柔性与硬性地工格网在砂土与风化泥岩围压下之力学特性

地工格网（以下称格网）用於加劲土壤时，除考虑无围压下的张力行为之外，围压下之力学性质更是设计考量的重点。实际工程应用而言，基於经济考虑，期以现地土壤作为回填材料。本研究分别以拉出、围压抗张与直剪三种试验来探讨格网放土壤中之力学行为；并利用凝聚性泥岩与非凝聚性细砂作为回填材料，评估两种回填材料对加劲成效之影响。结果显示，柔性格网之肋条在拉出过程中易扭曲，造成主应力面旋转的现象，以致拉出阻抗大放硬性格网；围压下格网抗张的应力-应变行为可分为三阶段，即束制阻抗期、张力发展期与破坏期。束制阻抗期大都於3％应变内即已完成；在低围压情况拉出阻抗达20％～60％之拉出强度（相同应变），在高围压下达150％。由直接剪力试验结果可以预测：（a）格网／泥岩加劲结构-低围压时，剪力破坏面应通过格网／泥岩之界面；而高围压时，剪力破坏面应通过泥岩上体。（b）格网／细砂加劲结构-低围压与高围压下剪力破坏面应通过格网／细砂之界面。     

长期荷载作用下土工格栅蠕变特性的试验研究

为探讨长期荷载作用下土工格栅的蠕变特性,在不同的外加荷载和环境温度的各种组合条件下,进行土工格栅的室内蠕变试验,以此获得格栅的蠕变关系曲线、载荷-应变等时曲线及拉伸模量的变化特征,并进行综合对比分析。根据试验与分析发现:荷载水平、环境温度和材料生产工艺是影响土工格栅长期蠕变特性的重要因素。进而,采用时温叠加原理,对于某一给定环境温度下确定土工格栅长期强度的经验估算模式和蠕变强度折减系数。试验结果与理论分析为土工格栅加筋结构长期工作性能的分析与评价提供参考依据。     

土工格栅施工损伤现场足尺试验研究

对两种不同规格型号的聚酯纤维单向土工格栅分别开展上设中粗砂保护层、上下均设保护层和无保护层 3 种工况的施工损伤现场足尺试验,共进行了 6 场试验,通过室内拉伸试验得到不同工况下格栅经施工损伤后的拉伸强度,给出了不同工况下格栅的施工损伤系数。通过对不同方式保护效果的对比分析,发现：在格栅上表面与粗颗粒填料间设置 10 cm 厚中粗砂保护层,对降低格栅筋材施工损伤程度以及提高其可用强度有明显效果;上设保护层和上、下均设保护层两种工况格栅的施工损伤系数较接近,说明格栅施工损伤主要源自上层填料的影响;填料粒径是影响格栅施工损伤程度的主要因素,而筋材自身技术规格的差异对施工损伤系数取值的影响较小。     

Displacement and load transfer mechanisms of geogrids under pullout condition

Reinforcing elements embedded within soil mass improve stabilization through a load transfer mechanism between the soil and the reinforcement. Geogrids are a type of geosynthetic frequently used for soil reinforcement, consisting of equally spaced longitudinal and transverse ribs. Under pullout conditions, the longitudinal ribs are responsible for tensile resistance, while transverse ribs contribute to a passive resistance. This paper describes a new analytical model capable of reproducing both load transfer and displacement mechanisms on the geogrid length, under pullout conditions. The model subdivides the geogrid into rheological units, composed by friction/adhesion and spring elements, mounted in line. Friction/adhesion elements respond to the shear component mobilized at the soil–geogrid interface. Spring elements respond to the geogrid's tensile elongation. Model parameters are obtained through tensile strength tests on geogrids and conventional direct shear tests on soil specimens. The need for instrumented pullout tests becomes therefore eliminated. Results predicted from this new model were compared to instrumented pullout test data from two types of geogrids, under various confining stress levels. The results revealed that the new model is capable of reasonably predicting load and displacement distributions along the geogrid.     

