钢-粉质黏土界面剪切特性及本构模型研究

在结构物与土体相互作用的研究中,分析界面剪切特性并建立合理的本构关系具有重要意义。利用改进的直剪设备,对粉质黏土与光滑或粗糙界面钢之间的剪切性能进行试验,分析剪切应力-剪切位移关系和界面抗剪强度指标,依据Gompertz曲线模型和Clough-Duncan双曲线模型对试验结果进行拟合,结果表明:剪切应力随剪切位移增大而增加,达到峰值剪切应力后基本不变,未出现弱化现象;峰值剪切应力和峰值剪切位移均随法向应力和粗糙度的增加而增大;粗糙界面的黏聚力和摩擦角较光滑界面的更大;在法向应力较低的情况下,按Clough-Duncan双曲线模型和Gompertz曲线模型拟合效果均较好,在法向应力较高的情况下,Gompertz曲线模型拟合结果更合理。     

高应力作用界面剪切性质的试验研究

通过在DRS-1型微机高压直残剪试验系统上所进行的不同土质与不同基底的界面剪切特性试验研究表明：高应力作用下不同结构接触面的峰值强度、残余强度与正应力之间符合库伦强度准则：基底性质对标准砂的抗剪峰值强度准则影响较小，但对其抗剪残余强度准则影响较大：混凝土界面下残余抗剪强度准则的选择与法向应力的大小有关，法向应力较低时剪切破坏发生在砂土中，法向应力较高时剪切破坏发生在界面上，而对于其他基底的剪切破坏始终发生在界面上：标准砂在剪切过程中的体积应变-剪切位移关系，呈现出应变硬化现象整体符合双曲线模型，表现为剪缩性。试验数据的直观分析和方差分析表明：对于残余强度和初始剪切刚度，法向应力是第一影响因素，其次是土体的性质，第三是界面的基底性质，剪切速率的影响最小。     

高应力状态砂土界面抗剪强度的试验研究

通过在DRS-1型微机高压直残剪试验系统上所进行的直接剪切试验研究表明高应力作用下砂-砂、混凝土-砂的抗剪峰值强度与正应力、残余强度与正应力之间的关系,符合库伦强度准则(高应力作用下混凝土-砂的剪切特性呈现出软化现象,混凝土-砂的剪切特性呈现出硬化现象.基底对砂土的抗剪峰值强度准则影响较小,但对抗剪残余强度准则影响较大,界面残余抗剪强度准则的选择与法向应力的大小有关,法向应力较低时剪切破坏发生在砂土中,法向应力较高时剪切破坏发生在界面上).     

剪切速率和法向加载频率对筋土界面剪切特性的影响

加筋土结构在道路工程中广泛应用,而车辆等动荷载对筋-土界面相互作用特性的影响不可忽略。采用动态直剪仪开展了一系列法向循环荷载作用下的砾石-土工格栅界面直剪试验。试验研究了4种剪切速率(0.1,1,5,10mm/min)、4种法向加载频率(0.1,0.5,1,2 Hz)以及3种法向初始应力(20,40,60 kPa)对筋土界面剪切特性的影响。试验结果表明:法向循环荷载下的剪切应力及法向位移呈周期性动态变化;上峰值应力、上残余应力及剪切应力幅值随频率的增大而减小,随剪切速率的增大而增大,而下剪切应力受影响较小;剪切应力和法向应力的峰值相对时间差分别在频率为0.5 Hz和速率为1 mm/min时最大,而摩擦系数与法向应力的相对时间差约为0.5个周期;上峰值摩擦角随频率的增加而减小,随剪切速率的增加而增大。     

