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.     

Multi-scale understanding of sand-geosynthetic interface shear response through Micro-CT and shear band analysis

To understand the process of mobilisation of shear strength in sand-geosynthetic interfaces at a fundamental level, it is essential to precisely characterize the size and shape of the grains and the shear-induced surface changes in geosynthetics. In the current study, shear behaviour of dilative and non-dilative geosynthetics interfacing with sands of different morphological characteristics was analysed through interface shear tests and a gamut of digital imaging techniques. 3D shape parameters of sands such as sphericity, convexity, roundness, aspect ratio, and roughness were quantified at different scales using X-ray micro computed tomography (μCT) and optical profilometry. Interface shear tests revealed higher peak and residual friction angles for particles with greater irregularity, angularity, and surface texture. The surface texture of geotextile surfaces resulted in higher interface friction and higher vertical displacement compared to geomembrane surfaces, which showed completely non-dilative behaviour. Surface changes in geomembranes were quantified using laser profilometry. High resolution images obtained at different stages of shearing were analysed for quantifying the shear zone thickness using digital image correlation (DIC). Thickness of the shear bands, microscopic shearing mechanisms and shear strength are correlated to the multi-scale shape parameters of sands and surface changes in geosynthetic surfaces.     

Investigation of mechanisms of bentonite extrusion from GCL and related effects on the shear strength of GCL/GM interfaces

Two groups of laboratory tests were carried out to investigate the effect of bentonite extrusion from a hydrated GCL on the shear strength of GCL/GM interfaces. All tests were performed with the woven geotextile side of GCL against the GM. The first group of tests were one-dimensional loading tests in which the GCL/GM specimens were subjected to hydration and vertical loading involving different sequences and loading rates. The second group of tests were large direct shear tests that studied the effect of shearing on bentonite extrusion and hence on shear strength reduction. It was found that bentonite extrusion occurs more readily from GCL/GM interfaces subjected to a swelling-loading sequence than those subjected to a loading-swelling sequence. The quantity of extruded bentonite during the normal loading showed an increasing trend with an increase in loading rate. The total mass/area of extruded bentonite during the normal loading ranged from 0 to 21.9 g/m², which was less than the quantity of bentonite extruded during the subsequent shearing (i.e., 10.7 – 81.1 g/m²). It was found that the volume of bentonite extruded at the large shear displacement caused a significant strength loss equivalent with 8° in terms of interface friction angle. The influence of bentonite extrusion on the peak shear strength showed a magnitude of 3.5° in terms of interface friction angle. The relatively insignificant bentonite extrusion during hydration and normal loading was observed to have a minor effect on the strength loss. Observations from the experimental results provide further insight into the mechanisms of bentonite extrusion.     

Relationships between Shape Characteristics and Shear Strength of Sands

In the Mohr-Coulomb criterion, the shear strength of sands is typically characterized by the internal friction angle, which depends on many factors such as grain size and distribution, the mineralogical origin of the particles, particle shape, unit weight, geological history, cementation, saturation, and overburden pressure. In this study, the empirical relationships among three particle shape indices, different fractal dimension definitions, and internal friction angles were investigated. Within this context, direct shear tests were conducted on 38 different types of sands from different origins and with various grain sizes. For each type of sand, image analyses were performed to find out the roundness, sphericity, regularity parameters belonging to individual grains. Additionally, several statistics of these parameters for different types of sands were determined. The results revealed that particle shape has a limited effect on the friction angle of sands in comparison to grain size distribution. Furthermore, it was found that decreasing regularity in particle shape caused an increase in the internal friction angle of uniform sands. These findings agree with the empirical relationship between the internal friction angle and particle shape suggested in the literature.     

机制砂钢筋混凝土梁受弯与受剪性能试验研究

为了从机制砂骨料的细观指标出发,研究机制砂混凝土梁的受弯与受剪性能,选取了天然砂和3种机制砂,对比了不同骨料的颗粒级配、细度模数、石粉含量、MB值等材性指标,利用图像分析处理技术获取不同骨料的形状参数,通过试验对比了不同细骨料混凝土的工作性能、抗压强度和体积稳定性能;分别设计了受弯和受剪天然砂及机制砂钢筋混凝土梁,通过荷载试验,比较不同细骨料混凝土梁的开裂荷载、挠度曲线和承载力等受力性能指标。研究结果表明：采用延性参数与圆度参数可以反映不同类型骨料间的颗粒形状差异,颗粒级配合理、形状参数良好的机制砂混凝土的强度和长期干缩性能可以达到甚至超过天然砂混凝土的,机制砂和天然砂钢筋混凝土梁的受弯、受剪破坏形态相似,承载力与其混凝土强度相关,可采用现行设计方法对机制砂混凝土梁进行承载力设计。天然砂钢筋混凝土梁抗裂能力略优于机制砂钢筋混凝土梁的,为此,采用现行开裂荷载计算方法会高估机制砂混凝土梁的开裂荷载,建议对机制砂钢筋混凝土梁的开裂荷载设计进行适度调整。     

