate the performance of geosynthetic-reinforced soil (GRS) piers. The numerical models were first calibrated and verified against test results available in the literature. A parametric study was then conducted under both 2D and 3D conditions to investigate the influences of reinforcement tensile stiffness, reinforcement vertical spacing, and a combination of reinforcement stiffness and spacing on the performance of GRS piers under vertical loading. Numerical results indicated that the effect of reinforcement spacing was more significant than that of reinforcement stiffness. The use of closely – spaced reinforcement layers resulted in higher global elastic modulus of the GRS pier, smaller lateral displacements of pier facing and volumetric change of the GRS pier, lower and more uniformly-distributed tension in the reinforcement, and larger normalized coefficients of lateral earth pressure. This study concluded that a 2D numerical model gave more conservative results than a 3D model.
A 109.5-Ft-long Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS) in Hawaii was instrumented to measure superstructure strains, vertical pressures below the footing, lateral pressures behind the end wall and modular block facing, and lateral displacements of the facing. Field surveys were also performed to measure the bridge footing settlement. The field data showed that: (1) with time the superstructure compressive concrete strains gradually increased and the end wall lateral pressures gradually decreased, evidence of superstructure concrete creep and shrinkage; (2) three years after construction, the total footing settlement was ≈ 1.2 in.; and (3) the bridge superstructure undergoes daily and seasonal thermal expansion and contraction cycles. Also seasonally, the vertical pressures beneath the footing, lateral pressures behind the end walls, and superstructure strains fluctuate cyclically. The vertical footing pressure closest to the stream experienced the greatest daily pressure fluctuation (≈ 2500−3000 psf), while the one nearest the end wall experienced the least. Based on the results of cyclic triaxial tests on a basalt aggregate similar to the GRS backfill to estimate permanent deformation of the abutment due to daily pressure fluctuations, it was estimated that the permanent strain ≈ 1%, comparable to what was observed in the bridge footing. After three years, the total settlement is about 1.6% of the GRS abutment height; ≈ 0.7% of this is due to the structure dead weight and the remaining 0.9% is due to cyclic loading, consistent with the 1% cyclic strain from laboratory permanent deformation tests. © National Academy of Sciences: Transportation Research Board 2019.
Geosynthetic reinforced soil (GRS) structures have gained popularity in replacing concrete rigid piles as abutments to support medium or small-spanned bridge superstructures in recent years. This study conducted 13 model tests to investigate the ultimate bearing capacity of the GRS mass when sand was used as backfill soil. The GRS mass was constructed and loaded to failure under a plane strain condition. Test results were compared with two analytical solutions available in literature. This study also proposed an analytical model for predicting the ultimate bearing capacity of the GRS mass based on the Mohr-Coulomb failure criterion. The failure surface of the GRS mass was described by the Rankine failure surface. The effects of compaction and reinforcement tension were equivalent to increased confining pressures to account for the reinforcing effects of the geosynthetic reinforcement. The proposed model was verified by the results of the model tests conducted in this study and reported in literature. Results indicated that the proposed model was more capable of predicting the ultimate bearing capacity of the GRS mass than the other two analytical solutions available in literature. The proposed model can be used to predict the ultimate bearing capacity of GRS structures when sand was used as backfill material. In addition, a parametric study was conducted to investigate the effects of friction angle of backfill soil, reinforcement spacing, reinforcement strength, and reinforcement stiffness on the ultimate bearing capacity of the GRS mass calculated with and without compaction effects. Results showed that the ultimate bearing capacity of the GRS mass was significantly affected by the friction angle of backfill soil, reinforcement spacing and strength. Compaction effects resulted in an increase in the ultimate bearing capacity of the GRS mass.
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 theMohr-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.
