Dynamic shear behaviors of textured geomembrane/nonwoven geotextile interface under cyclic loading

Textured geomembrane (GMB) and nonwoven geotextile (GTX) are usually used together in liner systems of MSW landfills, but the low shear strength of GMB/GTX interface is extremely detrimental to the stability of landfills, especially under earthquake loading. To study the dynamic shear strength of the GMB/GTX interface, a series of displacement-controlled cyclic direct shear tests are conducted with a large-scale direct shear machine. Normal stress levels ranging from 100 to 1000 kPa and displacement amplitudes ranging from 5 to 25 mm are considered. To compare the failure mechanism, GMB and GTX specimens are tested in not only hydrated but also dry conditions. Different waveforms and excitation frequencies are also applied to analyze the effects of test conditions. It can be seen that the shear deformation develops totally along the GMB/GTX interface when specimens are fully hydrated, while the internal failure of GTX is induced in dry condition. Equivalent linear analyses reveal that the shear stiffness depends on normal stress and displacement amplitude, while the damping ratio is only affected by displacement amplitude. Variations of shear strength during the shear process indicate that the softening behavior of the GMB/GTX interface is closely related to cumulative displacement and normal stress level. Furthermore, based on test results, a positive correlation is summarized between the shear strength and displacement rate of the interface.     

Dynamic shear behavior of GMB/CCL interface under cyclic loading

The dynamic shear behavior of composite liner interface is of great importance for landfill seismic analysis. In this study, an experimental investigation of the shear behavior of the interface between smooth high density polyethylene (HDPE) geomembrane (GMB) and compacted clay liner (CCL) is presented. A series of displacement-controlled cyclic shear tests were conducted to investigate the effects of displacement amplitudes, normal stress levels and number of cycles on the GMB/CCL interface shear behavior. Cyclic loading with higher displacement amplitude will produce greater vertical contraction and lower interface initial shear stiffness. Also, significant shear strength degradation was observed within the first 5 shearing cycles, then followed by slight interface reinforcement in subsequent cycles. The dynamic shear modulus of GMB/CCL interface is dependent on both normal stress levels and displacement amplitudes, while the damping ratio is only affected by displacement amplitudes. Finally, a method considering the GMB/CCL composite liner as an equivalent soil layer was proposed, which is useful for landfill seismic analysis.     

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.     

Performance of multicomponent GCLs in high salinity impoundment applications

The interface transmissivity (θ) of two multicomponent geosynthetic clay liners (GCLs) is investigated upon hydration and permeation with a highly saline solution (TDS ≈ 260,000 mg/l; Na⁺ ~ 95,000 mg/l; K⁺~12,000 mg/l) at two stress levels (10 kPa and 150 kPa). One GCL had a smooth 0.2 mm-thick coating whereas the second GCL had a textured 1 mm-thick coating. For both GCLs, the interface transmissivity after 2-weeks is shown to be higher than at steady-state. The lower the geomembrane's (GMB) stiffness, the lower interface transmissivity. However, the effect is generally diminished at steady state and higher stress. The effect of GMB stiffness at 10 kPa is shown to be 1.6-times that at 150 kPa. Similarly, the 2-week and steady state interface transmissivity for the textured GMB was higher at 10 kPa than at 150 kPa. Coating texture and coating orientation are shown to have a significant effect on GMB/multicomponent GCL interface transmissivity. A hole in the coating aligned with GMB hole creates an additional flow path at the coating/GCL interface (θ_Geofilm/GCL), however most of the flow occurs at the coating/GMB interface (θ_Geofilm/GMB).     

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.     

