Effect of geotextile ageing and geomembrane surface roughness on the geomembrane-geotextile interfaces for heap leaching applications

A series of large scale direct shear experiments is used to investigate the effect of the geomembrane (GMB) surface roughness, geotextile (GTX) properties, and GTX ageing, on the GMB-GTX interface shear behaviour. Interfaces involving smooth, coextruded textured, and structured surface GMBs underlying four different nonwoven needle-punched staple fibres (GTXs) with mass per unit areas between 200 and 2400 g/m², and a geocomposite drain (GCD) are examined at normal stresses between 250 and 1000 kPa. The results showed that the interlocking between the GMB and GTX increased with increasing the GMB asperity height and/or decreasing the mass per unit area of the GTX. For the interfaces that involved GTXs preaged prior to the shear box experiments for up to 2 years at 85 °C, it was found that the 2400 g/m² heat bonded two-layered GTX exhibited internal shear failure at low shear displacements. However, all the highly aged single layered GTXs showed an increase in the peak interface friction angles with the increase in their ageing. For these single layered GTX, the results suggest that assessing the interface friction angles using unaged GTXs for the stability analysis is conservative as long as the GTX remains intact in the field.     

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.     

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.     

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.     

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.     

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。     

UHPC键齿湿接缝直剪试验及湿接缝直剪承载力统一公式

为得到UHPC键齿湿接缝的直剪受力性能及直剪承载力统一公式,开展17个UHPC试件的直剪推出试验,考察键齿形状、侧向应力等对UHPC键齿湿接缝直剪破坏模式、裂纹发展情况、直剪强度以及剪切滑移性能的影响,并结合前期24个UHPC整体浇筑试件和24个UHPC平(湿)接缝试件的试验结果对UHPC界面直剪承载力计算方法进行研究。结果表明：UHPC键齿接缝试件的剪切破坏过程可分为线弹性阶段、裂纹发展阶段、破坏阶段和残余应力阶段;各键齿接缝直剪破坏界面与剪切界面几乎重合;梯形(键齿夹角113°)、矩形、倒梯形(键齿夹角65°)等三种键齿形式的直剪性能基本相同,为方便施工,工程中键齿宜选用梯形;键齿接缝试件的初裂强度、峰值强度提升值(相对于平接缝)与整体试件相应强度提升值之比远小于两者剪切面的整体面积之比;无侧向应力下,UHPC键齿接缝试件的剪切刚度和峰值强度均大于平接缝试件,逊于整体试件,且差距较大;键齿接缝试件峰值强度为整体试件的52.5%;在5MPa侧向应力下,平接缝试件与各键齿接缝试件剪切刚度已差别不大,峰值强度差值变小;键齿接缝试件的峰裂比(峰值强度与初裂强度之比)为1.173～1.319;键齿接缝试件的剪切刚度、延性系数、初裂强度、峰值强度及剪切滑移能力均随着侧向应力的增大而增大;UHPC湿接缝试件初裂强度和峰值强度随侧向应力的增长规律符合库伦准则,其摩擦系数可取1.26。最后,建立物理意义明确、形式简单、精度较高的可适用于未配抗剪钢筋UHPC整体界面、平湿接缝界面和键齿湿接缝界面的直剪承载力计算统一公式。     

非水反应高聚物与土工材料的界面剪切特性

基于单调直剪试验,研究了竖向应力、剪切速率对非水反应类高聚物-土工布界面及高聚物-砂土界面的剪切应力、剪切位移、抗剪强度和剪切模量等剪切特性的影响.结果表明:在给定竖向应力和剪切速率下,随着剪切位移的增加,高聚物-土工布界面、高聚物-砂土界面均表现出剪切软化的特性;竖向应力对高聚物-土工布界面抗剪强度及剪切模量的影响显著,在剪切速率v=2mm/min下,随着竖向应力由50kPa增加至150kPa,高聚物-土工布界面的抗剪强度由15kPa增至46kPa;在给定竖向应力下,剪切速率v=1~3mm/min时,剪切速率对高聚物-土工布界面的抗剪强度以及高聚物-砂土界面的抗剪强度和剪切模量的影响均较小.     

