Experimental study on the load bearing behavior of geosynthetic reinforced soil bridge abutments on yielding foundation

This paper presents an experimental study of the load bearing behavior of geosynthetic reinforced soil (GRS) bridge abutments constructed on yielding clay foundation. The effects of two different ground improvement methods for the yielding clay foundation, including reinforced soil foundation and stone column foundation, were evaluated. The clay foundation was prepared using kaolin and consolidated to reach desired shear strength. The 1/5-scale GRS abutment models with a height of 0.8 m were constructed using sand backfill, geogrid reinforcement, and modular block facing. For the GRS abutments on three different yielding foundations, the reinforced soil zone had relatively uniform settlement and behaved like a composite due to the higher stiffness than the foundation layers. The wall facing moved outward with significant movements near the bottom of facing, and the foundation soil in front of facing showed obvious uplifting movements. The vertical stresses transferred from the footing load within the GRS abutment and on the foundation soil are higher for stiffer foundation. The improvement of foundation soil using geosynthetic reinforced soil and stone columns could reduce the deformations of GRS abutments on yielding foundation. Results from this study provide insights on the practical applications of GRS abutments on yielding foundation.     

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

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

Seismic performance of a whole Geosynthetic Reinforced Soil – Integrated Bridge System (GRS-IBS) in shaking table test

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 S_v 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.     

Performance monitoring of Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS) in Louisiana

The Geosynthetic Reinforced Soil (GRS) Integrated Bridge System (IBS) is an alternative design method to the conventional bridge support technology. Closely spaced layers of geosynthetic reinforcement and compacted granular fill material can provide direct bearing support for structural bridge members if designed and constructed properly. This new technology has a number of advantages including reduced construction time and cost, generally fewer construction difficulties, and easier maintenance over the life cycle of the structure. These advantages have led to a significant increase in the rate of construction of GRS-IBS structures in recent years. This paper presents details on the instrumentation plan, short-term behavior monitoring, and experiences gained from the implementation of the first GRS-IBS project in Louisiana. The monitoring program consisted of measuring bridge deformations, settlements, strains along the reinforcement, vertical and horizontal stresses within the abutment, and pore water pressures. In this paper, the performance of instrumentation sensors was evaluated to improve future instrumentation programs. Measurements from the instrumentations also provide valuable information to evaluate the design procedure and the performance of GRS-IBS bridges. The instrumentation readings showed that the magnitude and distribution of strains along the reinforcements vary with depth. The locus of maximum strains in the abutment varied by the surcharge load and time that did not corresponds to the (45+?/2) line, especially after the placement of steel girders. A comparison was made between the measured and theoretical value of thrust forces on the facing wall. The results indicated that the predicted loads by the bin pressure theory were close to the measured loads in the lower level of abutment. However, the bin pressure theory under predicted the thrust loads in the upper layers with reduced reinforcement spacing. In general, the overall performance of the GRS-IBS was within acceptable tolerance in terms of measured strains, stresses, settlements and deformations.     

Full-scale mechanically stabilized earth (MSE) walls under strip footing load

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.     

Numerical evaluation of the performance of a Geosynthetic Reinforced Soil-Integrated Bridge System (GRS-IBS) under different loading conditions

