Experimental study on the load bearing behavior of geosynthetic reinforced soil bridge abutments with different facing conditions

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.     

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.     

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 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.     

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.     

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.     

Mitigation of seasonal temperature change-induced problems with integral bridge abutments using EPS foam and geogrid

Expansion of bridge girders in summer moves integral bridge abutments toward backfill, causing high lateral earth pressures behind the abutment. Some backfill material slumps downward and toward the abutment when the abutment moves away from the backfill due to bridge girder contraction in winter. Placement of geogrids within the backfill can increase stability of the backfill while placement of compressible inclusions (e.g., Expanded Polystyrene (EPS) foam) can reduce lateral earth pressures behind the abutment caused by bridge girder expansion. In this study, six physical model tests were conducted with 30 abutment top movement cycles due to simulated seasonal temperature changes to study the performance of integral bridge abutments with different mitigation measures. The test results showed that geogrid reinforcements caused higher maximum lateral earth pressures at the same abutment movement, but geogrids with wrap-around facing significantly reduced the backfill surface settlements. The combination of the EPS foam and geogrids could minimize lateral earth pressure increase and backfill settlement. The EPS foam reduced the abutment toe outward movement when the abutment top was pushed against the backfill; however, the mitigation effects by the EPS foam was limited due to its small thickness and relatively high elastic modulus in this study.     

采用不同下部结构形式的整体式无缝桥受力特征

建立考虑桥台 土、桩-土相互作用的整体式无缝桥有限元分析模型,并选取下部结构形式、温度作用、台后填土性质以及桥梁跨径为研究参数,对比分析了采用不同下部结构形式的整体式无缝桥受力特征。结果表明:下部结构刚度越大,其对上部结构的约束作用越强,桥梁纵向整体性更明显,但对主梁梁端和桥台的受力越不利;当下部结构刚度较大时,温度对桥梁内力和变形的影响更明显;随着桥梁跨径的增大,整体温度作用的影响逐渐成为温度作用中的主要因素;当下部结构采用矮桥台与桩基础时,台后填土密实度对梁端和桥台弯矩以及主梁轴力的影响不明显;当采用墙式桥台时,随着台后填土密实度的增大,温度作用下主梁轴力会快速增大;随着桥梁跨径的增大,整体式无缝桥的内力不断增大,且当采用刚度较大的下部结构时增大的速率更快;若以桥台在正常使用极限状态下的混凝土裂缝宽度为控制目标,应对整体式无缝桥的最大桥长进行限制,且下部结构刚度越大,最大桥长的限制越严格。     

Performance of a Preloaded-Prestressed Geogrid-Reinforced Soil Pier for a Railway Bridge

A new construction method, called “the preloaded and prestressed reinforced soil method”, proposed in this paper, aims at making reinforced backfill structures very stiff and stable. To make the deformation of a reinforced backfill nearly elastic, sufficiently large preload is first applied by introducing tension into metallic tie rods that penetrate the reinforced backfill and are connected to top and bottom reaction blocks. High tensile force in the tie rods functions as prestress, increasing the confining pressure in the backfill and thus keeping the stiffness and shear strength of the backfill soil sufficiently high. In 1996, in northern Kyushu, Japan, a prototype pier of preloaded and prestressed geogrid reinforced backfill was constructed for the first time to support a pair of simple beam girders for a temporary railway bridge. An abutment of geogrid-reinforced soil retaining wall, which was neither preloaded nor prestressed, was also constructed for the same bridge by otherwise the same construction method. The behaviours of the pier and the abutment were measured during the construction and the service period of about four and a half years and subsequently full-scale loading tests were performed. It is shown that the geogrid-reinforced backfill pier became substantially stiffer against static and dynamic load by having been preloaded and being prestressed when compared to the geogrid- reinforced backfill abutment.     

Case history on failure of geosynthetics-reinforced soil bridge approach retaining walls

Six geosynthetic-reinforced soil (GRS) retaining walls supporting bridge approach roads of an overpass bridge in China exhibited a series of structural problems after 18 years of service. Field investigations demonstrated that the major structural problems consist of excessive lateral facing displacement, settlement and damage of facing panels, and pavement cracks above the GRS retaining walls. The structural problems were mainly caused by inadequate backfill compaction behind the facing, rain water infiltration, the settlement of foundation soil, and reinforcement ageing. Among the six GRS walls, a 22-m-long section collapsed after mild rain in July 2016, and the failure surface in the collapsed zone was mainly located 0.5–0.9 m away from the back of facing panels along the wall height. The field investigation found that external water filtration into the backfill behind the facing panels, and the breakage of connection between reinforcement and facing panels were the main causes of the failure. The connection breakage resulted from the ageing of PP reinforcement strips, and the critical issue of PP reinforcement ageing in complex backfill environment was pinpointed. Remedial measures of the failed section and reinforcing techniques of the remaining GRS walls were briefly presented in the end.     

Reinforcement load and deformation mode of geosynthetic-reinforced soil walls subject to seismic loading during service life

A Finite Element procedure was used to investigate the reinforcement load and the deformation mode for geosynthetic-reinforced soil (GRS) walls subject to seismic loading during their service life, focusing on those with marginal backfill soils. Marginal backfill soils are hereby defined as filled materials containing cohesive fines with plasticity index (PI) >6, which may exhibit substantial creep under constant static loading before subjected to earthquake. It was found that under strong seismic loading reinforced soil walls with marginal backfills exhibited a distinctive “two-wedge” deformation mode. The surface of maximum reinforcement load was the combined effect of the internal potential failure surface and the outer surface that extended into the retained earth. In the range investigated, which is believed to cover general backfill soils and geosynthetic reinforcements, the creep rates of soils and reinforcements had small influence on the reinforcement load and the “two-wedge” deformation mode, but reinforcement stiffness played a critical role on these two responses of GRS walls. It was also found that the “two-wedge” deformation mode could be restricted if sufficiently long reinforcement was used. The study shows that it is rational to investigate the reinforcement load of reinforced soil walls subject to seismic loading without considering the previous long-term creep.     

