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基于平面应变试验的加筋土复合体极限承载力研究
Ultimate Bearing Capacity of Geosynthetic Reinforced Soil Composites Based on Plane Strain Tests
加筋土复合体凭借优异的承载性能已被广泛应用于承重式加筋土桥台中。为了进一步研究加筋土复合体的极限承载性能,设计并开展了9组土工织物加筋土复合体平面应变试验,探究了不同填料级配和加筋间距对加筋土复合体的极限承载性能的影响。试验结果表明:与不加筋相比,加筋可以显著提高加筋土复合体的极限承载力,且加筋间距越小,加筋土复合体的极限承载力越大,同时也表现出更大的刚度;在加筋间距相同,且填料粒径介于1~8 mm范围内时,填料级配对加筋土复合体极限承载力的影响很小,但对加筋土复合体刚度有一定影响。此外,现有加筋土复合体的承载力计算方法严重低估了复合体承载力,当填料级配不满足FHWA推荐值时,不宜直接采用该计算方法评估加筋土复合体承载力。研究成果为较小粒径填料在工程建设中的应用提供了参考。
[Objective] Due to the excellent load-bearing performance, geosynthetic reinforced soil (GRS) composites have been widely adopted in the construction of load-bearing GRS bridge abutments. Unlike conventional gravity or cantilever retaining walls, GRS abutments are required to bear significantly higher vertical loads transferred from the superstructure. Therefore, it is essential to investigate the ultimate bearing behavior of GRS composites to ensure the safety and reliability of the structures. [Methods] In this study, a series of plane strain model tests were conducted to evaluate the ultimate bearing capacity of GRS composites. Nine groups of tests were designed and conducted using geotextile as the reinforcement material, incorporating four types of backfill material gradations and three reinforcement spacings. The gradation of the backfill materials primarily varied in particle size distribution within the range of 1-8 mm, while the reinforcement spacing was set at 20 cm, 25 cm, and 33.3 cm. The test results were compared with those of unreinforced soil and analytical predictions based on the Federal Highway Administration (FHWA) design guidelines. [Results] The experimental results demonstrated that reinforcement significantly enhanced the ultimate bearing capacity of GRS composites. Under the same backfill material condition, the incorporation of reinforcement led to significant increases in ultimate bearing capacity compared with the unreinforced test. Specifically, with reinforcement spacings of 20 cm and 25 cm, the ultimate bearing capacity increased by 87.5% and 62.5%, respectively. These results clearly indicated that the reinforcement spacing played a critical role in the bearing performance of GRS composites. In addition, smaller spacings resulted in greater overall stiffness of the composite system. When the reinforcement spacing was constant and the backfill particle size ranged between 1 mm and 8 mm, the effect of gradation on the ultimate bearing capacity was relatively minor. However, differences in backfill material gradation led to noticeable variations in the overall stiffness of GRS composites. When the experimental results were compared with predictions obtained from the FHWA-recommended method for GRS composite bearing capacity, a significant discrepancy was observed. The FHWA method considerably underestimated the ultimate bearing capacity in all test cases. Therefore, it was not recommended to calculate the ultimate bearing capacity of GRS composites with finer graded backfill materials by directly applying the FHWA method. During post-test inspection, the locations of geosynthetic rupture were identified and analyzed. The observed failure surfaces within the reinforced soil mass approximately corresponded to a Rankine failure plane. The results indicated that the obvious composite behaviors were demonstrated in the GRS composites. [Conclusion] This experimental study provides a systematic analysis of the ultimate bearing capacity of GRS composites under plane strain conditions, emphasizing the roles of reinforcement spacing and backfill material gradation. The findings confirm that geosynthetic reinforcement can significantly enhance both the strength and stiffness of the soil composites, with closer reinforcement spacing resulting in better performance. The study reveals that the current design guidelines recommended by FHWA significantly underestimate the actual ultimate bearing capacity, particularly when the backfill material gradation differs from the recommended values. These findings offer valuable reference for future engineering design and construction, promoting more efficient and reliable use of fine-grained or narrowly graded soil in reinforced soil structures.
加筋土复合体 / 平面应变试验 / 极限承载力 / 影响因素 / 土工织物
geosynthetic reinforced soil composite / plane strain test / ultimate bearing capacity / influencing factors / geotextile
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以神农架机场60 m高挡墙为例, 论述了高加筋土挡墙基于极限平衡理论的设计方法, 为了确保设计方案安全合理, 采用强度折减有限元法对设计方案进行应力应变分析和稳定性验证, 并在挡墙关键部位布设了监测断面。根据监测资料可知:挡墙变形较小, 变化速率呈收敛趋势, 应力应变监测成果稳定。现场观测成果证明了该设计方法的合理性, 可为类似工程借鉴。
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The design method of reinforced high earth retaining wall based on limit equilibrium theory is expounded with the high retaining wall (60m) at Shennongjia airport as an illustration. To ensure the safety and rationality of the design, strength reduction and finite element were employed to analyze the stress-strain and stability of the retaining wall. Monitoring points were also arranged in the key sections of the retaining wall. The monitoring data revealed that the deformation of retaining wall was small, the deformation rate showed a tendency of convergence, and the stress and strain were all stable. The rationality of design method was proved by the monitoring results. This research provides reference for similar projects.