This paper presents the results of a finite element (FE) numerical analysis that was developed to simulate the fully-instrumented Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS) at the Maree Michel Bridge in Louisiana. Four different loading conditions were considered in this paper to evaluate the performance of GRS-IBS abutment due to dead loading, tandem axle truck loading, service loading, and abnormal loading. The two-dimensional FE computer program PLAXIS 2D 2016 was selected to model the GRS-IBS abutment. The hardening soil model proposed by Schanz et al., (1999) that was initially introduced by Duncan and Chang (1970) was used to simulate the granular backfill materials; a linear-elastic model with Mohr-Coulomb frictional criterion was used to simulate the interface between the geosynthetic and backfill material. Both the geosynthetic and the facing block were modeled using linear elastic model. The Mohr-Coulomb constitutive model was used to simulate the foundation soil. The FE numerical results were compared with the field measurements of monitoring program, in which a good agreement was obtained between the FE numerical results and the field measurements. The range of maximum reinforcement strain was between 0.4% and 1.5%, depending on the location of the reinforcement layer and the loading condition. The maximum lateral deformation at the face was between 2 and 9 mm (0.08%–0.4% lateral strain), depending on the loading condition. The maximum settlement of the GRS-IBS under service loading was 10 mm (0.3% vertical strain), which is about two times the field measurements (～5 mm). This is most probably due to the behavior of over consolidated soil caused by the old bridge. The axial reinforcement force predicted by FHWA (Adams et al., 2011b) design methods were 1.5–2.5 times higher than those predicted by the FE analysis and the field measurements, depending on the loading condition and reinforcement location. However, the interface shear strength between the reinforcement and the backfill materials predicted by Mohr-Coulomb method was very close to those predicted by the FE.     

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

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

Numerical study of geosynthetic reinforced soil bridge abutment performance under static and seismic loading considering effects of bridge deck

Although the use of Geosynthetic Reinforced Soil (GRS) bridge abutments has been increasing, the seismic performance of such structures has remained a significant concern due to their unknown behavior in load-bearing and stress distribution under bridge load and seismic conditions simultaneously. This paper investigates the static and dynamic response of GRS bridge abutment. A series of numerical models representing the realistic field conditions of these structures, including two reinforced soil walls and a single span deck that restrains the top of walls, rather than equivalent surcharge load, was developed. The calibrated numerical model in FLAC program was used to evaluate the effects of horizontal restraint from the deck on the GRS wall displacements and reinforcement loads at the end of construction and under harmonic base acceleration up to 0.5 g. Results indicated that the restraint mobilized from the bridge deck presence, considerably affected the results at both the end of construction and after the dynamic load was applied. Moreover, a series of the parametric studies were performed to investigate the influences of backfill soil relative compaction, reinforcement stiffness, reinforcement length, and reinforcement vertical spacing on the response of GRS abutments at the end of construction and post dynamic state.     

Numerical parametric study of geosynthetic reinforced soil integrated bridge system (GRS-IBS)

A 2-D finite flement model was developed in this study to conduct a FE parametric study on the effects of some variables in the performance of geosynthetic reinforced soil integrated bridge system (GRS-IBS). The variables investigated in this study include the effect of internal friction angle of backfill material, width of reinforced soil foundation (RSF), secondary reinforcement within bearing bed, setback distance, bearing width and length of reinforcement. Other important parameters such as reinforcement stiffness and spacing were previously investgated by the authors. The performance of GRS-IBS were investgated in terms of lateral facing displacement, strain distribution along reinforcement, and location of potential failure zone. The results showed that the internal friction angle of backfill material has a significant impact on the performance of GRS-IBS. The secondary reinforcement, setback distance, and bearing width have low impact on the performance of GRS-IBS. However, it was found that the width of RSF and length of reinforcement have negligible effect on the performance of GRS-IBS. Finally, the potential failure envelope of the GRS-IBS abutment was found to be a combination of punching shear failure envelope (top) that starts under the inner edge of strip footing and extends vertically downward to intersect with Rankine active failure envelope (bottom).     

Recent advances in geosynthetic-reinforced retaining walls for highway applications

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.     

Ultimate bearing capacity of strip footing resting on soil bed strengthened by wraparound geosynthetic reinforcement technique

In the recent past, the wraparound geosynthetic reinforcement technique has been recommended for constructing the geosynthetic-reinforced soil foundations. This paper presents the development of an analytical expression for estimating the ultimate bearing capacity of strip footing resting on soil bed reinforced with geosynthetic reinforcement having the wraparound ends. The wraparound ends of the geosynthetic reinforcement are considered to provide the shearing resistance at the soil-geosynthetic interface as well as the passive resistance due to confinement of soil by the geosynthetic reinforcement. The values of ultimate load-bearing capacity determined by using the developed analytical expression agree well with the model footing load test values as reported in the literature.     