动荷载作用下土工格栅应变软化特性

土工格栅的强度衰减特性对加筋路堤和加筋挡墙的稳定性有重要影响。对塑料土工格栅进行了应力控制式单向循环拉伸试验,研究了循环拉力、预拉力、加载频率等对格栅应变软化及变形的影响。试验结果表明,随着循环拉力、预拉力的增加,格栅的累积应变增大,软化指数增大,强度减弱;荷载振动频率的减小也会产生类似的结果。通过对试验数据的分析,总结了格栅应变软化的规律,并将其引入改进的Iwan模型中,建立了能描述循环拉伸荷载作用下土工格栅的拉力应变关系的模型,通过将模型计算结果与试验结果的对比,验证了模型的正确性。     

Rate-dependent mechanical behavior of jointed rock with an impersistent joint under different infill conditions

Transition in the rate-dependent mechanical response of rock was investigated due to the presence of impersistent joint with different infill conditions.Four types of samples,i.e.intact,jointed with no grouting,jointed and grouted with cement,and jointed and grouted with epoxy,were fabricated using model material.A series of dynamic split Hopkinson pressure bar(SHPB) tests was conducted on prepared samples with strain rates varying between 53-130 s^-1 along with static uniaxial compres...     

Evaluation of Tensile Strength of Cement-Treated Sand Based on Several Types of Laboratory Tests

There exist differences in the values of the tensile strength of cement-treated soils among results from direct tension, splitting tension, and bending tests. This paper presents numerical simulations that serve to explain such differences in the tensile strength properties obtained from the three types of tests. Finite element (FE)-analyses were performed to simulate the three tests as boundary value problems. A simple elasto-plastic model that simulates the behavior of cement-treated sands under a general three-dimensional stress state was used for these analyses. The analytical results showed that the direct tension tests yield reliable values of the actual tensile strength that will be mobilized under an ideal condition of uniaxial tension. On the other hand, in the splitting tension and bending tests, the tensile strength values are calculated from an external load by assuming a linear-elastic behavior in estimating the stress distribution. However, these values do not correspond to the actual tensile strength. The splitting tensile tests underestimate the tensile strength because the shear failure occurs below the loading strip and restrains further increase of the applied load. On the other hand, the bending tests overestimate the tensile strength because of the redistribution of stresses induced by the strain-softening behavior on the tensile side.     

基于特种筋材蠕变试验预应变加筋法 应用研究及计算模型

 通过对目前广泛应用于加筋土工程的特种筋材——CE131土工网、SDL25土工格栅进行了不同应力水平作用下的长期荷载蠕变试验。为比较不同强度土工合成材料的蠕变特性，研究制作6个CE131土工网试样和4个SDL25土工格栅试样。通过大量的试验结果分析，得出特种筋材CE131土工网、SDL25土工格栅的长期强度为抗拉强度的30%～40%，并提出在长期强度条件下特种筋材的预应变值大小及其计算公式。这对施工中的预应变加筋法技术有重要的参考价值。     

Evaluation of creep behavior of high density polyethylene and polyethylene-terephthalate geogrids

The tensile creep behavior of polyethylene-terephthalate (PET) and high density polyethylene (HDPE) geogrids was evaluated using five test methods: the short- and long-term stepped isothermal method (SIM), the short- and long-term time-temperature superposition (TTS), and the conventional method. SIM and TTS are acceleration tests using elevated temperatures. SIM uses a single specimen throughout all temperature steps in contrast to TTS in which a new specimen is employed for each temperature step. The test results indicate that at the same percentage of ultimate tensile strength, PET geogrid exhibited less creep deformation than the HDPE geogrid. The HDPE geogrid exhibited primary, secondary, and tertiary creep stages before rupture, whereas only primary creep and tertiary creep were detected in the PET geogrid. Furthermore, the strain rate of the primary creep stage was found to be independent of the applied loads for the PET geogrid, while it increased exponentially for the HDPE geogrid. The activation energies deduced from different accelerated creep tests were very similar for the PET geogrid. In contrast, the activation energies were higher from the short-term acceleration tests than from the long-term tests for the HDPE geogrid. The four-parameter Weibull model was able to predict the linear and non-linear creep behavior up to 100 years based on 10-h creep testing data. The creep reduction factor of 100 years design life was evaluated and higher values were resulted from the HDPE geogrid than from the PET geogrid.     