Microstructural investigation on mechanical behavior of soil-geosynthetic interface in direct shear test

Interface shear strength between soil and geosynthetics mainly depends on the mechanical and physical properties of soil, geosynthetics and the normal stress acting at the interface. This paper presents results of an extensive experimental investigation carried out on sand-geosynthetic interface using modified large direct shear box. The study focusses on the shearing mechanism at the sand-geosynthetic interface and the effect of different parameters on the shearing mechanism. Smooth HDPE geomembrane, nonwoven needle punched geotextile and two types of sand having different mean particle size, have been used in the present study. Microstructural investigation of deformed specimen through Field Emission Scanning Electron Microscope (FESEM) reveals the shearing mechanism which includes interlocking and fiber stretching for sand-geotextile while sliding, indentation and plowing for sand-geomembrane interface. The shearing mechanism for sand-geomembrane interface highly depends on the normal stress and degree of saturation of sand. The critical normal stress that demarcates the sliding and plowing mechanism for sand-geomembrane interface is different for dry and wet sand. The amount of scouring (or plowing) of the geomembrane surface reduces with increase in the mean particle size of sand. FESEM images revealed that the sand particles get adhered to the geotextile fibers for tests involving wet sands. The present microstructural study aided in understanding the shearing mechanism at sand-geosynthetic interface to a large extent.     

Evaluation of Interface Shear Strength between Geosynthetics Under Wet Condition

This paper presents direct shear testing data for interfaces between a nonwoven geotextile or two types of geosynthetic clay liners (GCL) (reinforced and unreinforced) and two types of geomembranes (smooth and textured). In this study, the effect of moisture on interface shear behavior was investigated by performing shear tests in both dry and wet (or hydrated) conditions because the geosynthetic interfaces in a landfill are easily exposed to rain, leachate and groundwater beneath the liners. The degree of strength reduction with increasing displacement and the effect of the normal stress level on friction angles were examined, and the modified hydration method applied for the GCL was also validated. The test results showed that the normal stress level, interface water presence and hydration methods dominated the interface shear strength and behavior. The relationship between the peak secant friction angle and the normal stress demonstrated that the friction angle decreased with increasing normal stress, implying that the shear strength for safe design should be determined by using the maximum value of the normal stress applied in landfills. Finally, comparisons with a few published test results were presented and some design implications for the geosynthetic-installed landfills were discussed.     

Shear strength of geosynthetic composite systems for design of landfill liner and cover slopes

Torsional ring shear tests were performed on composite specimens that simulate the field alignment of municipal solid waste (MSW) landfill liner and cover system components. Simultaneous shearing was provided to each test specimen without forcing failure to occur through a pre-determined plane. Composite liner specimens consisted of a textured geomembrane (GM) underlain by a needle-punched geosynthetic clay liner (GCL) which in turn underlain by a compacted silty clay. Hydrated specimens were sheared at eleven different normal stress levels. Test results revealed that shear strength of the composite liner system can be controlled by different failure modes depending on the magnitude of normal stress and the comparative values of the GCL interface and internal shear strength. Failure following these modes may result in a bilinear or trilinear peak strength envelope and a corresponding stepped residual strength envelope. Composite cover specimens that comprised textured GM placed on unreinforced smooth GM-backed GCL resting on compacted sand were sheared at five different GCL hydration conditions and a normal stress that is usually imposed on MSW landfill cover geosynthetic components. Test results showed that increasing the GCL hydration moves the shearing plane from the GCL smooth GM backing/sand interface to that of the textured GM/hydrated bentonite. Effects of these interactive shear strength behaviors of composite liner and cover system components on the possibility of developing progressive failure in landfill slopes were discussed. Recommendations for designing landfill geosynthetic-lined slopes were subsequently given. Three-dimensional stability analysis of well-documented case history of failed composite system slope was presented to support the introduced results and recommendations.     