应力历史对黏土–混凝土界面剪切特性的影响研究

 采用大型直剪仪,系统研究法向应力历史对黏土–混凝土界面剪切特性的影响。根据制定的加卸荷方案,对3个粗糙度等级(锯齿高为0,1,2 cm)的黏土–混凝土接触界面先加荷至初始法向应力,再卸荷至剪切法向应力进行剪切。从剪应力–剪切位移曲线、界面最大剪应力、剪胀性3个角度对试验结果进行对比分析。分析结果表明：黏土–混凝土界面剪应力–剪切位移曲线仍大体呈双曲线形式,并未出现应变软化现象。初始法向应力越大,相同剪切位移对应剪应力越大;初始法向应力越大对应的界面最大剪应力越大,根据Mohr-Coulomb准则通过线性拟合得出界面强度参数,并引入界面摩擦有效系数和黏聚有效系数。通过数据对比发现,界面黏聚有效系数随着初始法向应力的增大而增大,而摩擦有效系数则随初始法向应力的增大而减小。剪切过程中3个粗糙度等级的黏土–混凝土界面均发生不同程度的剪胀,界面越粗糙,剪胀量越大。同时,应力历史对界面剪胀性规律有明显的影响,未经历法向卸荷的界面剪切过程开始先剪缩然后再剪胀,而经历法向卸荷的界面剪切一开始便呈现剪胀,且初始法向应力越大,剪胀越明显。     

砂土与结构接触面粗糙度试验研究

利用大型多功能界面剪切仪进行了砂土与结构物的接触面剪切试验,研究了粗糙度对接触面力学性质的影响。试验结果表明:在法向应力不变条件下,随着粗糙度在增加,接触面的初始剪切模量和剪切应力的峰值都相应增大。不同粗糙度条件下,接触面的切向位移和剪切应力都表现出相近的非线性关系,并且随着法向应力的增加,切向位移和剪切应力关系曲线直接的距离越来越小。相同粗糙度条件下,接触面上的法向应力和剪切应力表现出明显的线性关系。     

法向循环荷载下筋土界面的剪切应力规律及预测

为了研究法向动荷载作用下筋土界面的剪切应力变化规律,采用大型动态直剪仪,对相对密实度为75%的砾石和土工格栅的界面进行了剪切试验,研究了4种法向初始应力（20,40,60,80 kPa）和4种法向荷载振动幅值（10,20,30,40 kPa）对筋土界面循环剪切特性的影响。在试验的基础上,建立了法向循环荷载作用下筋土界面在峰值前和残余阶段的剪切应力-法向应力表达式。同时,结合应力时间差规律,并考虑法向初始应力和荷载振幅等的影响,提出了界面剪切应力-剪切位移的表达式。将两种预测表达式与试验结果进行了对比,均具有较好的吻合度,验证了方法的正确性。     

垃圾填埋场土工合成材料界面摩擦特性的拉拔试验研究

土工合成材料与填料的界面特性是决定垃圾填埋场中衬垫系统与土工合成材料受力特性的重要因素。选择3种不同种类的土工合成材料,用砂土和黏土为填料,通过拉拔试验研究土工合成材料的界面特性。试验结果表明:界面的峰值剪切强度与峰值位移随着法向应力的增加而增加;土工合成材料与黏土之间的摩擦角较大;填料为砂土时,无纺布与填料间的摩擦角最大,EPDM次之,HDPE最小;当HDPE上下都铺无纺布时,界面的摩擦系数最小。     

CNL及CNS条件下结构面剪切特性试验研究

为比较CNL及CNS边界条件下结构面剪切力学特性的差异,开展2种法向边界条件下不同粗糙度结构面的直剪试验研究,系统阐述法向刚度、初始法向应力及结构面粗糙度对结构面剪切力学特性的影响。试验结果表明,低法向应力水平下,法向刚度效果显著,CNS条件下剪切位移曲线展现出明显的硬化特征,剪胀曲线被抑制,峰值及残余剪切强度、峰值剪切位移及峰值法向位移均大于CNL试验结果。随着法向应力增大,法向刚度效果逐渐减弱,高法向应力水平下,CNS和CNL试验结果相近。试验成果有助于进一步认识CNL和CNS条件下结构面剪切力学特性及其差异。     

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。     

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.     

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.     

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.     

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.     

