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).     

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。     

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.     

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.     

Performance of a 33m high geogrid reinforced soil embankment without concrete panel

Geosynthetic reinforced soil embankment are extensively applied in the construction of high-speed railway and highway in mountainous regions but limited field monitoring is conducted on high and steep cases. Aiming to acquire better understanding, a 33-m-high single-tiered wrapped-facing geogrid reinforced soil embankment with the slope of 1 V:0.5H in China was monitored for 2 years during and after construction. Vertical earth pressure, strain of geogrids, horizontal displacement and settlement per layer were recorded and analysed. The results show that the geogrid tensile strains gradually increased during construction. And they were still developing after completion due to creep and subsequent vehicle surcharge load. The predictions of reinforcement loads by the FHWA methods were much higher than the estimated ones from measured strains. The vertical earth pressures linearly grew during construction and then stabilized fast. The horizontal displacement increases with height and the largest value achieved around the top of the slope two years after the construction is 0.14% the total height approximately. The settlement per layer is larger in the lower and middle portion of the embankment and no obvious change is observed over time. This study hopes to serve as a case reference for design and construction of similar reinforcement projects in the future.     

加筋膨胀土地基极限承载力公式的确定

根据膨胀土产生膨胀性土压力的规律，并结合加筋土的加筋和破坏机理，由极限平衡分析理论，提出了考虑膨胀力作用的加筋膨胀土地基的极限承载力公式，为确定加筋膨胀土地基的承载力提供了理论依据，并可供设计时参考。     

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.     

弹性索理论在加筋土挡墙结构设计中的应用研究

在差异沉降现象下,现阶段关于加筋土挡墙面板与筋材连接处的结点应力及筋材的变形情况的研究尚少。例如软土地区地基较软处加筋土挡墙易发生差异沉降,其相关理论的不完善也导致了加筋土挡墙的区域适用性降低。为完善加筋土挡墙的设计理论,运用弹性索理论为理论基础,以加筋土挡墙面板与筋材的连接处为研究对象,结合现有的加筋土挡墙变形及其土压力计算理论,提出了一套差异沉降下加筋土挡墙筋材应力分布及其变形曲线的计算方法。通过理论分析,并结合室内模型试验数据验证了该方法的可行性和准确性。该方法具有计算简便,适用广泛等特点,能够较好地解释面板与筋材连接处的破坏形式,对完善加筋土挡墙的设计理论,特别是沉降控制设计方面具有一定的意义。     

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.     

Influence of cement-fly ash-gravel pile-supported approach embankment on abutment piles in soft ground

Abutment piles in soft ground may be subjected to both vertical and horizontal soil movements resulting from approach embankment loads. To constrain the soil movements, the soft soil ground beneath the approach embankment is often improved using composite pile foundations, which aim at mitigating the bump induced by high-speed trains passing through the bridge. So far, there is limited literature on exploring the influence of the degree of ground improvement on abutment piles installed in soft soil grounds. In this paper, a series of three-dimensional (3D) centrifuge model tests was performed on an approach embankment over a silty clay deposit improved by cement-fly ash-gravel (CFG) piles combined with geogrid. Emphasis is placed on the effects of ground replacement ratio (m) on the responses of the abutment piles induced by embankment loads. Meanwhile, a numerical study was conducted with varying ground replacement ratio of the pile-reinforced grounds. Results show that the performance of the abutment piles is significantly improved when reinforcing the ground with CFG piles beneath the approach embankment. Interestingly, there is a threshold value of the replacement ratio of around 4.9% above which the effect of CFG pile foundations is limited. This implies that it is essential to optimize the ground improvement for having a cost-effective design while minimizing the risk of the bump at the end of bridge.     

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.     

Numerical evaluation of the effect of foundation on the behaviour of reinforced soil walls

This study numerically investigates the influence of foundation conditions, in combination with other factors such as wall height and reinforcement and facing stiffness, on the behaviour of reinforced soil walls (RSWs) under working stress conditions. The foundation was simulated using different stiffnesses and geometries (with and without slope). The results highlight the importance of the combined effect of foundation conditions and the abovementioned factors on the performance of RSWs. The results of these analyses indicate that the shape of the distribution of the maximum reinforcement loads (T_max) with respect to wall height depends on the combined effect of the foundation condition, facing and reinforcement stiffness, and wall height, and varies from trapezoidal to triangular. Additionally, the results indicate that the effect of variations in foundation stiffness on reinforcement tension mobilisation decreases with wall height. Furthermore, the T_max prediction accuracy of three design methods were evaluated and some limitations of each method are presented and discussed.     

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.     