Evaluation of shear strength and mechanical properties of granulated coal ash based on single particle strength

The authors have been studying the mechanical properties of granulated coal ash formed by the milling process with a small amount of cement added, with particles almost equivalent in size to sand or fine gravel. The use of granulated coal ash has many advantages, such as the suppression of leaching of heavy metals and the possibility of outdoor curing. In addition, since granulated coal ash is produced artificially, their particle strength can be understood easily. Another advantage is that it is possible to control the particle strength, something which cannot be performed in natural sands. The present research was carried out in order to investigate the possibility of putting such advantages to good use.Single particle crushing tests were carried out on various kinds of granulated coal ash to evaluate the crushing characteristics of each individual grain. Also, the characteristics of the individual particle, such as shape, were investigated. Next, one-dimensional compression tests were conducted to investigate their compression characteristics. Finally, drained monotonic triaxial compression tests were performed under different confining pressures. The effects of the confining pressure on the shear characteristics, as well as their relation to shear strength, were examined with the single particle crushing strength taken into account.Contrary to observations in natural sands, the single particle crushing strength of granulated coal ash did not depend on particle size. Regarding compression characteristics, granulated coal ash compressed very easily because the crushing strength of a single particle was low. Therefore, the yield stress of granulated coal ash was found to be smaller than that of natural sands. Moreover, the crushing strength affected the shear characteristics, i.e., as the crushing strength increased, the shear stiffness became higher and the compressive volumetric strain became smaller. Therefore, crushing strength can be a useful parameter in evaluating the shear characteristic of granulated coal ash.     

Effect of gas-oil contamination on the mechanical behavior of sand-woven geotextile interface: Experimental investigation and constitutive modeling

The accurate estimation of the frictional resistance of interfaces between soils and geosynthetics plays a central role in stability and serviceability of geosynthetic reinforced earth structures. Contamination with hydrocarbons generally impairs soil geotechnical properties; however, its effect on the behavior of soil-geosynthetic interfaces has seldom been examined precisely. For this reason, an extensive series of direct shear tests was performed to investigate the consequence of gas-oil contamination on the mobilization of shear strength and volume change response of gas-oil contaminated angular sand in contact with woven geotextile (WGTX). Complementary tests on the interfaces between glass beads as a replicate for sands with high degree of sphericity and roundness in contact with WGTX were also performed to explore the effect of particle shape. Gas-oil contamination is observed to causes decrease of the peak and critical state friction and dilation angles in both the sand-WGTX and glass bead-WGTX interfaces. However, gas-oil contamination-induced decrease in the frictional efficiency in the glass bead-WGTX interfaces was greater than that in the angular sand-WGTX interfaces. Calibration of a state-dependent sand-structure interface model against the laboratory data of gas-oil contaminated soils-WGTX interfaces results in a reasonable agreement between the model simulations and the laboratory data.     

Shear strength and failure mechanism of needle-punched geosynthetic clay liner

The internal shear strength of a geosynthetic clay liner (GCL) within composite liner systems is crucial for the stability of landfills and should be carefully considered in the design. To explore the shear strength and failure mechanism of the extensively used needle-punched GCL, a series of displacement-controlled direct shear tests with five normal stress levels (250–1000 kPa) and eight displacement rates (1–200 mm/min) were conducted. The shear stress to horizontal displacement relationships exhibit well-defined peak shear strengths and significant post-peak strength reductions. The monitoring results of the thickness change indicate that the degree of volumetric contraction is related to the reorientation of fibers and dissipation of pore water pressure. Furthermore, the peak and residual shear strengths both depend on the displacement rate because of the rate-dependent tensile stiffness of needle-punched fibers and shear strength of the soil/geosynthetic interface. Through additional tests and lateral comparison, it was discovered that the shear behavior of sodium bentonite, degree of hydration, and pore water pressures all affect the shear mechanisms of the NP GCL. In particular, the failure mode transfers from fiber pullout to fiber rupture with the increase in water content as the hydrated bentonite particles facilitate the stretching of needle-punched fibers.     