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为探究包裹式加筋土桥台结构性能,依托安徽省明(光)巢(湖)高速公路包裹式加筋土桥台项目,进行现场原位监测。通过工后330 d的现场监测,研究了包裹式加筋土桥台面板后水平土压力、桩侧水平土压力、面板水平位移、面板沉降,以及土工格栅应变等分布和变化规律,同时探讨了绕桩格栅截断对面板后水平土压力分布的影响。结果表明:面板后水平土压力沿墙高呈非线性分布,面板中部水平土压力大于其他位置,同时墙趾约束会使底部水平土压力逐渐增大,格栅绕桩截断对面板后水平土压力未造成显著影响;桩侧水平土压力沿高度呈近线性分布,盖梁的存在会使桩侧上部水平土压力偏小;面板水平位移和沉降在工后150~180 d内达到稳定状态,最大变形值均出现在道路中线附近;墙背土工格栅应变随墙高呈非线性分布,随桥台运营时间出现收缩现象。总体上,包裹式加筋土桥台在工后较为稳定,服役期间的监测数据可为今后类似工程设计施工提供参考。
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Four pairs of large-scale instrumented geosynthetic reinforced soil (GRS) square columns were load tested to study the effects of varying reinforcement strength to spacing ratio, to discern the lateral pressures during construction and during load testing, and to derive shear strength parameters of the GRS composite. Each pair was identical in every respect, except one was loaded with a dry-stacked concrete masonry unit (CMU) facing in place and the other without. Lateral pressures during construction were found to be small for the facing type used in this study. Also, based on the derived GRS composite shear strength parameters, it was found that (1) the GRS composite Mohr–Coulomb envelopes are not parallel to those for the unreinforced soil; (2) the reinforcement increased the composite cohesion compared to the unreinforced soil (cohesion increases with decreasing spacing and increasing reinforcement strength); (3) the composite friction angle is less than that of the unreinforced soil (friction angle increases with decreasing reinforcement strength and increasing spacing); (4) as the composite friction angle increases, the active lateral earth pressure coefficient decreases; and (5) the benefits of reinforcing a soil become increasingly significant as the reinforcement spacing decreases.
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This paper presents a case study of the first geosynthetic reinforced soil-integrated bridge system (GRS-IBS) with full-height rigid facings in China. Open graded gravel and biaxial geogrid were used for the GRS-IBS. Steel frames and three-dimensional (3D) vegetation nets were used as temporary facing support during construction of the GRS abutments. Full-height rigid facings were cast in place on strip foundations. Field monitoring results of vertical stress distribution for different construction stages and loading conditions are presented and discussed. For both bridge dead load and truck loads, measured incremental vertical stresses under the beam seat increase significantly with increasing elevation, especially for higher applied vertical stress. The calculated incremental vertical soil stresses using the Boussinesq solution are in reasonable agreement with the measured values, while the 2 : 1 stress distribution method overestimates the incremental stresses in the lower section of the abutment. The transferred vertical stresses from bridge load application for the GRS abutment with full-height rigid facing are larger than those for the GRS abutment with modular block facing near the top of the abutment, but are smaller near the bottom.
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Bridge abutments made of geotextile-reinforced soil have been shown to support the bridge load without the use of piles. However, current design procedures are considered to be conservative. To determine the strength, and to understand better the behavior of reinforced soil, large unconfined cylindrical soil samples reinforced with geosynthetics were axisymmetrically loaded. Samples were 2.5 ft (0.76 m) in diameter and 5 ft (1.52 m) in height. Peak strengths of 4.8 kips/ft2 (230 kPa) to 9.6 kips/ft2 (460 kPa) at 3% to 8.5% vertical strain were obtained from cylinders reinforced with geotextiles at 6-in. (152-mm) vertical spacing. A strength reduction occurred after the peak strength, but most of the loads were sustained up to at least 10% strain before yielding. Tension in the reinforcement appears to be mobilized first in the middle layers, as determined from the location of tears in the geotextile. An equation to calculate the tensile force in the reinforcement, Tmax, in a reinforced bridge abutment is proposed. The normalized strains led to the development of the strain distribution factor incorporated in the proposed equation. The proposed equation is slightly more conservative or almost equal, depending on the type of facing, when compared with the Ko-stiffness method, but gives values approximately one-half of those obtained using the National Concrete Masonry Association and FHWA Demonstration Project 82 methods.
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This paper presents two-dimensional (2D) and three-dimensional (3D) numerical simulations of a half-scale geosynthetic reinforced soil (GRS) bridge abutment during construction and bridge load application. The backfill soil was characterized using a nonlinear elastoplastic model that incorporates a hyperbolic stress–strain relationship and the Mohr–Coulomb failure criterion. Geogrid reinforcements were characterized using linearly elastic elements with orthotropic behavior. Various interfaces were included to simulate the interaction between the abutment components. Results from the 2D and 3D simulations were compared with physical model test measurements from the longitudinal and transverse sections of a GRS bridge abutment. Facing displacements and bridge seat settlements for the 2D and 3D simulations agree well with measured values, with the 2D-simulated values larger than the 3D-simulated values due to boundary condition effects. Results from the 3D simulation are in reasonable agreement with measurements from the longitudinal and transverse sections. The 2D simulation can also reasonably capture the static response of GRS bridge abutments and is generally more conservative than the 3D simulation.
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