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。     

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。     

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。     

土工格栅界面摩擦特性试验研究

土工格栅与土的界面作用特性直接影响着加筋土挡墙的安全与稳定性。因此,土工格栅与填料的界面技术指标在加筋土挡墙的设计中至关重要。本文在从试验方法、加载方式、试验箱侧壁边界效应和尺寸效应、填料厚度、压实度以及筋材夹持状况等几方面分析土工格栅界面摩擦特性影响因素基础上,进行了土工格栅在砂砾料和粘性土中的拉拔试验和直剪试验。试验结果表明:土工格栅与砂砾料接触面抗剪强度较高,而与粘土接触面抗剪强度很低;对于加筋土挡墙拉拔力较大的层位,应选用刚度大的土工格栅和砂砾料为填料。直剪摩擦试验不适合确定土工格栅接触面的抗剪强度。该试验结果对土工格栅加筋土挡土墙的设计具有重要的参考价值。     

土工格栅加筋挡土墙工作性能的非线性有限元数值分析

针对土工格栅加筋挡土墙发展了有限元数值分析方法,依此对某一足尺试验墙进行具体的有限元数值计算,将计算结果与试验实测结果进行了对比分析,验证了加筋挡土墙结构应力与变形有限元分析的可靠性,进而通过变动填土的强度特性和刚度特性以及加筋的刚度、长度和间距等参数,进行了数值计算,通过对比分析探讨了填土及加筋对土工格栅加筋挡土墙工作性能的影响程度,为加筋挡土墙的设计提供了参考依据。     

Bearing force mobilisation in pull-out tests on geogrids

This paper presents a study on the mobilisation of bearing forces in geogrids subjected to pull-out solicitation. A theoretical model incorporating the effects of interference between grid bearing members on grid pull-out behaviour is presented and used for the interpretation of the results of large-scale pull-out tests on grids with varying geometrical and mechanical properties. The results obtained in this study show the influence of parameters such as free reinforcement length, test speed and interference between members on the pull-out response of geogrids. It is also shown that the load–displacement curve obtained in pull-out tests is not sufficient for an accurate investigation of soil–grid interaction and bearing force degradation mechanisms must be incorporated in the analysis of grid pull-out response if accurate predictions of pull-out strength and grid deformations are to be made.     

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.     

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。     

A shakedown limit calculation method for geogrid reinforced soils under moving loads

A method to calculate the elastic shakedown limit of transportation systems (e.g. pavements and railways) supported by geogrid reinforced soils is presented. For the first time, lower-bound shakedown theory is combined with a strength-based geogrid simulation approach, resulting in a rapid method to quantify the benefit of geogrids on the elastic shakedown limit. It allows decoupling of elastic stress generation and shakedown calculations, meaning it is straightforward to implement, and requires minimal computational effort. Therefore it presents a useful tool to optimise geogrid design for transportation structures such as highway pavements and railways. To show the capability of the method, shakedown limits are calculated for a variety of geogrid configurations using elastic stresses induced by a moving Hertz load. The effect of geogrid depth, soil cohesion, soil friction angle and loading type (normal versus tangential) are investigated for reinforced and non-reinforced soils. It is found that the optimum depth is sensitive to the soil strength properties. Regarding loading, it is shown that for highly tangential loads, shallower geogrids are effective, while for loads with a minimal tangential component, deeper geogrids are effective.     

Numerical analysis of tensile behavior of geogrids with rectangular and triangular apertures

Geogrids, made of polymeric materials, have been used as a construction material for many applications, such as walls, slopes, roads, building foundations, etc. In the past, geogrids were manufactured with apertures in a rectangular or square shape. Recently, geogrids with a triangular aperture shape have been introduced into the market. The new geogrids are manufactured with ribs oriented in three equilateral directions and expected to have a more stable grid structure, which can provide more uniform resistance in all directions. In this study, the numerical software - FLAC was adopted to investigate the responses of geogrids with rectangular and triangular apertures when subjected to a uniaxial tensile load at different directions relative to the orientations of ribs in air. The geogrid ribs were modeled using beam elements jointed rigidly at nodes (i.e., the angle between two adjacent ribs did not change) and subjected to tension in one direction. The numerical results showed that the stress-strain responses of the geogrids were different at different loading directions relative to the orientations of ribs. The effects of aperture shape of geogrid, and elastic modulus and cross-section area of geogrid ribs on the tensile stiffness of the geogrid were also evaluated. The geogrid with triangular apertures had more uniform tensile stiffness and strength distributions than the geogrid with rectangular apertures. An increase of the elastic modulus and cross-section area of the geogrid ribs could increase the stiffness of the geogrid with triangular apertures. The numerical results were verified by experimental data for geogrids with rectangular and triangular apertures.     