Full scale investigation of GCL damage mechanisms in small earth dam retrofit applications under earthquake loading

This paper reports results of full scale testing to further explore potential GCL damage mechanisms in earth dam retrofit applications in seismically active areas; in particular, to a) investigate whether shear displacements could reduce the magnitude of GCL panel overlap during earthquake shaking; b) explore the influence of gravel particles on GCL thickness at localised point of contact; and c) observe the consequences of an accidental exposure of an uncovered GCL to short duration rainfall in terms of moisture content and effects during subsequent compaction. The results of these experiments indicate that even under severe shaking no movements were detected at the GCL panel overlap. Whereas gravel particles were observed to locally reduce the thickness of the GCL to 2.2 mm, no plowing of the particle into the GCL occurred due to a lack of shear displacement at the interface, resulting in no localised internal erosion through the barrier. Furthermore, hydration of GCL panels during construction due to surface wetting was observed to result in a state of hydration less than its post-construction state. These results indicate that although each of the three GCL damage mechanisms cannot be ruled out to ever be relevant in practice, the performance of the GCL retrofitted earth dam tested was satisfactory under even severe Level 2 earthquake shaking, and suggests that the retrofitting of small earth dams with GCLs is a promising strategy to improve their static and seismic resistance.     

水泥混凝土用机制砂的级配探讨与试验

概括了国内外不同国家和地区的标准对机制砂级配、石粉含量限值和细度模数规定的差异。在对比国内外机制砂标准的差异及分析我国不同地区生产的69种机制砂颗粒级配、石粉含量和细度模数数据的基础上,提出了机制砂生产控制的颗粒级配、石粉含量和细度模数的I、II区级配范围。并通过配制不同强度等级的混凝土拌合物工作性试验,确定了一个机制砂全级配标准,同时对配制不同强度混凝土用的机制砂石粉含量和细度模数的要求进行了规定。     

Influence of particle shape on the shear strength and dilation of sand-woven geotextile interfaces

The influence of particle shape on the mechanical behavior of sand-woven geotextile interfaces over a wide domain of soil density and normal stress is studied. A uniformly graded angular fine sand, and a blend of well rounded glass beads with identical particle size distributions, were selected as granular material. Experiments revealed the impact of particle shape on peak and residual friction angles as well as the maximum dilation angle of interfaces between both granular media and woven geotextile. It was observed that the residual friction angles of interfaces between angular sand/glass-beads and woven geotextile are very similar to the residual friction angles of angular sand and glass-beads in soil–soil direct shear test. It is understood that the peak friction angle and maximum dilation angle of angular sand-woven geotextile were slightly lower than corresponding values for angular sand in soil–soil direct shear test. While the peak friction angle and maximum dilation angle of angular sand-woven geotextile interface decrease with the increase in normal stress, experiments showed that these factors are insensitive to normal stress for glass beads-woven geotextile interfaces, at least for the range studied herein. All interfaces with woven geotextile as the contact surface exhibit an abrupt loss of shear strength in the post-peak regime of behavior. Finally, a unified stress-dilation law for the angular sand-woven geotextile, glass beads-woven geotextile, and angular sand-roughened steel interfaces is obtained.     

水化状态对含针刺GCL复合衬里抗剪强度的影响分析

针刺GCL和HDPE土工膜(GM)在防渗工程中应用广泛,含多层界面的复合衬里整体抗剪强度是边坡稳定性分析的关键。介绍了含针刺GCL复合衬里的大单剪试验方法,并且对比分析了针刺GCL初始状态分别为干燥和完全水化两种情况下的复合衬里抗剪强度。结果表明,复合衬里的剪切破坏不会发生在干燥针刺GCL内部界面,而GCL干燥状态下的复合衬里单剪强度未必高于GCL完全水化状态下的复合衬里单剪强度。结合含GCL复合衬里的剪切破坏机理,阐述了针刺GCL的水化状态对复合衬里抗剪强度的影响。含GCL复合衬里在不同水化状态下的界面滑移稳定性都应引起工程人员的重视。     