Coupled effect of UV ageing and temperature on the diffusive transport of aqueous,vapour and gaseous phase organic contaminants through HDPE geomembrane

High-density polyethylene geomembranes (HDPE GMBs) are indispensable in constructing composite barriers of modern engineered landfills. GBMs would be exposed to various weathering conditions apart from the interaction of organic pollutants in landfill gas and leachate. Though many studies have investigated the contaminant's migration, the transport of volatile organic compounds (VOCs) concerning the synergistic effect of thermal and UV exposure has not been thoroughly explored. Given this, the present study investigated the diffusive transport of four VOCs (aqueous and vapour phase) at 10 °C, 25 °C, 40 °C, and methane diffusion through an HDPE GMB subjected to accelerated UV weathering. The results indicated that the contaminant phase (aqueous, vapour, gaseous), its properties (aqueous solubility, molar mass, polarity), temperature, and the extent of ageing significantly influenced the transport behaviour. The standard oxidative induction time decreased appreciably with ageing duration following a first-order exponential decay. The change in tensile properties and melt flow index indicated the dominance of UV degradation by cross-linking, resulting in an increased crystallinity of GMB that was identified using Fourier transforms infrared (FTIR) spectroscopy and Differential scanning calorimeter (DSC). The crystallinity increased with UV ageing and caused the sorption and diffusion coefficients to decrease.     

Interface shear strength properties of geogrid-reinforced steel slags using a large-scale direct shear testing apparatus

This research evaluates the shear strength properties of unreinforced and geogrid-reinforced ladle furnace slag (LFS), electric arc furnace slag (EAFS) and a blend comprising 50% LFS and 50% EAFS (LFS50+EAFS50) using the large direct shear testing apparatus (DST). The large DST results of unreinforced steel slags indicated that LFS had the lowest shear stress ratio at the peak shear strength among all samples, while LFS50+EAFS50 samples (both unreinforced and reinforced) demonstrated the highest shear stress ratio amongst the tested samples. A higher apparent cohesion value was achieved with the inclusion of biaxial geogrid in LFS and EAFS samples as compared to the triaxial geogrid interface. The observed behavior can be attributed to the larger aperture size of the biaxial geogrid compared to the triaxial geogrid leaving more void planar space for a direct interaction between slag particles. In contrast, the apparent cohesion of LFS50+EAFS50 without a geogrid interface was high and did not change significantly with the insertion of geogrid. Given, the range of internal friction angles for ordinary soils, studied slag by-products achieved internal friction angles in excess of 59° (with no geogrid interface) and these significant values proved highly beneficial application for these waste materials in pavement construction.     

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.     

Determination of geomembrane – protective geotextile friction angle: An insight into the shear rate effect

This study investigates how the shear rate can affect the geomembrane – protective geotextile friction angle. Four types of geomembranes (GMB) were considered (EPDM, HPDE, PP, and PVC) and a single nonwoven needle-punched geotextile (GTX_nw) was used to make the interfaces with the geomembrane. Three shear devices were used: a large-scale inclined plane (IP), a shear box (SB), and a small-scale shear device (ssSD). The ssSD allows two shear modes to be compared: one mode involves incrementally increasing the shear stress, and the other involves imposing a constant tangential velocity at the interface. Only the PP GMB- GTX_nw was tested with the SB and the ssSD. Inclined plane standardised tests show that for the three interfaces that undergoes gradual sliding (EPDM, PP and PVC GMB- GTX_nw), it is shown that a step-by-step experimental procedure gives significantly lower interface friction angle than that given by the procedure from the current international standard, which is explained by the increase of interface shear stress with sliding speed. These observations are confirmed by shear box tests. One major practical result is that, following the nature of geosynthetics, the shear rate applied in large-scale shear box tests should be adapted to assess a safety value of a geosynthetic - geosynthetic interface friction angle.     