Behavior of compacted clay-concrete interface

Tests of interface between compacted clay and concrete were conducted systematically using interface simple shear test apparatus. The samples, having same dry density with different water content ratio, were prepared. Two types of concrete with different surface roughness, i. e., relatively smooth and relatively rough surface roughness, were also prepared. The main objectives of this paper are to show the effect of water content, normal stress and rough surface on the shear stress-shear displacement relationship of clay-concrete interface. The following were concluded in this study: 1) the interface shear sliding dominates the interface shear displacement behavior for both cases of relatively rough and smooth concrete surface except when the clay water content is greater than 16% for the case of rough concrete surface where the shear failure occurs in the body of the clay sample; 2) the results of interface shear strength obtained by direct shear test were different from that of simple shear test for the case of rough concrete surface; 3) two types of interface failure mechanism may change each other with different water content ratio; 4) the interface shear strength increases with increasing water content ratio especially for the case of clay-rough concrete surface interface.     

Evaluation of interface shear strength of composite liner system and stability analysis for a landfill lining system in Thailand

The paper presents the case history of laboratory evaluation of the interface shear strength properties of various interfaces encountered in a modern day landfill with emphasis on proper simulation of field conditions and subsequent use of these results in the stability analyses of liner system. Over 70 large direct shear tests were systematically conducted to evaluate the interface shear strength properties of composite liner system using project specific materials under site specific conditions, being used at non-hazardous and hazardous landfills project situated in Sa Kaeo Province, Thailand. The critical interfaces were located between the geotextiles and the smooth geomembrane (GM), the smooth GM and the geosynthetic clay liner (GCL), and the smooth GM liner and the compacted clay liner (CCL) with the interface friction angles ranging from 6.5° to 10.5° for dry conditions and 6.5° to 9.5° in wet conditions. The residual shear stress for these interfaces was attained at a displacement less than 4 mm. Three methods, namely, limit equilibrium method (LEM), limit method (LM), and the simple composite column (SCC) approach were used to evaluate the tensile loads induced in the geosynthetic components. The SCC approach proposed by Liu, C.N. [2001. Tension of geosynthetics material regarding soils on landfill liner slopes. Proceedings, National Science Council ROC(A), 25(4), 211–218] that takes into account the force equilibrium as well as displacement compatibility yielded satisfactory results. The factor of safety for geosynthetic components in the liner was found to be greater than 3.0 for both types of landfill.     

Analysis of cyclic shear characteristics of reinforced soil interfaces under cyclic loading and unloading

The cyclic properties of geosynthetic soil interface are crucial for reinforced soil structures subject to seismic loading. A series of cyclic direct tests under cyclic normal loading was conducted on geogrid-gravel interface. The relationship among the amplitudes of cyclic normal loading and shear displacement and frequencies in the horizontal and vertical directions with interface shear strength and volume change was investigated. Test results showed that the relative time shift, shear stiffness, and enhance coefficient increased with increasing amplitude of cyclic normal loading. The interface exhibited shear hardening and softening with increasing amplitude of shear displacement. The vertical displacement decreased with increasing amplitude of cyclic normal loading but increased with increasing amplitude of shear displacement. Furthermore, three patterns were analysed for different frequencies in two loading directions. The value of vertical displacement was largest when the normal loading impact frequency was larger than the cyclic horizontal shear frequency, and smallest at equal frequencies in two loading directions. The shear stiffness was positively correlated with the amplitude of cyclic normal loading. However, it was negatively correlated with the amplitude of shear displacement. The value of the damping ratio was smallest under constant normal loading at a shear displacement amplitude of 0.5 mm.     

