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。     

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。     

不同温度下水化针刺GCL+GM复合衬里的剪切特性

针刺GCL和糙面土工膜(GM)组成的复合防渗衬里应用广泛,其在用于处置矿物冶炼废物或固体废弃物等情况时可能面临变化的温度环境.针对复合衬里剪切强度的温度效应问题,利用大尺寸温控水浴直剪仪对水化针刺GCL+GM复合衬里开展了不同温度下不限定剪切破坏面的整体剪切试验.获得了复合衬里在10℃～70℃温度范围内的剪切强度,揭示...     

土工膜/GCL界面剪切强度特性的试验研究

土工复合膨润土垫(GCL)是一种新型复合材料,其良好的防渗隔气功能以及抗张拉的能力使其与HDPE土工膜(GM)的联合使用在填埋场具有广泛的应用前景,但GM/GCL界面较低的剪切强度易导致填埋场衬垫系统等失稳。利用大尺寸界面直剪仪进行三种不同土工膜与GCL界面的剪切试验,重点研究GCL不同加载水化顺序对膨润土挤出及界面强度的影响。试验结果表明:干燥状态下,粗糙土工膜/GCL界面的峰值强度较大于光滑土工膜/GCL界面的峰值强度,但粗糙土工膜/GCL界面表现出强烈的应变软化特性,其残余强度接近于光滑土工膜/GCL界面的强度;不同加载水化顺序是影响膨润土挤出的重要因素,并严重影响界面剪切强度;膨润土挤出造成粗糙土工膜/GCL界面的峰值摩擦角降低3.5°,大位移摩擦角降低7.6°,接近光滑土工膜/GCL界面的摩擦角。     

竖向应力作用下GCL的膨胀特性和渗透性能

近年来,土工织物膨润土垫(GCL)被越来越多地应用到各种防渗工程之中,它的防渗有效性也成为了设计人员和研究人员所关注的焦点。GCL的防渗有效性包括渗透性能、吸附能力和内部剪切强度三个方面。通过水化膨胀试验和渗透试验研究了GCL在竖向应力作用下的膨胀特性和渗透性能,并分析了正应力和加压水化顺序的影响。试验结果表明:(1)随着竖向应力的增大,GCL的膨胀量不断减小,而GCL的渗透系数则出现先减小后略有增大的规律;(2)水化加压顺序对GCL的膨胀量和渗透系数均有影响;(3)在实际工程应用中,GCL铺设完成后在堆载之前最好完全水化,这样能够大大提高GCL的防渗有效性。     

GCL/GM界面膨润土挤出机理研究

土工复合膨润土垫(GCL)具有良好的防渗隔气及抗张拉等性能,在填埋场中具有较好的应用前景。GCL/GM界面由于膨润土挤出而导致其剪切强度较小,将影响GCL在填埋场边坡中的使用。因此,采用改进的固结仪进行了膨润土挤出机理的研究,重点探讨了正应力、加载速率、加载与水化的顺序、土工织物类型以及渗滤液等因素对膨润土挤出的影响,并在实验结果的基础上提出了超孔隙水压力挤出机理:快速加载时,GCL内部产生了较大的超孔隙水压力,导致膨润土颗粒在该渗透力的作用下通过土工织物中的孔隙往GCL/GM界面的间隙迁移。     

纤维加筋膨润土在大剪切位移下的性状

通过正应力为20—400kPd下的系列环剪试验，研究了聚丙烯纤维加筋膨润土在大剪切位移下的性状。膨润土和聚丙烯纤维制成初始含水量170％的试样，纤维长12mm或24mm，厚0．023mm，在试样中干重含量为1．5％或3％。纤维在膨润土中随机分布，提高了试样的最大抗剪强度，但当变形继续增大时，加筋土的强度加剧变小，使得纤维加筋膨润土和素膨润土残余强度相似。纤维长度和含量的增加都会提高峰值剪切强度。试验结束后将纤维取出，发现纤维体出现拉长和破坏。     