In this study, a three-dimensional (3D) Finite Element (FE) analysis was developed to simulate the fully-instrumented geosynthetic reinforced soil integrated bridge system (GRS-IBS) at Maree Michel Bridge in Louisiana. The 3DFE computer program PLAXIS 3D 2016 was selected to simulate the GRS-IBS behavior under different loading conditions. A second order-hyperbolic elasto-plastic soil model was used to simulate the granular backfill materials. The soil-structure interaction was simulated using zero thickness interface elements, in which the interface shear strength is governed by Mohr-Coulomb failure criterion. Three different loading conditions were considered in this study: (a) at the end of bridge construction (Case 1); (b) surface loading (Case 2); and (c) at abnormal loading (Case 3), which is equal to the dead load of the bridge structure plus three times the service loading. The predicted results were compared with the field measurements at the end of bridge construction. Moreover; the predicted results of the 3D-FE analysis were compared with those predicted using the 2D-FE analysis. A good agreement was obtained between the 3D-FE and 2D-FE numerical results and the field measurements. The predicted results using the 3D-FE showed that the range of maximum reinforcement strain under service loading ranges between 0.6% and 1.5%, depending on the location of the reinforcement layer. The maximum lateral deformation at the face was between 3 mm (0.07% lateral strain) under service load case and 7 mm (0.3% lateral strain) for abnormal load case. The maximum settlement of the GRS-IBS due to the service loading was 9 mm (0.3% vertical strain). The axial reinforcement forces predicted by FHWA (Adam et al., 2011) design methods were compared with those predicted by the 3D-FE and 2D-FE analysis. The results showed that the FHWA analytical method is 1.5–2.5 times higher than those predicted by the FE analysis, depending on the loading condition and reinforcement location.
This study analyses two full-scale model tests on mechanically stabilized earth (MSE) walls. One test was conducted with a rigid and one with a flexible wall face. Other parameters were the same in these two tests, like the number and type of geogrid layers, the vertical distance between the layers and the soil type. The loads and strains on the reinforcement are measured as function of the horizontal and vertical earth pressure and compared with analytical models. Specifics regarding the behavior of the geogrids under the compaction load during the construction of the model and under strip footing load are included in the study. Results are compared with AASHTO and the empirical K-stiffness method. In this study, an analytical method is developed for the MSE walls taking into account the facing panel rigidity both after backfill construction and after strip footing load. There is good agreement between the proposed analytical method and the experimental results considering the facing panel rigidity. The results indicate that the tensile force on reinforcement layers for rigid facing is less than the flexible facing. The maximum strains in the reinforcement layers occurred in the upper layers right below the strip footing load. The maximum wall deflection for the flexible facing is more than for the rigid facing. The maximum deflection was at the top of the wall for the rigid facing and occurred at z/H = 0.81 from top of the wall for the flexible facing.
This paper presents an experimental study on the response of a half-scale geosynthetic reinforced soil (GRS) bridge abutment subjected to shaking in the direction transverse to the bridge beam. The specimen geometry, reinforcement stiffness, soil modulus, applied surcharge stress and characteristics of the earthquake motions were scaled according to established similitude relationships for shaking table tests in a 1g gravitational field. The GRS bridge abutment was constructed using modular facing blocks, well-graded angular sand and uniaxial geogrid reinforcement, in both the longitudinal and transverse directions. Facing displacements, bridge seat settlements, accelerations, vertical and lateral soil stresses, reinforcement strains, and bridge seat and bridge beam interactions were measured during a series of input motions. The average incremental residual bridge seat settlement was 4.7 mm after the Northridge motion, which corresponds to a vertical strain of 0.22% for the lower GRS fill. After the series of motions, the maximum residual tensile strains occurred near the facing block connections for the lowermost reinforcement layer and under the bridge seat for higher reinforcement layers.
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.
This paper presents numerical simulations of the performance of the Geosynthetic Reinforced Soil-Integrated Bridge System (GRS-IBS) under static loading conditions. Simulations were conducted using a finite-difference program and realistic conditions for system geometry, backfill soil, geosynthetic reinforcement, and applied loads. Simulation results, including lateral facing displacements, settlements, lateral and vertical earth pressures, and reinforcement tensile strains and forces, indicate good performance of GRS-IBS during construction and under traffic loads. A parametric study was conducted to investigate the effects of backfill soil compaction, reinforcement length, reinforcement stiffness, bearing bed reinforcement, bridge seat setback distance, bridge load, and abutment height on the performance of GRS-IBS. Results indicate that reinforcement stiffness, bridge load, and abutment height have the most significant influences on lateral facing displacements and bridge seat settlements. Differential settlements between the bridge and approach roadway were small for all conditions.
This 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 spacing, and bridge load have the most significant influence on lateral facing displacements and bridge footing settlements for GRS abutments. Differential settlements between the bridge footing and approach roadway were small for all simulated conditions.
