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.     

黑龙江呼兰河大桥的光纤光栅智能监测技术

对光纤光栅波长变化与应变的关系进行了理论分析,并通过材料试验和等强度梁试验加以验证;研究了光纤光栅的布设工艺,并在黑龙江省呼兰河大桥预应力箱形梁的施工过程中成功地布设了12个光纤光栅应变传感器与3个温度传感器;布设的传感器监测了预应力箱形梁张拉过程的钢筋应变历程,以及箱形梁静载试验的钢筋应变增量与分布;大桥建成后,利用布设的光纤光栅应变和温度传感器对呼兰河大桥进行了阶段性运营监测,监测了车辆荷载下的应变历程和大桥温度变化过程。监测结果表明埋入的光纤光栅可以方便地监测汽车的流量及其可能的疲劳损伤,为桥梁结构的健康诊断提供依据。一年多的考验表明,光纤光栅的稳定性与耐久性满足钢筋混凝土桥梁结构长期健康监测的要求,性能明显优于传统的电阻应变片。     

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.     

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.     

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。     

Toward an Integrated Smart Sensing System and Data Interpretation Techniques for Pavement Fatigue Monitoring

Abstract: Currently, pavement instrumentation for condition monitoring is done on a localized and short‐term basis. Existing technology does not allow for continuous long‐term monitoring and network level deployment. Long‐term monitoring of mechanical loading for pavement structures could reduce maintenance costs, improve longevity, and enhance safety. In this article, on‐going research to develop and validate a smart pavement monitoring system is described. The system mainly consists of a novel self‐powered wireless sensor based on the integration of piezoelectric transduction with floating‐gate injection capable of detecting, storing, and transmitting strain history for long‐term monitoring and a novel passive temperature gauge. A technique for estimating full‐field strain distributions using measured data from a limited number of implemented sensors is also described. The ultimate purpose is to incorporate the traffic wander effect in the fatigue prediction algorithms. Preliminary results are shown and limitations are discussed.     

Structural monitoring of an onshore wind turbine foundation using strain sensors

To date, the vast majority of onshore wind turbines are cast integrally with the foundations through embedded rings. Local damage around the bottom flange of the embedded ring was observed in a large number of existing foundations. Wind turbine foundations lack structural redundancy. The tower-foundation interface is the weak point and the deterioration of the interface will reduce the foundation’s effectiveness. This study is focused on structural health monitoring of a 1.5-MW onshore wind turbine foundation with embedded ring, on the basis of strain signals acquired continuously from the turbine foundation. A strain sensor monitoring system was installed in field to monitor the concrete status by measuring strain patterns and subsequently alerting any abnormal state of concrete in long term. It was integrated with a dynamic strain acquisition system to develop a real time relationship between the behaviour of local concrete deformation and the characteristics of loads exerted on the foundation. The performance of the foundation was simulated through numerical calculations. The results obtained from the sensors, numerical model and environmental operating data showed that the wind speed and direction dominated the loads exerted on foundation and local concrete deformation inside the foundation. The concrete deformation varied around the circumference of the embedded ring resulting from changes of wind direction and rotor position. The overall distribution pattern correlated roughly with peaks and troughs of calculated forces as well as the mean wind speed. Moreover, cracks were detected to occur in both sides of the bottom flange of the embedded ring and those cracks outside the flange were observed to fluctuate correlated to the change of wind speed. The field test demonstrates that the long-term monitoring of the local concrete deformation is necessary to ensure the safety of the foundation.     

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.     

土工合成材料加筋土柔性桥台复合结构及应用

土工合成材料加筋土柔性桥台复合结构是美国“未来桥梁计划”中针对50多万座面临“更换问题”的中小型单跨桥梁而提出的一种新型技术，是土工合成材料加筋土柔性桥台技术的优化与提升。这种新型技术整体性好，可彻底避免传统桩承桥台与引道路基之间的差异沉降，近10年来已逐步在美国各州推广应用。在我国，尽管加筋土理论和技术应用取得了很大发展，但尚无真正的加筋土柔性桥台复合结构的工程实践方面的报道。文章收集了已有的加筋土柔性桥台复合结构工程案例和现场监测成果，整理归纳其结构特点和工作特性，结果表明：该结构通常具有统一的结构形式和技术特点，如加筋间距一般为0.2m、面层型式一般为模块式等；该结构具有施工简便快速、造价节省等优点，并表现出工后沉降小等良好的服役性能。     

Health monitoring of flexural steel structures based on distributed fibre optic sensors