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.     

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.     

土工格栅嵌锁力的理论计算方法探讨

针对土工格栅嵌锁力提出了一套简化的理论计算方法。通过对碎石填料和格栅作用模型的简化处理,给出了一种格栅与填料颗粒的单元嵌锁模式,并通过对该嵌锁模式下的填料颗粒的两种运动趋势—平移和转动进行讨论和分析,推导了不同条件下格栅的极限拉力,得出了最终的单元嵌锁力。     

Lateral earth pressure behind an integral abutment

Integral abutment bridges have gained increasing attention in the past few decades. They provide a cost-effective solution to the high maintenance expenses associated with the joints and bearings found in conventional bridges. This paper describes the observed behaviour of granular soil backfill retained behind an integral abutment subjected to cyclic loading. Significant pressure build-up was observed in the soil behind the abutment in most locations. The pressure build-up is attributed to several mechanisms such as sand particle flow and densification due to cyclic loading, and the shearing of dense sand during bridge expansion. Therefore, the applicability of using a linear soil pressure distribution assumed by the classical theories in designing the integral abutment system is discussed. Furthermore, the vertical and lateral distribution of the soil pressure behind the abutment has also been analysed. Results from the data measured show that bridge skew resulted in bigger soil pressures at the obtuse side of the abutment compared to the acute. The conclusions of this paper highlight several new design aspects, which are usually overlooked by the common design methodologies of integral abutments, that more accurately predict the vertical and lateral variation in the soil pressure behind abutments.     

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。     

Force equilibrium-based finite displacement analyses for reinforced slopes: Formulation and verification

Formulation and verification for a force equilibrium-based finite displacement method (FFDM) using test results of reinforced model slopes subjected to increasing pseudo-static seismic forces are reported. The FFDM requires, in addition to force equilibrium for a sliced potential failure mass, a hyperbolic shear stress–displacement constitutive law for the backfill soils, a hyperbolic pull-out force–displacement constitutive law for the reinforcement, and a displacement compatibility requirement for adjacent soil slices. As a result, the mobilized reinforcement force is an analytical output, rather than an empiricism-based input as required in conventional limit equilibrium analyses. Analytical results from the FFDM also indicated that a brittle failure is associated with the lightly reinforced failure surface; a ductile failure is associated with the heavily reinforced failure surface, regardless of the extensibility of reinforcement investigated in the present study. Good agreements between the measured and the computed slope displacements and reinforcement forces in response to increases in pseudo-static seismic forces suggest that the FFDM can be used as an analytical tool for evaluating displacements of reinforced slopes subjected to pseudo-static seismic loads.     

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.     

Numerical modelling of two full-scale reinforced soil wrapped-face walls

The paper reports the details of numerical models used to predict the performance of two 3.6 m-high well-instrumented wrapped-face walls. The walls were nominally identical except that the reinforcement material in one wall was a steel welded wire mesh and in the other a biaxial polypropylene geogrid. The backfill soil was modelled using both linear and nonlinear elastic-plastic constitutive models. A general hyperbolic (nonlinear) axial load-strain-time model was used for the reinforcement. The numerical results show good agreement with measured performance features for the welded wire mesh wrapped-face wall. Agreement between numerical predictions of facing displacements and maximum reinforcement loads was less accurate for the very flexible geogrid wrapped-face wall. The discrepancies are believed to be related to the unusually flexible wrapped face used in the geogrid wall construction. Numerically predicted and measured maximum reinforcement loads are compared to loads using the AASHTO reinforcement strength-based design approach (Simplified Method) and the Simplified Stiffness Method which is an empirical reinforcement stiffness-based method. The paper provides physical test data that can be used to benchmark other numerical models, highlights lessons learned during the development of the models, and identifies reasonable expectations for numerical model accuracy for models of similar complexity used to simulate the performance of mechanically stabilized earth (MSE) wall structures under operational conditions.     

Stability of existing bridges improved by structural integration and nailing

To examine whether and how the seismic stability of existing bridges can be substantially improved by integrating the girder, the abutments and the backfill, a series of shaking table tests were performed in 1 g. The tested small bridge models are (1) a conventional-type comprising a girder, supported by a pair of gravity-type abutments (without pile foundation) via bearings (fixed and movable), and unreinforced backfill, (2) the girder and the abutments of the above are integrated (without using bearings), (3) the backfill of the above is reinforced with two layers of large-diameter nails connected to the abutment top and the toe or the heel of the abutment footing and (4) the bottom nails of the above are replaced with longer ones connected to the toe of the abutment footing. Their dynamic behavior was analyzed as a damped single-degree-of-freedom system. The dynamic stability of the bridge was found to increase with an increase in (i) the dynamic strength against the response acceleration, (ii) the initial stiffness, (iii) the dynamic ductility (i.e., a smaller decreasing rate of stiffness during dynamic loading) and (iv) the damping ratio. When factors (ii) and (iii) are high enough, the natural frequency of a bridge can be kept much higher than the input frequency, and thus, the response acceleration can be kept low. All these factors can be improved by integrating the girder, the abutments and the backfill together with part of the supporting ground. In a series of static model tests, lateral cyclic displacements, caused by the seasonal thermal deformation of the girders with prototypes, were applied to the top of a small abutment model. The active failure in the backfill and the detrimental effects of large passive pressure, both developing due to the dual ratchet mechanism, can be effectively restrained by reinforcing the backfill and supporting the ground with nails connected to the top and the bottom of the abutments.