经编土工格栅施工损伤现场试验研究

经编土工格栅因施工损伤导致强度折减现象较为严重,但国内现场试验较少。比较了双向拉伸经编格栅在4种填料下压实后的纵横向拉伸强度、伸长率、应力应变曲线,对施工损伤导致的强度折减影响因素进行了分析,并给出了相应的折减系数,为设计人员提供了适合国内施工工艺、施工器械、经编格栅产品的施工损伤折减系数的参考值。     

Strain distribution along geogrid-reinforced asphalt overlays under traffic loading

While there is significant field evidence of the benefits of geosynthetic-reinforced asphalt overlays, their use has focused on minimizing the development of reflective cracks. Yet, geogrids in asphalt overlays are also expected to develop reinforcement mechanisms that contribute to the pavement structural capacity. Specifically, the use of geosynthetics in asphalt overlays may also improve the mechanical behavior of paved roads by controlling permanent displacements and reducing strains in the pavement layers. While relevant advances have been made towards identifying the mechanisms in geosynthetic stabilization of base courses, such mechanisms may differ from those that develop in geosynthetic-reinforced asphalt overlays. This paper investigates the development and distribution of tensile strains along geogrids used to reinforce asphaltic layers. Experimental data was collected from large-scale paved road models subjected to the repeated loading imparted by wheel traffic. Specifically, the study examines both the elastic and permanent components of displacements induced in geogrids by using mechanical extensometers attached to the geogrids. The testing program includes a number of geosynthetic-reinforced paved road models, as well as a control (unreinforced) section that was also instrumented for comparison purposes. Asphalt strain gauges were used to measure strains within the asphalt concrete layer, providing an additional source of information that proved to be highly consistent with the results obtained from the extensometers. The experimental results showed a progressive mobilization of permanent geogrid strains that reached a final profile beyond which additional traffic loading did not result in additional straining. In comparison, higher strains developed in the unreinforced model, which showed a continuously increasing trend. Elastic tensile strains in the asphalt mixture and rutting under the wheel load were comparatively smaller when using geogrids. Overall, the results generated in this study indicate that the presence of geogrids in asphalt overlays results in a lateral restraining mechanism that influences on the mechanical behavior of flexible pavements.     

Behavior of bolted angle connection under catenary condition in fire

The behavior of bolted angle connections under the combination of shear and tensile forces is studied in this paper to simulate the force applied on a connection in a real fire. First, ansys is used to develop a 3D model of these connections. These models are analyzed in a similar condition to experimental tests, ignoring the tensile force, and the results are compared with those of the experimental tests. Having assured of the accuracy, we studied the connection models in several conditions under the combination of shear and tensile forces. The results show that the strength of connection is rapidly decreased when the temperature is increased, and the decrease pattern of connection strength is similar to decrease pattern of bolts used in the connection. Moreover, investigation of strength reduction value of these connections by the increase of temperature under shear and tensile forces obviously shows that it is possible that the failure of steel frames at elevated temperatures occurs at the connections, and thus utilization of catenary action to enhance the fire resistance of structural steel beams requires investigation of the capacity of steel connections to resist the tying forces generated at the ends of the beams. Copyright © 2012 John Wiley & Sons, Ltd.     