In our previous study, a distributed long-gauge fibre optic sensing system (with the ability to obtain effective average strain, or macro-strain, distributions) for practical adaptation in civil structural health monitoring was developed and verified. The present paper is devoted to proposing an integrated health monitoring scheme for elastic beam-like structures, where flexure dominates structural responses, based on static testing and measurements using the developed sensors. A series of experimental investigations on steel beams with different levels of damage are first carried out. The static strain measurements from distributed sensors are characterised and some concerns on the data processing and feature extraction are discussed. On the basis of the extracted features, structural health monitoring (SHM) investigations are deployed in three parts: damage identification with no requirement for a structural analytical model, parametric estimation based on finite element (FE) models, and evaluation of structural global behaviour. By comparing to traditional transducers such as linear variable displacement transducers (LVDTs) and foil ‘point’ strain gauges, the ability and ascendancy of the sensors developed here for SHM purposes are verified. A comprehensive health monitoring strategy for steel flexural structures based on the distributed strain sensors array is proposed finally.     

Wireless Monitoring of a Multispan Bridge Superstructure for Diagnostic Load Testing and System Identification

Abstract: This article focuses on the deployment of a wireless sensor system (WSS) developed at Clarkson University for structural monitoring purposes. The WSS is designed specifically for diagnostic bridge monitoring, providing independent conditioning for accelerometers, strain transducers, and temperature sensors in addition to high‐rate wireless data transmission and is capable of supporting large‐scale sensor arrays. A three‐span simply supported structure was subjected to diagnostic load testing as well as ambient vibration monitoring. A total of 90 wireless and several wired sensors, including accelerometers and strain transducers were used in the deployment. Strain measurements provided capacity and demand characteristics of the structure in the form of neutral axis locations, load distributions, and dynamic allowances which ultimately produced an inventory and operating load rating for the structure. Additionally, modal characteristics of the structure, including natural frequencies and mode shapes, were derived from measured accelerations and discussed briefly.     

Applications of FBG-based sensors to ground stability monitoring

Over the past few decades, many optical fiber sensing techniques have been developed. Among these available sensing methods, optical fiber Bragg grating(FBG) is probably the most popular one. With its unique capabilities, FBG-based geotechnical sensors can be used as a sensor array for distributive(profile) measurements, deployed under water(submersible), for localized high resolution and/or differential measurements. The authors have developed a series of FBG-based transducers that include inclination, linear displacement and gauge/differential pore pressure sensors. Techniques that involve the field deployment of FBG inclination, extension and pore-pressure sensor arrays for automated slope stability and ground subsidence monitoring have been developed. The paper provides a background of FBG and the design concepts behind the FBG-based field monitoring sensors. Cases of field monitoring using the FBG sensor arrays are presented, and their practical implications are discussed.     

现有铁路钢桥应变监测系统评估

现有铁路线中旧式钢桥所用的是目前已不再使用的材料,常常会出现老化问题,使用中常遇到与设计荷载完全不同的荷载情况。因此,应变监测是保障结构性能和进行安全评估的重要工具。介绍并讨论了安装在Trezói大桥的应变监测装置系统,用该系统评估铁路钢桥结构。采用两种不同的传感器(电子传感器和光纤传感器)进行结构性能的现场观测。所设计的电子监控系统安装在桥梁上,为研究项目提供试验数据,光纤光学显示器系统用于评估系统的可靠性以确保其安全,并对试验点的测量值进行筛选。对结果进行分析,描述桥梁特性和性能,讨论两种传感器在获得桥梁使用情况和疲劳评估相关数据方面的局限性,也即关注其精确采集结构的静态与动态响应及频率的能力。     

Evaluation of a strain monitoring system for existing steel railway bridges

The old steel bridges that integrate the existing railway lines are structures built with materials that are no longer used and whose knowledge has being lost over the years, often presenting severe problems of deterioration and subjected to loading environments very different from those for which they were designed. In this context, adequate strain monitoring is a crucial tool in supporting the behavior characterization and safety assessment of these structures.This article presents and discusses the monitoring systems installed in the Trezói Bridge, within a research project aimed at developing and applying procedures for evaluation of the structural integrity of steel railway bridges. The field observations of the structural behavior were accomplished by using two different types of sensors: electric and fiber optic strain sensors. The electric monitoring system was designed and installed on the bridge to supply the experimental data for the research project, while the fiber optic monitoring system was firstly applied to evaluate the reliability of the former and to check its efficiency, and secondly to provide some redundancy of the measurements at critical locations. The obtained results are analyzed to characterize the bridge behavior and the capabilities and limitations of both types of sensors to acquire the relevant data for the bridge service condition and fatigue assessment are discussed, namely in what concerns the ability to accurately capture the static and dynamic components of the structural response and the frequency content of interest.     