基于单根纤维拉拔试验的波形纤维加筋土界面强度研究

纤维/土界面间的力学作用特性是决定纤维加筋土工程性质的关键因素。为了改善纤维/土界面作用力,开发了一种新型的波形纤维作为加筋材料,并自主设计了一套拉拔试验装置,对单根纤维加筋土开展了多组拉拔试验,定量获得了波形纤维加筋土的拉拔特性及界面剪切强度,通过与传统直线形纤维对比,分析了波形纤维/土界面的力学作用机理,并从理论上探讨了波形纤维的最大临界加筋长度。结果表明：提出的单根纤维拉拔试验方法及设计的试验装置为研究纤维/土界面力学作用提供了有效的途径,试验结果具有较好的可重复性;直线形纤维的拉拔曲线呈典型的单峰特征,拉力达到峰值后迅速减小到残余值并逐渐趋于稳定,而波形纤维的拉拔曲线呈显著的多峰特征,曲线波长与纤维的波长基本一致;通过对比,波形纤维/土界面剪切强度明显高于直线形纤维,强度值提高了178%,极大改善了纤维的加筋效果,此外,波形纤维拉拔曲线各峰值对应的界面剪切强度及残余剪切强度随拉拔位移呈指数递减趋势;利用测得的纤维/土界面剪切强度,结合纤维自身的抗拉强度和一些假设条件,能确定纤维的最大临界加筋长度,对实际工程设计有一定参考意义。     

加筋红砂岩风化土强度和变形特性

红砂岩风化土是湖南公路路堤工程中应用较多的填筑材料,采用直剪试验研究了不同压实度的红砂岩风化土的强度和变形特性,以及加筋对其工程性质的影响。试验表明,随压实度增大,红砂岩风化土的峰值抗剪强度明显提高,但主要由粘聚力的增大引起,随剪切位移增大,粘聚力减小,抗剪强度大幅度降低,其应力–应变曲线呈现随应变软化型。加筋提高了红砂岩风化土的峰值抗剪强度和残余强度,更重要的是明显减小了峰值后强度的降低幅度,且达到峰值抗剪强度的剪切位移增大,峰值区域增宽,土体延性提高,改善了红砂岩风化土的强度和变形特性;对于不同的加筋层数和不同的筋材模量,以及在不同的压实度和试验竖向压力下,加筋对红砂岩风化土的强度和变形特性的改变不同;根据试验结果,还对红砂岩风化土的工程性质以及加筋的抗剪作用机理进行了初步探讨,阐述了加筋材料在土的应力–应变关系中的主要功能和作用。     

考虑渗透吸力影响膨润土的修正有效应力及其验证

普遍认为膨润土是高放废物地质处置库缓冲/回填层的理想材料,膨润土在地下水溶液中的膨胀变形和剪切强度关乎高放废物地质处置库的安危。地下水溶液中膨润土膨胀变形和剪切强度受到渗透吸力影响,根据膨润土表面分形模型,建立考虑渗透吸力应力影响的修正有效应力公式,用竖向应力p和渗透吸力π表示为■,并采用膨润土在NaCl溶液中的膨胀变形和剪切强度进行验证。NaCl溶液中膨润土的膨胀变形与修正有效应力p~e表示为同一曲线■,峰值剪切强度与修正有效应力p~e表示为同一直线■,符合Mohr–Coulomb准则。     

Stress-controlled direct shear testing of geosynthetic clay liners II: Assessment of shear behavior

This paper is the second of a two-paper set on stress-controlled direct shear testing of geosynthetic clay liners (GCLs). Design of the apparatus, preliminary experiments, and shear deformation mechanisms in heat-treated and non-heat treated needle-punched (NP) GCLs were discussed in Part I. The objective of Part II (this paper) was to evaluate the effects of physical factors (i.e., peel strength and initial normal stress, σ_ni), environmental factors (i.e., temperature and hydration solution), and creep on the internal shear behavior of NP GCLs. In addition, failure conditions of GCLs in the stress-controlled direct shear tests were compared to displacement-controlled direct shear tests to verify results. An increase in internal shear strength developed from increased GCL peel strength or increased normal stress. Elevated temperatures were observed to decrease internal shear strength for both non-heat treated and heat-treated NP GCLs. Specimens hydrated with a calcium-rich synthetic mining solution experienced increased internal shear strength due to cation exchange in the bentonite, whereas specimens hydrated with a highly alkaline synthetic mining solution experienced decreased internal shear strength. Creep tests revealed an increase in time-to-failure with decrease in applied shear stress. Finally, stress states at failure from stress-controlled and displacement-controlled shear tests corresponded to a unique failure envelope, which validates the efficacy of using stress-controlled direct shear tests to assess internal shear behavior and shear strength of NP GCLs.     