Methoden zur Analyse des Bruchverhaltens hochfester Stahlfaserbetone

Methods for analyzing the fracture behavior of high‐strength steel fiber‐reinforced concretes High‐strength and ultra‐high strength fiber‐reinforced concretes are most suitable for applications with extreme mechanical loads. These extreme conditions require a ductile behavior under tensile loading, which is obtained solely by the addition of steel fibers and their working mechanism. Profound know ledge on the working mechanism of the steel fibers is necessary for optimizing this material. Usually, this knowledge is obtained by means of classical measuring techniques of destructive tests. Adopting measuring techniques from non‐destructive material testing helps to analyze and to identify the different stages of the fracture mechanism of high‐strength and ultra‐high strength fiber‐reinforced concretes in detail. The application of different non‐destructive measuring techniques is shown exemplary on tensile tests conducted on an ultra‐high strength fiber‐reinforced concrete and its applicability for analyzing the fracture behavior is discussed. The main focus is on the characterization of the relevant failure modes under tensile loading by the different measuring techniques and the comparison with classical measuring techniques (e. g. extensometer). The tensile tests have been analyzed by optical deformation measurements using digital image correlation (DIC), acoustic emission analysis (AE), and 3D computed tomography (CT).     

Strength enhancement of clay by encapsulating geogrids in thin layers of sand

Large size direct shear tests (i.e.300 × 300 × 200 mm) were conducted to investigate the possibility of strength enhancement of clays reinforced with geogrids embedded in thin layers of sand. In this paper test results for the clay, sand, clay–sand, clay–geogrid, sand–geogrid and clay–sand–geogrid samples are presented and discussed. Thin sand layers with thicknesses of 4, 6, 8, 10, 12 and 14 mm were used to quantify their effect on the interaction between the clay and the geogrids. In this regard effects of sand layer thickness, normal pressure (i.e. confinement) and transversal members of geogrids were investigated. All the tests were conducted using saturated clay with no drainage allowed. Test results indicate that provision of thin layers of sand for encapsulating the geogrids is very effective in improving the strength and deformation characteristics of saturated clay. Maximum strength enhancement was derived at an optimum sand layer thickness of 10 mm which proved to be independent of the magnitude of the normal pressure used. For a particular sand layer thickness, increasing the normal pressure resulted in enhanced strength improvement. Results also showed that removal of the geogrid transversal members resulted in reducing the strength of the reinforced samples by 10% compared to geogrids with transversal members. Encapsulating geogrids in thin layers of sand not only will improve the performance of clays if used as backfill it would also provide drainage paths preventing pore water pressure generation on saturation of the backfill.     

Linear viscoelastic behaviours of bituminous mixtures and fiberglass geogrids interfaces

One major research topic is to characterize the mechanical behaviour of actual reinforced pavement structures from laboratory experimentation and take it into account for the design. This investigation aims to verify the effect of fiberglass geogrid presence on interface linear viscoelastic (LVE) behaviour separately and as a system along with the bituminous mixture layers. To conduct the research, two different fiberglass geogrids, with ultimate tensile strength (UTS) of 100 and 50 kN/m, and tack coat made of straight-run bitumen and modified by polymer were combined for the fabrication of three reinforced configurations. In addition, two unreinforced configurations were also fabricated. The first was a single layer slab and the second was a double-layered slab composed of two bituminous mixtures (same type) bonded layers by a tack coat. Complex modulus tests were carried out in specimens cored in two different directions, vertically (V) and horizontally (H) cored. The experimental data were fitted using the 2 Springs, 2 Parabolic Elements and 1 Dashpot (2S2P1D) model. The test results showed that all interfaces’ complex modulus obtained for V specimens were LVE. Moreover, complex viscous properties of the interfaces were obtained from the used binder. The interface containing polymer modification presented the highest stiffness.     

焊接不锈钢工字形截面受弯构件整体稳定与设计方法

总结了作者在清华大学完成的一批焊接不锈钢梁在弯矩荷载作用下的整体稳定性能试验,试验以试件的长细比为主要变化参数,对8根工字形截面梁进行了研究.试验的材料是奥氏体型316冷轧板,材料应力-应变关系通过拉伸试验获得;同时采用有限元软件ANSYS建立包含初始缺陷的有限元模型,对试件的极限承载力和荷载-变形关系进行分析,并与试验结果进行了对比,以证明有限元分析的合理性和适用性,为进一步研究奠定基础.同时,还将试验结果与欧洲规范和作者的建议设计方法进行了对比.分析结果表明:通过合理建模的有限元分析结果与试验结果吻合良好,有限元分析能够合理的模拟焊接不锈钢梁的整体稳定性能,作者所建议的设计方法的计算结果与欧洲规范相近,能够应用于工程设计并保证足够安全.     