The effects of geosynthetic reinforcement type on the strength and stiffness of reinforced sand were evaluated by performing a series of drained plane strain compression tests on large sand specimens. The reinforcement type is described in terms of the degree of unification of the constituting components (for geocomposites) as well as the tensile strength and stiffness, the covering ratio and others (for geocomposites and geogrids). Sand specimens reinforced with different geosynthetic reinforcement types exhibited significantly different reinforcing effects. A geocomposite made of a woven geotextile sheet sandwiched firmly with two sheets of non-woven geotextile, having a 100% effective covering ratio, exhibited reinforcing effects higher than typical stiff and strong geogrids. With some geocomposite types, the reinforcing effects increase substantially by better unifying longitudinally arranged stiff and strong yarns and non-woven geotextile sheets. When fixed firm to the yarns, the non-woven geotextile sheets function like the transversal members of a geogrid by locally transmitting load activated by interaction with the backfill to the yarns. These geocomposites can exhibit reinforcing effects equivalent to those with stiff and strong geogrids. Local strain fields of the specimens are presented to show that, for reinforced sand, the peak stress state reached is always associated with the development of shear band(s) in the sand and a higher peak strength is achieved when the strain localisation starts at a larger global axial strain due to better reinforcing effects.
The geosynthetic reinforced soil (GRS) performance test (PT), also called a mini-pier experiment, was developed by the Federal Highway Administration (FHWA) to evaluate the material strength properties of GRS composites built with a unique combination of reinforcement, compacted fill, and facing elements. The PT consists of constructing a 1.4-m square column of alternating layers of compacted granular fill and geosynthetic reinforcement with a facing element that is frictionally connected up to a height of 2 m, then axially loading the GRS mass while measuring deformation to monitor performance. The results can be directly used in the design of GRS abutments and integrated bridge systems. Considering that the geometry of the PT is square in plan, the equivalency of the results to a bridge application, which more resembles a plane strain condition, is evaluated and presented in this paper. The analysis indicates that the PT closely approximates the bearing resistance, or capacity, of a typical GRS abutment, and is a conservative estimate when predicting stiffness. These results indicate that the PT can be used as a design tool for GRS abutments at both the strength and service limit states.
The use of geosynthetic-reinforced soil (GRS) as a bridge abutment foundation has been adapted by the Eastern Federal Lands Highway Division in several projects. This adaptation is considered ideal and appropriate because of the nature of these projects. The design, construction, and performance of a GRS abutment system to support two bridges in the Mattamuskeet National Wildlife Refuge in Hyde County, North Carolina, are illustrated. The bridge foundations were constructed over soft, silty fat clay (A-7-6[CH]) soils. The GRS abutment alternative was favored over a deep foundation system of driven prestressed concrete piles for economic and constructability reasons. A cellular confinement system (CCS) filled with gravel was used for the GRS abutment facing instead of the types of flexible facings typically used with GRS abutments. CCS facing has the advantage of providing abutment face protection against erosion and shallow scour, and the outer cell on both sides of the abutment can be filled with topsoil and seed to provide an aesthetically pleasing vegetated face. A geotechnical instrumentation monitoring program consisting of settlement plates and piezometers was established to evaluate the GRS abutment system performance during and after completion of construction. The foundation soils supporting the GRS abutments were preloaded before construction of the bridge footings to reduce anticipated long-term settlement and improve bridge performance. Design procedures and initial performance monitoring results indicated that the GRS abutments were performing satisfactorily.
In order to refine Geosynthetic Reinforced Soil Integrated Bridge System technology (developed by the FHWA), girders from a 42.7 m single span bridge were instrumented with strain gages, end pressures were measured using horizontal pressure cells, and the abutments were instrumented with vertical pressure cells and survey targets to measure girder footing and abutment wall movements. After a six month monitoring period, vertical deflections are within tolerable limits (ranging from 1.1 to 4.6 cm) and there are no visible cracks at the bridge-approach interface. The strain gages and earth pressure cells continue to collect meaningful data in terms of magnitude and trend. A change in ambient temperature causes a temperature induced strain in the steel, which affects the lateral pressure measured behind the steel girders as expected. Vertical pressures in the abutment are also affected by the thermal cycle. This paper will display the preliminary results from this project.