Stress-controlled direct shear testing of geosynthetic clay liners I: Apparatus development

The use of geosynthetic clay liners (GCLs) in waste containment applications can induce long-term normal and shear stresses as well as expose GCLs to elevated temperatures and non-standard hydration solutions. Considering the importance of GCL internal shear strength to the design and integrity of waste containment barrier systems, innovative laboratory testing methods are needed to assess shear behavior of GCLs. There were two main objectives of this study: (i) develop a stress-controlled direct shear apparatus capable of testing GCLs exposed to elevated temperatures and hydrated in non-standard solutions; and (ii) assess internal shear behavior of GCLs under varying experimental conditions (e.g., stress, temperature, solution). These two objectives were partitioned into a two-paper set, whereby Part I (this paper) focuses on the shear box design and Part II focuses on an assessment of shear behavior. The direct shear apparatus includes a reaction frame to mitigate specimen rotation that develops from an internal moment within needle-punched reinforced GCLs. Rapid-loading shear tests were conducted to assess functionality of the apparatus and document baseline shear behavior for a heat-treated and a non-heat treated needle-punched GCL with comparable peel strength. These two GCLs failed at comparable applied shear stress; however, the heat-treated GCL yielded lower shear deformation and failure occurred via rupture of reinforcement fiber anchors, whereas the non-heat treated GCL yielded larger shear deformation and failure via pullout of reinforcement fibers.     

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.     

Interface shear strength between geosynthetic clay liner and covering soil on the embankment of an irrigation pond and stability evaluation of its widened sections

Geosynthetic clay liners (GCLs) are typically used for widening sections of an embankment. They are also used as low permeability liners to minimize water leakage from reservoirs such as irrigation ponds. However, few investigations have been carried out on the specific properties of GCLs, such as granulated bentonite sandwiched between geotextiles, their internal shear strength, and the shear strength at the interface between a GCL and an embankment body. In this study, a series of direct box shear tests were performed to determine the shear strength properties of bentonite and compacted soils as well as at the interface between a GCL and bentonite or compacted soil. In addition, a series of field-loading tests were conducted to investigate the failure behaviour of an embankment body containing a GCL when changes in the water content of the bentonite of the GCL in a real embankment occur. Furthermore, the stability of widened embankment bodies that incorporated GCLs were evaluated. The main conclusions of this study are as follows: (1) The shear strength of the interface between the covering soil and geotextiles varied according to the soil type, geotextile type, and the submergence period, (2) the maximum safety factor was observed at the interface between decomposed granite soil and the geotextiles, while the minimum safety factor was observed at the interface between the bentonite and the geotextiles, and (3) the influence of GCLs on the instability of a widened embankment was extremely small.     

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.     

Factors affecting the down-slope erosion of bentonite in a GCL

Leaving a composite liner exposed for an extended period can sometimes lead to down-slope bentonite erosion from geosynthetic clay liners (GCLs). This laboratory study examines a number of factors that can affect the erosion of bentonite particles with an imposed flow of water for one particular geotextile-encased, needle-punched GCL. The factors examined include the effect of an initial wet/dry cycle, water chemistry, flow rate, slope, prior cation exchange, and the effect of no-drying phase in the test cycle. No erosion was observed unless the GCL had been hydrated and dried to create a wet/dry cycle. The most critical factor was found to be the water chemistry. No erosion was observed with tap water (39 ppm calcium) with up to 360 cycles and a flow of 3 L/hour. Tests simulating the evaporation and condensation of water below an exposed composite liner by imposing deionized water on the GCL surface developed erosion holes within 5–6 cycles.     

根系作用下的基材土–岩接触面原位剪切试验研究

采用自制原位剪切仪,对不同粗糙度、不同多花木蓝根系密度的基材土–岩接触面进行原位剪切试验,研究了基材土–岩接触面的剪切变形特征。研究表明,基材土–岩接触面剪切应力–位移具有典型的软化特性;受根系加筋、锚固作用影响,含根试样的剪切破坏面粗糙、破碎。根系作用对土–岩接触面峰值剪切强度、峰值剪切位移及残余剪切强度影响明显,对比发现,不同粗糙度下,相同根系面积比对提高界面峰值剪切强度的贡献率差异较小;根系面积比较小时,增强界面峰值剪切强度的贡献率低于粗糙度,随着根系面积比增大,根系提高界面强度的作用更为明显,其贡献率显著增加。