两种介质接触面剪切力学性能的试验研究

采用直剪试验研究一体两介质和两体两介质接触界面力学性能的影响,以砂浆和混凝土构造宽度方向一致不同形状因子λ的不规则接触面,用以模拟工程体和地质体的相互作用。根据直剪试验,分析了一体两介质试件和两体两介质试件的破坏状态及应力–位移曲线,得到结论:界面黏结力和正应力对两介质接触面的力学性能有很大影响;形状因子λ在中低等法向力下对于两体两介质接触面的剪切力学性能有着重要的影响,在高法向应力下接触面的剪切力学性能则由弱介质的强度等因素决定;由于破坏的状态和受力的方式不同,在中低等法向力下,一体两介质的峰值抗剪强度大于两体两介质的峰值抗剪强度;在高法向应力下,恰恰相反。     

基于直剪试验的金坛盐岩力学特性研究

 采用RMT–150C试验机对江苏金坛拟建储气库埋深段盐岩试样进行一系列直剪试验研究。将盐岩作为多晶聚合体考虑,深入分析试验过程中的剪切应力、剪切变形、剪胀以及破坏特性。试验结果表明：盐岩剪切破坏是一种延性破坏,盐岩剪切峰值对应的剪切位移一般为4～6 mm,个别试样可达7～8 mm,且整个剪切应力–剪切位移曲线都较平缓,剪切峰值不明显,这主要是由于局部位错交替导致的;得到盐岩抗剪强度参数c,? 值分别为3.16 MPa,44.7°;发现残余强度基本上与法向应力呈正比例关系,且残余应力较大,约为峰值应力的50%～80%,表明盐岩摩擦承载能力较强;盐岩剪胀终止发生在峰值应力之后、残余应力之前,且在较大法向应力作用下剪胀起始应力与剪胀终止应力接近;盐岩的剪切破坏位置不是一个面,而是一个破碎带,破碎带上下一定范围内有不同程度损伤;表面局部有明显擦痕,类似于摩擦学的“犁沟效应”,有利于提高其抗剪能力。试验研究成果对深入理解金坛盐岩破坏机制及地下盐岩储气库稳定性研究具有一定的参考价值。     

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。     

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 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.     

裂隙岩体的一体两介质模型抗剪性能研究

 裂隙薄弱层与岩石接触面的抗剪强度是影响裂隙岩体稳定的重要参量。采用一体两介质力学模型研究裂隙岩体接触面的力学性能。按照著名的Barton试验所采用的粗糙界面,以砂浆和混凝土构造5种JRC曲线的接触面,并采用自创的双指标描述系统--考虑粗糙度的改进分维值指标Rd和考虑起伏度的形状因子? 对接触面形貌进行定量描述。Rd-? 描述系统的优点在于：不仅能描述粗糙程度,还能解释同一粗糙界面上当受力方向不同时,其界面抗剪力学性能显著不同的事实。通过直剪试验,分析岩体接触面的破坏状态及剪切应力–剪切位移曲线,确定影响抗剪强度的主要因素为接触面粗糙起伏形貌、法向力、界面材料的黏结强度等,并借助试验结果,拟合得到考虑上述影响因素,以Rd-? 描述系统表达的剪切强度公式。将提出的剪切强度公式与其他试验获得的剪切试验结果做对比分析,证实该剪切强度公式的正确性。     

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。     

剪切速率对黏性土混凝土界面抗剪强度影响的试验研究

通过自行研制的大型恒刚度桩土界面直剪仪,进行6种剪切速率的黏性土混凝土界面剪切试验,探讨剪切速率对黏性土混凝土界面抗剪强度的影响规律。结果表明:在黏性土混凝土界面,超孔隙水压力随着剪切速率的提高而增大;法向应力和剪切速率通过影响超孔隙水压力大小,决定黏性土混凝土界面剪切峰值强度和剪切破坏位移的大小;剪应力剪切位移关系曲线由基本一致变化到一定范围内产生偏离,且法向应力和剪切速率越大偏离越显著,并出现明显的应变软化现象;剪切速率从0.4mm/min增加至5.0mm/min,黏性土混凝土界面抗剪强度减小幅度增大,摩擦系数减小0.1,有效黏着力的变化介于0.81～5.93kPa之间。