Cyclic shear behavior of GMB/GCL composite liner

The composite liner system consisting of geomembrane (GMB) and geosynthetic clay liner (GCL) has been widely used in landfills. Although there have been a lot of studies on the monotonic shear behavior of GMB/GCL composite liner, the dynamic test data are still very limited and consequently, the dynamic shear mechanism is not clear. A series of displacement-controlled cyclic shear tests were conducted to study the shear behavior of GMB/GCL composite liner, including the shear stress versus horizontal displacement relationships, backbone curves, and shear strengths. Hysteretic loops in the shape of parallelogram were obtained and equivalent linear analyses revealed that the secant shear stiffness decreased and the damping ratio increased with the rise in loading cycles. According to the test results, it is generally acceptable to predict the dynamic peak strength of a GMB/GCL composite liner with its static strength envelope. Furthermore, the dynamic softening mechanism and rate-dependent shear stiffnesses were well described by the proposed equations, which also facilitate the accurate modeling of the cyclic shear behavior.     

Effect of prehydration,permeant, and desiccation on GCL/Geomembrane interface transmissivity

A laboratory investigation was conducted on two different conventional GCLs (one with fine granular and another one with powdered bentonite) to explore the effect of prehydration and permeant fluid; GCL desiccation on the interface transmissivity, θ, between the interfaces of a 1.5 mm-thick high-density polyethylene (HDPE) geomembrane (GMB) and a GCL. The study also aimed to assess the self-healing capacity of desiccated GCLs for three different permeant solutions under a range of applied stresses (10–150 kPa). It was found that at stresses less than 70 kPa, θ was dominated by variability in the initial contact condition between the GMB-GCL interfaces. The effect of other factors was largely masked by the contact variability. At 100–150 kPa, the effects of initial variability were largely eliminated, but there was no notable effect of other factors on θ in the absence of desiccation. GCL desiccation increased θ by up to three orders of magnitude than an intact specimen at 10–100 kPa. Even at 150 kPa, desiccated specimens had a θ ≤ 8.0 × 10⁻⁹ m²/s for all specimens tested. The chemical composition of the permeant solutions, crack width, and nature of bentonite could play an important role in healing the cracks of desiccated GCLs.     

Influence of soil inherent anisotropy on behavior of crushed sand-steel interfaces

This paper presents the results of an extensive series of direct shear tests covering a broad spectrum of bedding plane inclination angles with respect to the shearing plane to study the outcome of inherent anisotropy on the mechanical behavior of sand-steel interfaces. As a part of the research, it is shown that the peak friction and the maximum dilation angles of the tested inherently anisotropic sand are highly influenced by the bedding plane inclination angle. However, for smooth, intermediate, rough, and very rough sand-steel interfaces, the test results indicate that the variation in peak friction angle with the bedding plane inclination angle is meaningfully less than that for the same sand. Moreover, it is observed that the extrema of peak friction and maximum dilation angles in sand-steel interface tests are attained at bedding plane inclination angles that are significantly different from those obtained from direct shear tests on sand samples.     

Tragverhalten von geschichteten Deckenelementen aus Normal‐ und Porenleichtbeton

Load bearing behaviour of layered ceiling elements made of regular and porous lightweight concrete Lightweight and efficiently bearing steel reinforced elements may be achieved through the application of regular and porous concrete in a three‐layer cross‐section. While exterior layers of higher strength carry bending moments, a lightweight core layer material holds up to shear stresses. In order to quantify the potential of this construction method, the bearing behaviour of 18 layered ceiling elements with six different geometries was investigated. The goals were both to identify different failure modes, as well as evaluate the suitability of commonly used calculation procedures. The following paper shows that an optimal usage of cross‐sections of ceiling elements can already achieved by using concrete with strengths between 5 MN/m² and 20 MN/m². The efficiently bearing elements are characterized by the fact, that both the concrete, the reinforcing steel and the layer's interface are highly stressed both under pressure and tension. The tested specimens showed both a tensile bending and interface failure with a partly very high utilization of the flexural compressive zone.     