土工格栅控制液化土体流动变形的试验研究

液化导致的土体大变形以及侧向流动是地震引起建筑物破坏的主要原因。采用土工格栅作为主要加固材料,开展建筑物荷载作用下液化场地流动变形的振动台试验研究,考虑水平层状土工格栅、包裹状土工格栅和土工格栅+无纺布联合处理等3种加固方案对结果的影响,从超孔隙水压力发展、建筑物沉降量以及格栅应变特性等分析加固方案对液化变形的处理效果。试验表明:采用上述3种加固方案所得的相同埋深处超孔隙水压力峰值基本相等,表明土工格栅的加入基本不能改变地基的液化状态,而后期超孔隙水压力在土工格栅+无纺布联合加固方案下消散速度最快。与其它两种加固方案相比,土工格栅+无纺布联合加固方案下建筑物沉降量最小,相比未加固工况沉降量减少24%,土工格栅中间位置的应变峰值小于边缘位置的应变峰值。采用土工格栅+无纺布联合加固时,具有较大表面积的无纺布对该覆盖区域液化土体有较好的约束作用,限制了砂土颗粒的竖向移动。此外,砂土颗粒对无纺布的作用力将由土工格栅承担,这种作用力将有利于土工格栅与砂土之间的摩擦效应,进一步限制液化砂土的流动变形。     

On the determination of the chemical reduction factor for PET geogrids

Data from four samples of commercially available PET geogrids (made either of yarns or bars), which were measured by BAM or other institute, are analyzed to discuss the procedure and problems of determining the chemical reduction factor RF_CH associated with a certain service life. Estimates from Arrhenius extrapolation usually have very large statistical errors. The level of confidence must therefore be specified. A reliable estimate requires data from immersion tests below the glass transition temperature of PET. To extrapolate the time of reductions for each reduction factor at such low temperatures, one has to know the functional form of the mechanical degradation curve. It is shown how the degradation curve of the tensile strength may be obtained by determining the relation between increase in concentration of carboxyl end group (CEG) and decrease in tensile strength. Therefore, experimental studies to determine the chemical reduction factor should be accompanied by the measurements of the CEG concentration and the intrinsic viscosity. Furthermore, such measurements allow a non-ambiguous determination of the molecular mass. Hydrolytic molecular degradation will proceed continuously even at 20 °C with half-life of the inverse of the CEG concentration of 40–100 y. Nevertheless, small chemical reduction factors at a lifetime of 100 y are obtained with high level of confidence for materials with low initial CEG concentration and high molecular mass. This is shown by pooling data from samples with comparable CEG concentration, molecular mass and above all comparable intrinsic relation between increase in CEG concentration and decrease in strength. Therefore, the recommendation of ISO TR 20432, Table 2, for chemical reduction factors seems to be applicable to PET geogrids with index properties well below the one specified by the technical report. Whether these index properties are actually a sufficient condition to have small chemical reduction factors even at a very long service life is still an open question. The determination of chemical reduction factor should be based on aging experiments, at least for products with index properties close to the limiting values for the following reasons. (1) Even so standards are available, results of different laboratories on absolute values of CEG concentration and number averaged molecular mass differ to a certain extent. (2) Other factors, like crystallization, affect the mechanical degradation significantly. (3) There is no universally applicable form of the mechanical degradation curve.     

季节性冻土中土工格栅加筋特性试验研究

针对东北地区典型粉质黏土,通过一系列土工格栅在冻土中的拉拔试验,重点分析了土壤含水率及冻融循环作用对土工格栅加筋性能的影响,并基于拉拔摩擦强度、表观摩擦系数、界面摩擦阻力和端承被动阻力等理论,对试验数据进行了详细分析。研究发现:含水率对土工格栅加筋效果存在明显抑制作用,含水率从20%提高至32%时,筋土界面摩擦阻力和端承被动阻力均减小60%以上。而当含水率一定时(w=24%),冻融循环作用反而提高了土工格栅的加筋效果,经历7次冻融循环后,土工格栅加筋效果增幅为30%左右,主要是横肋前端承被动阻力的提高,而筋土间界面摩擦阻力变化不大。上述试验成果可为土工格栅在冻土地区的推广应用提供一定的理论依据。     

多层土工织物加筋的沥青路面力学性能分析

应用复合材料力学原理和有限元法对单层和多层土工织物加强沥青混凝土路面进行了力学分析,在行车荷载条件下,研究了不同土工织物层数加筋层数和弹性模量对加筋的沥青路面层底弯拉应力和路面弯沉的影响,以达到优化道路设计的目的。     

Performance control for seismic design of moment frames

Performance control (PC) is the important mental task that is or should be the cornerstone of earthquake-resistant structural design. The fundamental notion behind PC is that the seismic structural response is largely a function of design and detailing, rather than conventional analysis. PC is a design strategy in which the strength, stiffness and other characteristics of groups of members are induced in accordance with predetermined objectives rather than investigated with respect to certain design criteria. PC methodology enables engineers to predict and control structural damage at preselected response stages such as at first yield, any fraction of the failure load or allowable drift ratio, etc. PC provides a wealth of important information that may not be readily available through traditional methods of design. The ultimate failure load solutions are “unique” and suitable for plastic design treatment in that they include P-delta and stiffness degradation effects, and satisfy the prescribed yield criteria as well as boundary support and static equilibrium conditions. The proposed procedures for seismic design of moment frames are entirely suitable for manual computations. The paper does not address irregularities in earthquake-resistant moment frames.     