Experimental and theoretical studies on the ultimate bearing capacity of geogrid-reinforced sand

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.     

Bearing capacity of square footings on geosynthetic reinforced sand

The results from laboratory model tests and numerical simulations on square footings resting on sand are presented. Bearing capacity of footings on geosynthetic reinforced sand is evaluated and the effect of various reinforcement parameters like the type and tensile strength of geosynthetic material, amount of reinforcement, layout and configuration of geosynthetic layers below the footing on the bearing capacity improvement of the footings is studied through systematic model studies. A steel tank of size 900 × 900 × 600 mm is used for conducting model tests. Four types of grids, namely strong biaxial geogrid, weak biaxial geogrid, uniaxial geogrid and a geonet, each with different tensile strength, are used in the tests. Geosynthetic reinforcement is provided in the form of planar layers, varying the depth of reinforced zone below the footing, number of geosynthetic layers within the reinforced zone and the width of geosynthetic layers in different tests. Influence of all these parameters on the bearing capacity improvement of square footing and its settlement is studied by comparing with the test on unreinforced sand. Results show that the effective depth of reinforcement is twice the width of the footing and optimum spacing of geosynthetic layers is half the width of the footing. It is observed that the layout and configuration of reinforcement play a vital role in bearing capacity improvement rather than the tensile strength of the geosynthetic material. Experimental observations are supported by the findings from numerical analyses.     

Soil-reinforcement interaction: Stress regime evolution in geosynthetic-reinforced soils

Understanding the stress regime that develops in the vicinity of reinforcements in reinforced soil masses may prove crucial to understanding, quantifying, and modeling the behavior of a reinforced soil structures. This paper presents analyses conducted to describe the evolution of stress and strain fields in a reinforced soil unit cell, which occur as shear stresses are induced at the soil-reinforcement interface. The analyses were carried out based on thorough measurements obtained when conducting soil-reinforcement interaction tests using a new large-scale device developed to specifically assess geosynthetic-reinforced soil behavior considering varying reinforcement vertical spacings. These experiments involved testing a geosynthetic-reinforced mass with three reinforcement layers: an actively tensioned layer and two passively tensioned neighboring layers. Shear stresses from the actively tensioned reinforcement were conveyed to the passively tensioned reinforcement layers through the intermediate soil medium. The experimental measurements considered in the analyses presented herein include tensile strains developed in the reinforcement layers and the displacement field of soil particles adjacent to the reinforcement layers. The analyses provided insights into the lateral confining effect of geosynthetic reinforcements on reinforced soils. It was concluded that the change in the lateral earth pressure increases with increasing reinforcement tensile strain and reinforcement vertical spacing, and it decreases with increasing vertical stress.     

土工格栅加筋土柔性桥台结构性能的试验研究

基于静载作用下加筋土柔性桥台结构工作性能的试验研究,综合对比分析桥台基础距下部挡墙面板的距离D对柔性桥台结构极限承载力、下部挡墙变形特点、筋材应变和土压力的影响。试验结果表明：当下部加筋挡墙中筋材长度为整体桥台高度时,桥台结构极限承载力随偏移距离D增加呈现先增加后减小趋势,且在D为0.4H_L(H_L为下部挡墙高度)时达到最大值;加筋柔性桥台整体结构加载至破坏前一级载荷时,桥台基础沉降与台背加筋土顶部沉降均呈近似线性变化,且D/H_L为0.4时二者差异沉降最小;挡墙面板顶部的水平位移明显大于中、底部,且挡墙水平位移与挡墙高度比值均小于1%;挡墙中各层筋材应变最大值随D增加而逐渐向远离面板方向发展,且D为0.4H_L时台背加筋土和下部挡墙加筋中筋材的应变相差不大,整体柔性桥台结构工作性能达到最佳状态。     