剪切路径对土与结构接触面三维循环特性影响研究

运用80t大型三维接触面试验机,通过改变往返圆弧剪切路径的半径和旋转角度幅值研究不同剪切路径下接触面三维循环力学特性,重点分析了剪切路径的影响规律。剪切路径由于影响两正交切向位移大小、方向转变及结构面附近土颗粒运动形态,从而导致接触面剪切体变及可逆性剪切体变-x或y向位移关系、切向应力位移关系及切向应力间关系形式差异显著。随旋转角度幅值的增大,接触面x向应力位移关系形式由双曲线向椭圆形转变;残余摩擦角由35°逐渐减小到33°,而后趋于稳定;峰值摩擦角则保持35°不变。不同剪切路径下接触面力学特性亦有诸多相同之处：抗剪强度具有各向同性;往返型路径下均产生明显的不可逆性和可逆性剪切体变,随循环剪切的进行,前者单调增长,后者峰值逐渐减小而后趋于稳定;不同剪切路径下接触面可逆性剪切体变-主切向位移关系及主剪应力-主切向位移关系具有良好的一致性;且后者曲线形式随循环剪切的进行由双曲线形式逐渐向理想弹塑性模式转变。     

粗粒土与混凝土接触面特性单剪试验研究

采用大型单剪仪进行粗粒土与混凝土接触面在膨润土以及混合土(膨润土中掺入水泥)泥皮条件下的剪切试验。通过对不同水泥浆含量的混合土泥皮接触面进行试验,揭示不同泥皮条件下接触面的力学特性。结果表明,与无泥皮或膨润土泥皮时不同,存在混合土泥皮时,剪应力与剪应变关系曲线存在明显软化段,峰值强度的位置与水泥含量以及法向应力大小有关。水泥含量越大其相应的强度越大,水泥含量由10%提高到40%时,其相应的内摩擦角提高约3.2倍,水泥含量为40%时,其强度达到无泥皮时的84%。剪切破坏时,在同一高度处,法向应力越大,切向位移也越大;同样的法向应力及高度处,切向位移随水泥含量的提高而增大。无泥皮、低法向应力下,试样出现明显的剪胀现象,而泥皮条件下试样均表现为剪缩。试样的有效高度对粗粒料的强度及变形有一定的影响,最大粒径为20mm时,高度分别为100与30mm的试样相比,其内摩擦角及水平位移偏差分别为3%和6%左右。与最大粒径为60mm试样相比,最大粒径20mm试样的内摩擦角要小1.9°,减幅4.8%。     

Role of soil inherent anisotropy in peak friction and maximum dilation angles of four sand-geosynthetic interfaces

Using a modified direct shear apparatus, an extensive experimental investigation is conducted into the influence of the inherent anisotropy of sand on the mobilization of the peak and critical state friction angles as well as the maximum dilation angle of the interfaces between an inherently anisotropic crushed sand and two woven geotextiles, one nonwoven geotextile, and one geomembrane. Experimental findings confirm that both peak and maximum dilation angles of sand-geosynthetic interfaces are affected from soil inherent anisotropy depending on the bedding plane inclination with respect to the shear plane. However, a unique critical state (residual) friction angle is attained for each interface type irrespective of the bedding plane inclination angle. Compiling results of a total of 141 tests, it is shown that a unique rule describes stress-dilation relationship of four different dense crushed sand-geosynthetic interfaces. The experimental data indicate that the ?_p vs. θ and ψ_max vs. θ curves are symmetrical with respect to θ?=?90° for the sand-woven geotextile and sand-geomembrane interfaces. Finally, it is shown that a constitutive equation by Pietruszczak and Mroz (2001) can predict the variation of ?_p with θ for the sand-woven geotextile and sand-geomembrane interfaces.     