Geosynthetic-reinforced soil (GRS) integrated bridge systems (IBS) integrate conventional bridge superstructures with a GRS abutment foundation and GRS approach for a cost-effective, rapid construction alternative. A 42.7 m long single span GRS IBS was constructed and instrumented to monitor the thermally induced behaviors and better understand the interaction between the superstructure and substructure within the limits of this system. Strain gauges were attached to the steel girders, and lateral end pressures were monitored using earth pressure cells to determine the level of stress thermally induced in the GRS approach over a 3.5 year monitoring period and evaluate the rigidity of the boundary conditions that exist at the interface. During this 3.5-year monitoring period, the data show that the GRS approach is engaged with the superstructure and experiences both active and passive lateral pressures during each thermal cycle without displaying an increase in passive pressure with time. The stress-strain data acquired during this project indicate that the GRS IBS is behaving significantly more like a system with unrestrained boundaries due to the flexibility of the GRS approach at each end. The tightly spaced reinforcements create a composite material at the ends of the superstructure that enable the approach fill to move successfully with thermally induced superstructure deformations without creating a failure within the soil or at the surface of the roadway (interface included).
An innovative alternative to conventional bridge support technology, the Federal Highway Administration's (FHWA) geosynthetic-reinforced soil-integrated bridge system (GRS-IBS) utilizes closely spaced layers of geosynthetic reinforcement and compacted granular fill material to provide direct bearing support for structural bridge members. This new technology has a number of unique advantages, including reduced construction time and cost, generally fewer construction difficulties, and easier maintenance over the life cycle of the structure. This technology can also perform well under a variety of static and dynamic loading conditions if designed and constructed properly. These advantages have led to a significant increase in the rate of construction of GRS-IBS structures in recent years. This paper provides details about the implementation of a GRS-IBS project in Delaware, the first project of this type in the state. An overview of the design and construction process for this project is provided, along with a brief description of a custom-designed instrumentation system for monitoring the long-term performance of the GRS-IBS.
Geosynthetic-reinforced soil (GRS) for bridge support was developed through the cooperative efforts of the University of Colorado at Denver, the Colorado Department of Transportation, and the Federal Highway Administration (FHWA), and was supported in recent years by a nationally funded research program. Most recently, the FHWA Turner-Fairbank Highway Research Center developed the integrated bridge system (IBS), a relatively fast, cost-effective method of bridge support that blends the roadway into the superstructure using GRS technology. Through the FHWA's Every Day Counts (EDC) initiative, the IBS is being rapidly deployed across the country, with about 100 in design or construction to date. This paper provides the instrumentation plan and the initial performance observations of the first wall of this type constructed in Minnesota. A robust monitoring program was developed to capture performance related to settlement, wall distortion, and pressure and deformation associated with the concrete box beams under thermal cycling. Automated monitoring will continue for a three-year period following construction.
This paper will discuss the design, construction, and performance of a geosynthetic—reinforced soil (GRS) integrated abutment used for Bowman Road Bridge, built in Defiance County, Ohio, during the fall of 2005. The project was part of the Federal Highway Administration's (FHWA) "Bridge of the Future Program" to develop technologies to build more efficient, durable 70 to 90 foot single-span bridges. One of the most significant aspects of this project was the evaluation of the "integrated abutment" concept; this simple method of combining the substructure with the superstructure is different from the "integral abutment" method of construction. The bridge consists of prestressed concrete box beams supported on GRS abutments without the use of a deep foundation to support the superstructure. The GRS abutments were built on a Reinforced Soil Foundation (RSF) over the clay subsoil. The bridge has no cast-in-place concrete. The bridge also does not have an approach slab; the intent was to allow the bridge and the adjacent road to settle together, providing a bump free, smooth ride for drivers traveling over the bridge. The cost to construct this bridge was about 20 percent less than the quoted price of a bridge supported on pile-capped abutments with 2:1 slopes. The bridge was instrumented and surveyed to evaluate performance and to refine the "integrated abutment" design concept. To date, the performance of the bridge is excellent and the angular distortion of the superstructure is well within AASHTO criteria for simple supported bridges.
This article presents a numerical approach for modeling of the compaction-induced stresses on the analyses of geosynthetic reinforced soil (GRS) walls. The modeling of the backfill compaction stresses was described and analyses were performed using this suggested procedure. Two distribution loads at the top and bottom of each soil layer were used to simulate the vertical induced stress due to backfill soil compaction. The suggested procedure was validated with the results of a wrapped-faced full-scale reinforced soil wall performed at the Geotechnical Laboratory of COPPE/UFRJ. The results of the simulation using this procedure were compared with another procedure reported in the literature. Parametric studies were carried out to verify the effect of compaction induced stress and surcharge loads on the behavior of GRS walls. Results show that the compaction procedure suggested in the present paper was able to properly represent the measured values of the summation of the maximum tension in the reinforcement and lateral movements. It was verified that the compaction procedure used in the literature overestimated the measured values, and this discrepancy increases with depth and also with compaction effort.