A preliminary study of the application of the strain-self-sensing smart geogrid rib in expansive soils

Flexible conductive materials are widely used in structural health monitoring; it is also known in geotechnical engineering. In this preliminary study, a strain-self-sensing smart geogrid rib was proposed to monitor the induced strain by wetting-drying cycles of the expansive soil. After the calibration, a physical modeling test was conducted with the smart geogrid rib reinforced in expansive soils under three wetting-drying cycles. Results demonstrated: that the smart geogrid rib was capable of self-sensing its strain; the strain self-sensed by the smart geogrid rib was in good agreement with that measured by FBG strain sensors before cracks were generated; it could capture the crack propagation of expansive soils during wetting-drying cycles by the discrepancy compared to FBG sensors. Further study will be continued for the mechanism of the geogrid instead of the geogrid rib and the application to real-time monitoring of the performance of the geosynthetic expansive soil slopes.     

Strengthen and real-time monitoring of RC beam using “intelligent” CFRP with embedded FBG sensors

The reinforced concrete (RC) structures strengthened with carbon fiber reinforced polymers (CFRP) have been widely used since they have the prominent characteristics that the structures strengthened with traditional methods cannot compare with. FBG sensors have excellent performances of high sensitivity, interference resistance, corrosion resistance, micro-volume, lightweight and variable form, one-line multi-point monitoring, long distance monitoring etc. Due to the compatibility with CFRP sheets, they can be embedded into the CFRP-strengthened RC beams. In the paper the strain change of CFRP is calculated through monitoring the wavelength change of FBG sensors during load-bearing process of the tested RC beams. Then the theoretically calculated strain of tensile steel and compressive concrete is obtained according to the plane cross-sectional geometric relationship function. It is found that the theoretically calculated strains agree well with the experimental value monitored by FBG sensors. The wavelength signal of FBG sensors embedded in CFRP has a uniform linear relationship with the strain measured by strain gauge. The real-time theoretically calculated load agrees well with the applied load.     

Structural health monitoring of a steel stringer bridge with area sensing

The Federal Highway Administration Long-term Bridge Performance Programme initiated an International Bridge Study by selecting a steel stringer bridge as a benchmark structure for structural health monitoring. As a part of this programme, the authors studied the application of the Long-Gauge Fibre Bragg Grating (LG-FBG) sensors on this bridge. This paper aims at illustrating the LG-FBG-related state-of-the-art technologies by taking the bridge as the test bed. (1) The concept of the LG-FBG sensor for area sensing is presented. Most fibre optic sensors measure point strains for local monitoring. In contrast, the developed LG-FBG area sensor has a long gauge (e.g. 1–2 m), and it can be connected to each other to make a sensor array for distributed strain measuring; (2) spectral analyses of the macro-strain time histories are performed to identify structural frequencies, and the results are compared with those estimated from acceleration measurements; (3) the neutral axis position of the girder of the investigated bridge is estimated from the recorded macro-strain time histories, and the results are compared with those from static truck tests and (4) a modal macro-strain-based damage index is applied for damage detection of the steel stringer bridge.     

Monitoring for close proximity tunneling effects on an existing tunnel using principal component analysis technique with limited sensor data

A realistic field monitoring application to evaluate close proximity tunneling effects of a new tunnel on an existing tunnel is presented. A Principal Component Analysis (PCA)-based monitoring framework was developed using sensor data collected from the existing tunnel while the new tunnel was excavated. The developed monitoring framework is particularly useful to analyze underdetermined systems due to insufficient sensor data for explicit relations between force and deformation as the system input and output, respectively. The analysis results show that the eigen-parameters obtained from the correlation matrix of raw sensor data can be used as excellent indicators to assess the tunnel structural behaviors during the excavation with powerful visualization capability of tunnel lining deformation. Since the presented methodology is data-driven and not limited to a specific sensor type, it can be employed in various proximity excavation monitoring applications.     

基础局部沉降下土工格栅加筋衬垫系统变形特性的试验研究

基础局部沉降会引起垃圾填埋场衬垫系统中的土工膜产生较大的拉应变,有可能导致衬垫系统性能下降,因此正确评价衬垫系统的应变就显得非常重要。通过模拟试验,采用应变片和位移计对基础发生局部沉降后土工格栅加筋衬垫系统的变形进行试验研究。试验结果表明:环境温度对衬垫系统的变形影响较大;相同组成材料下土工格栅和土工膜叠放在一起比其他方案更能降低土工膜的应变;衬垫系统刚度对沉陷范围影响不大,但对最大应变值影响较大。所得结果对垃圾填埋场衬垫系统的设计具有一定的指导意义。