Effects of transverse members on geogrid pullout behavior considering rigid and flexible top boundaries

Geogrid pullout tests have been regarded as the most direct and effective way to describe the interfacial behavior between geogrid and soil. To investigate the coupled effects of geogrid transverse members and top-loading boundaries on the geogrid-soil interaction, numerical simulations of geogrid pullout tests using the Discrete Element Method (DEM) were carried out in this study. The rigid top boundary was simulated by a rigid wall, while the flexible top boundary was modeled with a string of bonded particles that could rotate and move up and down freely. The coupled effects of geogrid transverse members and top boundary conditions on the geogrid-soil interaction under pullout loads were visualized not only by the force distributions along the geogrids and in the specimens but also by the displacements of soil particles and geogrids. Additionally, the quantitative geogrid force and strain distributions along the geogrids, the lateral force distributions on the front walls, and the vertical displacements of top boundaries also showed the influence of transverse members on the geogrid pullout behavior considering the rigid and flexible top boundaries. The DEM investigation results of this study may provide helpful guidelines for regulating the geogrid pullout test apparatus and methods.     

Evaluation of Extent of Damage to Geogrid Reinforced Soil Walls Subjected to Earthquakes

The aim of this study is to establish a simple method for evaluating the extent of damage to geogrid reinforced soil walls (GRSWs) subjected to earthquakes. Centrifuge tilting and shaking table tests were conducted to investigate the seismic behaviour of GRSWs, with special focus on the effects of the tensile stiffness of the geogrids, the pullout characteristics and the backfill materials. As a result, it was found that GRSWs showed large shear deformation in the reinforced area after shaking, that such deformation was influenced by the tensile stiffness of the geogrids, the pullout resistance and the deformation modulus of the backfill material, and that finally slip lines appeared. However, the GRSWs maintained adequate seismic stability owing to the pullout resistance of the geogrids, even after the formation of slip lines. It is considered that such slip lines appeared due to the failure of the backfill material. Since the maximum shear strain occurring in the backfill can be roughly estimated from the inclination of the facing panels, using a simple plastic theory, it is possible to evaluate whether the backfill has reached its peak state or not. The formation of slip lines observed in the centrifuge model tests could be well explained by this method. Finally, the method is proposed to estimate the failure sections in the GRSWs using a Two Wedge analysis.     

Dynamic tensile testing of fabric–cement composites

Dynamic tensile tests were conducted using a high speed servo-hydraulic testing machine on three types of fabric reinforced cement composites. The high speed testing procedure and data processing method are presented. Quasi-static tests were also conducted on the composites. Effects of strain rate on the mechanical properties of fabric–cement composites are noted. A good correlation was found between the properties of the fabrics and the composites, with the carbon fabric exhibiting the highest strength and ductility performance in high speed tensile tests. The differences in tensile behavior of the various composites were correlated with the differences in the role of the fabric materials. Composites tested under high speed loading exhibited different responses as compared to similar composites tested under quasi-static condition.     

Evaluation of tensile load-strain characteristics of geogrids through in-soil tensile tests

This paper evaluates in-soil tensile load-strain characteristics of geogrids with the help of a custom designed and developed in-soil tensile setup in the laboratory. Displacement controlled in-soil tensile tests were carried out to evaluate the effect of normal stress, soil type, and presence of sand-sandwiched layer, on the tensile load-strain characteristics of geogrid. Confinement of geogrid within the soil and application of normal stress were found to increase the mobilized tensile load and secant tensile stiffness of geogrid. Secant stiffness improvement factors were determined to quantify the improvement in tensile load-strain characteristics of geogrid under confinement, on comparison to in-isolation values. Geogrid was observed to exhibit lower secant tensile stiffness when embedded in marginal soil, moist-compacted at wet of optimum. However, the concept of sand-sandwiched geogrid was found to improve the tensile load-strain behaviour of geogrids embedded in marginal soil compacted at wet of optimum.