Shear strength of a fibre-reinforced clay at large shear displacement when subjected to different stress histories

The topic of fibre-reinforced soil has been introduced and studied increasingly in the past few decades. However, the shear strength response of fibre-reinforced clay soils with different initial void ratio values when subjected to large shear displacement has not been explored in the literature. The purpose of this study is to evaluate the shear strength responses of fibre-reinforced clay soils when remoulded with relatively small and large initial void ratio and subjected to large shear displacement. In order to exclude the composition variability of the fibre-reinforced samples when subjected to various normal effective stresses, a series of multi-stage reverse drained direct shear test was undertaken with four reverse cycles of ±7 mm, ±7 mm, ±7 mm and +14 mm to achieve an accumulative horizontal shear displacement up to 56 mm that is 93% of the sample dimension. The first stage of the testing programme was carried out on soil samples consolidated at normal effective stress of 600 kPa and unloaded to 50 kPa, followed by 4 shear cycles at normal effective stresses of 50, 100 and 200 kPa, respectively. The results of these tests confirmed significant effective stress ratio improvement with fibre reinforcement, even at large shear displacement to the fourth cycle. However, the rate of improvement decreased with normal effective stress and initial void ratio. Based on the experiments carried out in this study, the optimum fibre content to increase the shear strength of the clay soil with initial void ratio of 0.64 was found to be 0.25% with 140%, 81% and 23% increase in the stress ratio over that of the unreinforced soil at normal effective stresses of 50, 100 and 200 kPa, respectively. The second stage of the testing programme was conducted on a set of samples consolidated and sheared at normal effective stresses of 50, 100 and 200 kPa, respectively. The optimum fibre content was found to be related only to the initial void ratio of the soil, irrespective of the stress history of the soil and the applied normal effective stress. The shear stress ratio of the fibre-reinforced clay soils at large shear displacement was found to be relatively independent of the stress history of the soil. For all soil samples tested in this study, the stress ratio at 200 kPa normal effective stress was found to remain between 0.45 and 0.60.     

粉土界面恒刚度循环剪切试验研究

竖向循环荷载作用下桩土界面的作用机理是研究桩土摩擦疲劳的关键。针对循环荷载作用下桩-粉土界面的剪切性能,使用改进的剪切试验装置在恒刚度条件下进行循环剪切试验,研究循环次数、累积位移和法向刚度对其摩擦疲劳性能、循环后单调剪切性能的影响。试验结果表明,粉土在循环剪切过程中,法向应力和剪应力在初始10个循环内随循环数增加快速衰减,随着循环进行,逐渐趋于稳定;单次循环内在剪切位移方向变化时,土体呈现表现出剪缩-剪胀-剪缩交替现象,总体变形呈现剪缩的趋势;循环荷载作用下,粉土界面的法向应力和剪应力随法向刚度增大衰减速率增大,达到稳定的累积循环位移越小;粉土循环后的单调剪切、法向应力恢复的单调剪切的剪应力比小于首次单调剪切试验值,且法向应力恢复的循环后剪切试验的剪胀程度较小,表明循环剪切过程中界面处粉土颗粒棱角破碎,颗粒变得光滑。在对试验数据分析的基础上,提出了与累积位移、法向刚度和初始应力相关的无量纲累积位移,建立了法向应力和界面摩擦角随累积位移的衰减方程。     

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.     

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。     

土工合成材料大型直剪界面作用宏细观研究

利用大型直剪模型试验设备，在不同竖向压力下进行一系列的土工合成材料直剪试验，应用数码可视化跟踪技术，结合土体变形无标点量测技术来研究双向土工格栅与砂土直剪界面作用的宏细观特性，同时分析界面附近土压力分布规律，并研究界面颗粒运动变化规律和细观组构演化特征与宏观特性的关联。分析结果表明，直剪筋土界面附近竖向压力分布从前端依次向后端减少；直剪界面位移达25 mm时，形成了稳定的剪应变集中带；在筋土界面（6～8）D₅₀粒径厚度范围内，界面颗粒以旋转和平动方式同时位移，该范围外颗粒以平动方式沿剪切方向位移，且位移较小；在剪切过程中，界面颗粒发生旋转，土体发生剪胀，孔隙率增大，平均接触数减小，颗粒重新被压密，孔隙率减小，平均接触数增多，颗粒长轴排列趋于水平方向，各细观组构处于相对稳定状态。