颗粒形状对砂土抗剪强度及桩端阻力影响机制试验研究

 为研究颗粒形状对砂土的力学及变形性质的影响,以3种不同颗粒形状的均粒砂及相似材料“玻璃球”为研究对象,通过光学显微镜获取颗粒数字图像,借助图形处理系统获取颗粒单元体几何参数,定义构建2种不同层次的形状描述指标(球形度、磨圆度),并通过计算机辅助程序完成颗粒形状参数的量化。通过4种颗粒材料直剪试验,获得临界摩擦角、剪胀角及峰值摩擦角等强度指标,并分析其随颗粒形状参数的变化,通过饱和试样在不同密实度、不同离心加速度下的静力触探测试来模拟不同砂土地基上的中等圆形闭口桩静压贯入过程,并探讨桩端阻力随颗粒形状的变化关系。直剪试验结果表明：在矿物组分相同的情况下,砂土临界摩擦角随颗粒的磨圆度和规则性的增加呈线性减小,剪胀性随颗粒的不规则性的增大而增大,并可通过修正的Bolton公式来量化表述形状系数对于峰值摩擦角的作用。离心试验结果表明：静力触探锥端阻力可直接用于桩端阻力(qb)的估算,具有很好的可信度,砂颗粒形状越偏离标准球状,表面棱角度越突出,桩贯入所受阻力越大,桩端承载力越高。通过理论分析,提出一种迭代计算方法,并量化分析桩端阻力(qb)、相对密度(Dr)、应力水平(?v′)和颗粒形状(?)四者的相互作用关系,该方法计算结果与试验实测数据具有良好的吻合度。     

Brief note on the influence of shape and percentage of gravel on the shear strength of sand and gravel mixtures

The paper records the influence of the shape and the percentage of gravel on the shear strength/frictional angle of sand and gravel mixtures using direct shear tests. The shear strength is mainly derived from the frictional forces developed due to sliding and interlock; they depend on the maximum particle size and shape, the uniformity coefficient, density and the effective normal stress. As the size of material in a mixture is variable, the shear strength also depends upon the ratio of the specimen diameter to the maximum particle size. In this study, two different shapes of limestone were used, angular and rounded, and the maximum gravel size was 6.3 mm in diameter. Air-dried samples were used in the tests. It is concluded that the shape and percentage of gravel have an important influence on the shear strength properties. Electronic Publication     

砂土颗粒级配对筋土界面抗剪特性的影响

为了研究砂土与土工合成材料相互作用时筋土界面的抗剪强度以及剪胀特性,采用3种不同级配的砂土分别与土工格栅和土工织物进行室内大型直剪试验,研究不同颗粒级配、密实度、筋材种类以及竖向应力对界面剪切特性的影响,并对界面剪胀系数进行分析。试验结果表明：粗砂和细砂与筋材的界面剪切强度要明显大于粗细混合砂;松砂剪切过程中只有剪缩效应的存在,但密实砂土呈现出明显的剪胀过程;当竖向应力较大时,筋土界面达到峰值剪切强度所需的剪切位移比低应力时大;粗砂与土工格栅作用时达到峰值剪切强度所需的剪切位移比与土工织物作用时大,而细砂则相反。     

Failure mechanisms of geosynthetic clay liner and textured geomembrane composite systems

The objective of this study was to evaluate shear behavior and failure mechanisms of composite systems comprised of a geosynthetic clay liner (GCL) and textured geomembrane (GMX). Internal and interface direct shear tests were performed at normal stresses ranging from 100 kPa to 2000 kPa on eight different GCL/GMX composite systems. These composite systems were selected to assess the effects of (i) GCL peel strength, (ii) geotextile type, (iii) geotextile mass per area, and (iv) GMX spike density. Three failure modes were observed for the composite systems: complete interface failure, partial interface/internal failure, and complete internal failure. Increasing normal stress transitioned the failure mode from complete interface to partial interface/internal to complete internal failure. The peak critical shear strength of GCL/GMX composite systems increased with an increase in GMX spike density. However, the effect of geotextile type and mass per area more profoundly influenced peak critical shear strength at normal stress > 500 kPa, whereby an increase in geotextile mass per area enhanced interlocking between a non-woven geotextile and GMX. Peel strength of a GCL only influenced the GCL/GMX critical shear strength when the failure mode was complete internal failure.     

水化针刺GCL+GM复合衬里的单剪破坏特征

针刺GCL和HDPE土工膜(GM)广泛应用于填埋场防渗衬里,GCL的内部剪切强度和GCL/GM界面剪切强度是填埋场复合衬里边坡滑移稳定性的控制因素。通过开展不限定剪切破坏面的水化针刺GCL+GM复合衬里大单剪试验,获得了剪切过程中GCL/GM界面位移和GCL内部位移发展规律,分析了GM的糙面分别与GCL的有纺面和无纺面接触时的峰值强度,揭示了GCL+GM复合衬里的整体剪切破坏特征。试验结果表明:大单剪试验能够正确和合理地模拟GCL与GM间的相互作用,GCL+GM复合衬里中的极限破坏面不仅会随着法向应力的增加而发生转移,甚至出现GCL内部和GCL/GM界面同时成为剪切破坏面的临界状态。