长期反复荷载作用下软黏土地基的变形特性

软黏土地基在长期反复荷载作用下容易产生变形大、沉降时间长且难于预测等问题。针对该问题,通过太沙基一维固结理论,求解了矩形及梯形反复荷载作用下软黏土地基的一维固结解析解;利用ABAQUS有限元软件,提出了一种反复荷载作用下软黏土地基长期固结变形的数值分析预测方法。利用该方法结合工程实例详细分析了反复荷载下软黏土地基沉降、孔隙压力和有效应力及孔隙比等随时间的长期发展变化规律,发现沉降量与反复荷载的水平加载段时间成正比;孔隙水压力的最终发展趋势是围绕0值上下波动;有效应力随着加载次数逐渐增加;孔隙比的变化与土层深度、加载大小与加载次数有关,并将不同荷载类型下的模拟值、理论值和实测数据进行了分析比对,发现等效的反复荷载下的沉降曲线与实测值吻合较好。     

加筋地基土压力测试与机理分析

在素垫层内铺设土工材料加筋后,筋土的界面摩擦作用使加筋垫层的模量提高、应力扩散范围增大,有效发挥下卧土层的承载力、减小地基沉降。通过土工带加筋现场原位试验垫层底的土压力分布测试与结果分析得出：加筋薄垫层（Z/B=0.2）地基垫层底土压力分布是不均匀的,应力集中于基础的边缘,基础中心下应力较小。加筋地基强度和变形与加筋参数有关,通过引入应力扩散系数,分析不同加筋参数下加筋地基的应力扩散能力,研究筋土界面摩擦作用的应力扩散加筋机理,结合工程实际提出太原地区应力设计扩散角取值范围,为加筋地基的设计提供理论依据。     

软土地基的桥涵设计

介绍了软土地基桥涵病害的症状,从桥台设计、桥台软基处理、涵洞软基设计等方面探讨了软土地基桥涵设计的有关注意事项和病害治理对策,为广大桥梁工作者提供了参考。     

Behaviour of geogrid reinforced soil retaining wall with concrete-rigid facing

Monitoring was carried out during construction of a cast-in-situ concrete-rigid facing geogrid reinforced soil retaining wall in the Gan (Zhou)-Long (Yan) railway main line of China. The monitoring included the vertical foundation pressure and lateral earth pressure of the reinforced soil wall facing, the tensile strain in the reinforcement and the horizontal deformation of the facing. The vertical foundation pressure of reinforced soil retaining wall is non-linear along the reinforcement length, and the maximum value is at the middle of the reinforcement length, moreover the value reduces gradually at top and bottom. The measured lateral earth pressure within the reinforced soil wall is non-linear along the height and the value is less than the active lateral earth pressure. The distribution of tensile strain in the geogrid reinforcements within the upper portion of the wall is single-peak value, but the distribution of tensile strain in the reinforcements within the lower portion of the wall has double-peak values. The potential failure plane within the upper portion of the wall is similar to “0.3H method”, whereas the potential failure plane within portion of the lower wall is closer to the active Rankine earth pressure theory. The position of the maximum lateral displacement of the wall face during construction is within portion of the lower wall, moreover the position of the maximum lateral displacement of the wall face post-construction is within the portion of the top wall. These monitoring results of the behaviour of the wall can be used as a reference for future study and design of geogrid reinforced soil retaining wall systems.     

“柔性基础”刚性桩复合地基试验分析与变形计算

为了建立路堤下刚性桩复合地基变形计算模型,通过对14组刚性4桩复合地基的现场试验结果的分析,研究柔性基础条件下刚性桩复合地基的工作性状。试验研究表明采用一定强度或刚度的加筋垫层配合合适的桩帽设置,可改善柔性基础条件下刚性桩复合地基的工作性状,有效减小复合地基变形。复合地基承载力相同时,"柔性基础"沉降量为刚性基础的1.1~1.5倍。在一定荷载或承载力条件下,垫层中不设加筋材料时"柔性基础"短桩复合地基变形接近加筋垫层天然地基。垫层加筋有帽桩复合地基变形明显小于其他情况。对"柔性基础"短桩复合地基,设桩帽、垫层加筋,桩土应力比与不加筋相比增加了1~3倍;对刚性基础长短桩复合地基,碎石垫层中加筋与不加筋相比,桩土应力比范围得到延伸,其中长桩桩土压力比增大明显。提出了"柔性基础"条件下刚性桩复合地基承载力确定,以及考虑桩"上刺入"的刚性桩复合地基变形计算方法。该方法采用力平衡方程计算桩身中性点位置,将复合土层变形计算分为中性点平面     

加筋复合砂石垫层在软弱地基土中的应用

介绍了加筋复合砂石垫层处理软弱地基土的工作原理。结合工程实例，从地基承载力和沉降变形两方面进行了验算，结果均能满足设计规范要求。指出加筋复合砂石垫层处理软弱地基安全合理，经济可行。     

基于离心模型试验的路桥变形耦合特性数值模拟研究

基于离心模型试验,采用有限差分仿真法建立路桥过渡段三维数值分析模型。通过数值模拟计算结果和离心模型试验结果的对比分析,证明了所建立的模型可合理预测路、桥台变形耦合特性。数值分析结果表明:①距台背不同位置过渡段路基面沉降均随过渡段地基处理程度的加强而减小,离台背距离越近,路基面的沉降坡度越大;②桥台沉降稳定后再填筑路基有利于减小桥台自身的水平位移及台背处的差异沉降值。填筑时应注意填筑速率,重点关注桥台及过渡段路基的整体稳定性和过度的不均匀下沉,防止地基因填土堆载发生滑动破坏及路基层状几何形态的变坏所引起的路基刚度的急剧下降;③不同地基处理程度情况下路桥衔接处均存在差异沉降,因此,有必要在桥头设置搭板将路桥交界处的台阶式跳跃沉降变成斜坡式连续沉降;④过渡段路基收敛沉降值与桥台的水平位移和倾角成正相关性,在数值计算的基础上提出了台后20m处过渡段路基收敛沉降值这一控制指标,并将台后过渡段填料的容许剪应变引入到过渡段设计中。