A method for correcting field strain measurements to account for temperature effects

Measurements of deformation and strain are of critical importance for the structural health monitoring of geosynthetic reinforced soil (GRS) structures. For some commonly used wiring configurations and sensors, field strain measurements can exhibit apparent strains that are caused by changes in wire temperature or changes in gauge temperature, which do not correspond to changes in actual strain in the geosynthetic. Correcting for these effects is important for separating out real structural behavior from wiring and sensor issues, and is imperative for avoiding false trigger warnings in automated instrumentation reporting systems. The current paper consequently presents a method for correcting field strain measurements to account for temperature effects. Data collected from sensors embedded in a heavily-instrumented GRS abutment over a two-year monitoring period is presented. The distributed thermistor array embedded in the GRS abutment is shown to be particularly useful for understanding temperature effects on the measured foil strain gauge data. The presented approach and associated framework for data correction are useful for practicing engineers and other researchers, as the general concepts from this study can be applied to data collected from many instrumented geosynthetic field projects.     

Experimental and numerical investigations of the behaviour of footing on geosynthetic reinforced fill slope under cyclic loading

This paper investigates the cyclic loading responses of a strip footing supported by a geosynthetic reinforced fill embankment. A series of large-scale model footing tests were conducted first to investigate the accumulation of permanent footing displacement and residual vertical soil stress over large number of load cycles. The embankment fill was a heavily compacted silty sand and the reinforcement was a flexible geogrid, so that the model test configurations were representative of actual field conditions. Both permanent displacement and residual stress accumulated asymptotically with load cycles and majority of the build-up occurred over the first few hundred cycles. The potential effect of load interruptions was part of the study. Depending on how cyclic load interruption was implemented, it may or may not induce a trailing effect on subsequent cyclic loading responses. To have more in-depth understanding, these footing tests were also investigated numerically based on a soil model that can capture the unload-reload stress-strain loop over large number of load cycles. Reasonably good agreement between experimental observations and numerical predictions was also achieved.     

整体式桥台桥梁台后土压力的季节性变化研究

建立了整体式桥台与土共同作用的有限元模型 ,计算分析了季节性温度变化下台后土压力的分布曲线。以三跨连续混凝土板桥为工程实例 ,提出并验证台后设置EPS压缩层和置换轻填土两种方案 ,同时配合台后土体加筋 ,为整体式桥台设计提供理论依据     

Load-bearing performance of model GRS bridge abutments with different facing and reinforcement spacing configurations

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.     

Seismic effects on reinforcement load and lateral deformation of geosynthetic-reinforced soil walls

Current design methods for the internal stability of geosynthetic-reinforced soil (GRS) walls postulate seismic forces as inertial forces, leading to pseudo-static analyses based on active earth pressure theory, which yields unconservative reinforcement loads required for seismic stability. Most seismic analyses are limited to the determination of maximum reinforcement strength. This study aimed to calculate the distribution of the reinforcement load and connection strength required for each layer of the seismic GRS wall. Using the top-down procedure involves all of the possible failure surfaces for the seismic analyses of the GRS wall and then obtains the reinforcement load distribution for the limit state. The distributions are used to determine the required connection strength and to approximately assess the facing lateral deformation. For sufficient pullout resistance to be provided by each reinforcement, the maximum required tensile resistance is identical to the results based on the Mononobe–Okabe method. However, short reinforcement results in greater tensile resistances in the mid and lower layers as evinced by compound failure frequently occurring in GRS walls during an earthquake. Parametric studies involving backfill friction angle, reinforcement length, vertical seismic acceleration, and secondary reinforcement are conducted to investigate seismic impacts on the stability and lateral deformation of GRS walls.