Effects of Geosynthetic Reinforcement Type on the Strength and Stiffness of Reinforced Sand in Plane Strain Compression

The effects of geosynthetic reinforcement type on the strength and stiffness of reinforced sand were evaluated by performing a series of drained plane strain compression tests on large sand specimens. The reinforcement type is described in terms of the degree of unification of the constituting components (for geocomposites) as well as the tensile strength and stiffness, the covering ratio and others (for geocomposites and geogrids). Sand specimens reinforced with different geosynthetic reinforcement types exhibited significantly different reinforcing effects. A geocomposite made of a woven geotextile sheet sandwiched firmly with two sheets of non-woven geotextile, having a 100% effective covering ratio, exhibited reinforcing effects higher than typical stiff and strong geogrids. With some geocomposite types, the reinforcing effects increase substantially by better unifying longitudinally arranged stiff and strong yarns and non-woven geotextile sheets. When fixed firm to the yarns, the non-woven geotextile sheets function like the transversal members of a geogrid by locally transmitting load activated by interaction with the backfill to the yarns. These geocomposites can exhibit reinforcing effects equivalent to those with stiff and strong geogrids. Local strain fields of the specimens are presented to show that, for reinforced sand, the peak stress state reached is always associated with the development of shear band(s) in the sand and a higher peak strength is achieved when the strain localisation starts at a larger global axial strain due to better reinforcing effects.     

Geogrid connections

Connections can have a significant impact on the performance of geosynthetic systems whether their intended function is separation or reinforcement. When reinforcement is the function, the connection must be considered as a limiting strength factor and evaluated by the same performance standards as the reinforcement material alone. If used in critical structures with design lives of more than 1 or 2 years, the geosynthetic and its connections must be evaluated for durability limitations such as creep, site damage and chemical degradation. Geogrid connections typically fall into this category.

Geogrid connections are accomplished by either overlapping or mechanical connections. Overlap connections must be designed and constructed to assure proper overlap lengths. Overlap length is governed by soil interaction behavior. Geogrid-soil interaction is measured by pull-out testing and pull-out performance is a function of geogrid dimensional stability. Overlaps can be secured by a variety of tyiing techniques but these ties are not efficient for load transfer. Geogrid Bodkin Joints are the preferred mechanical connection for assuring full load transfer between adjacent layers of some geogrids.

This paper discusses geogrid connections, the techniques used, and the factors that affect their performance.     

Sand-geogrid interfacial shear response revisited through additive manufacturing

Though it is known that the geometric features of geogrids are crucial for deriving optimal interface shear strength, not much work is done to optimize the size and shape of the apertures relative to the particle size of the soils in contact. Most of the commercial geogrids have rectangular or square apertures, which are many times bigger than the soil particles. The present study explores the effects of aperture size and shape of geogrids relative to the size of the sand particles on their interface shear response through direct shear tests and digital image analysis. Geogrids of different aperture sizes and shapes were manufactured using a 3D printer. Shear tests were carried out on three sands of different grain sizes interfacing with geogrids of five different aperture sizes and three different aperture shapes. Through these tests, interface shear response with a wide range of aperture ratio and different shapes of geogrids is understood. Shear zone thickness of different sand-geogrid interfaces was computed through Particle image velocimetry (PIV). Based on the tests and analyses, triangular apertures are found to be more efficient compared to other apertures. The optimal range of aperture ratio is found to be 2–11.29.     

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。     

加筋风积砂地基承载力试验研究及计算分析

针对沙漠地区风积砂土特殊的物理力学性质，以土工格栅为加筋材料对风积砂土进行加固。通过室内模型试验，对未加筋的风积砂土和15种布筋方式下的加筋风积砂地基承载力进行了试验研究。测定了各种布筋方式下加筋风积砂土的极限破坏荷载，分析了加筋土的变形以及应力扩散情况。根据试验结果，总结了不同布筋方式及不同埋深条件下，加筋风积砂地基承载力的变化规律，并推荐片式双层格栅为施工中有效的布筋方式，此布筋方式下的加筋风积砂地基承载力较风积砂地基承载力增加1．2倍。提出了加筋风积砂土的强度机理和破坏模式，建立了无埋深条件下片式单层格栅加筋风积砂地基承载力的计算公式。经试验验证，所得结果具有实用价值。