粉土—混凝土界面与粉土剪切对比试验研究

采用自制的大型直剪仪开展不同含水量条件下黄泛区粉土-混凝土界面剪切及粉土直剪试验,研究不同含水量下粉土-混凝土界面及粉土剪切力学特性。试验结果表明:粉土-混凝土界面和粉土直剪的剪切应力-剪切位移曲线均为应变硬化型;粉土直剪的剪缩变形明显大于界面剪切结果;相同条件下,粉土-混凝土界面的剪切强度大于粉土的直剪强度,随着含水量的增大,粉土-混凝土界面与粉土剪切强度的差异逐渐减小,粉土趋近饱和时,两者强度基本相同;粉土直剪破坏时的剪切位移大于界面剪切破坏时的剪切位移;粉土直剪的黏聚力和界面黏聚力、摩擦角随含水量的增大均近似呈线性减小趋势,粉土直剪的内摩擦角随含水量增加呈现先缓慢后加速减小的变化趋势。     

Shear behavior of geocell-geofoam composite

In design, internal stability of EPS lightweight fills are provided either by applying load distributing mechanisms (thick pavements or concrete slabs) or using more strong lightweight material through denser EPS geofoam blocks. However, unit weight of the EPS geofoam is a limited parameter. As an attempt to improve the mechanical properties of EPS geofoam, geocell-geofoam composite (GGC) is introduced in this study. Geocell mattresses were infilled with solidified geofoam beads in the factory to fabricate GGC. The EPS geofoam and GGC samples were tested using a large-scale shear test apparatus of size measuring 1 m³. Results indicate that inclusion of the geocell leads to a considerable increase in the shear strength and a great decline in the compressibility of the geofoam. In comparison with EPS blocks, up to 72% rise in the shear strength and 67% decline in the vertical displacement were observed in GGC samples at the normal stress of 35 kPa. In addition, incorporation of the geocell was found to change the resisting mechanism of the EPS geofoam from cohesive to cohesive-frictional. While there was only 4.5% decline in the cohesion, the internal friction angle of the tested geofoam increased six-fold due to the involvement of the geocell.     

Analytical and experimental studies on composite slabs utilising palm oil clinker concrete

The utilisation of waste materials in the construction industry is an effective way to sanitise the environment and reduces the cost of construction. In this research, palm oil clinker (POC) aggregates was used to fully replace normal aggregates to produce structural lightweight concrete. This concrete was used in the construction of composite slabs with profiled steel sheet. A total of eight full scale composite slabs, six palm oil clinker concrete (POCC) slabs and two conventional concrete slabs were constructed and tested in accordance to Eurocode 4: Part 1.1 and BS 5950: Part 4: 1994. Two shear spans were used, 450 mm for short shear span and 900 mm for long shear span. The structural behaviour of the slabs was investigated and compared. The horizontal shear-bond strength between the concrete and the steel was determined according to two methods; m–k and partial shear connection methods. Test results show that the structural behaviour and the horizontal shear-bond strength of the POCC slabs are nearly similar to the conventional concrete slabs. The mechanical interlock (m) and the friction (k) between the steel and the concrete are 117.67 N/mm² and 0.0973 N/mm², respectively and the design horizontal shear-bond strength using m-k and PSC methods is 0.248 N/mm² and 0.215 N/mm², respectively. The difference between the two methods is 13.3%. POCC is therefore suitable to be used for structural applications with a reduction in weight of 18.3% compared to conventional concrete composite slabs.     

干燥及饱和橡胶砂压缩和剪切特性

将废弃轮胎作为建筑材料应用在土木工程领域是对其回收处理最有前景的方法之一。为了研究轮胎橡胶颗粒改良砂土的效果,选取橡胶颗粒与福建砂的混合土为研究对象,研究了橡胶砂的压缩和抗剪特性,分析了橡胶含量与干湿状态对橡胶砂力学特性的影响,并建立了双曲线模型预测橡胶砂受剪过程的剪应力与剪切位移关系。结果表明：橡胶砂的压缩变形随橡胶含量增大而增大,饱和橡胶砂的压缩变形明显大于干燥橡胶砂;橡胶砂的剪应力剪切位移曲线表现出应变硬化特征;橡胶砂的抗剪强度与橡胶含量的关系不大;干燥试样内摩擦角随着橡胶含量的增大而降低,饱和试样的内摩擦角随着橡胶含量的增大而轻微升高;通过双曲线模型预测的剪应力剪切位移关系曲线与试验结果吻合。