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.     

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.     

A new constitutive model for textured geomembrane/geotextile interfaces

The paper presents a study of the frictional behaviour of geosynthetics used for municipal solid waste landfills. Direct shear tests of several geomembrane/geotextile interfaces were performed to investigate the shear behaviour. Furthermore, analytical and numerical models were developed to describe the observed behaviour, especially to simulate progressive geomembrane/geotextile interface failure and the factors controlling its significance.     

Experimental investigations and constitutive modeling of cyclic interface shearing between HDPE geomembrane and sandy gravel

This paper presents the results of experimental investigations and constitutive modeling of cyclic interface shearing between HDPE geomembrane and cohesionless sandy gravel. A series of cyclic interface shear tests was performed using a large-scale cyclic shear apparatus with servo controlled system. Particular attention was paid to the influences of the amount of shear-displacement amplitude, number of cycles, shear rate and the normal pressure on the mechanical response. The experimental results show that the path of the shear stress against the cyclic shear displacement is strongly non-linear and forms a closed hysteresis loop, which is pressure dependent, but almost independent of the shear rate. For small shear-displacement amplitudes, the obtained damping ratio is significantly greater than zero, which is different to the behavior usually observed for cyclic soil to soil shearing. In order to describe the pressure dependency of the hysteresis loop using a single set of constitutive parameters, new approximation functions are put forward and embedded into the concept of the Masing rule. Further, a new empirical function is proposed for the damping ratios to capture the experimental data for both small and large cyclic shear-displacement amplitudes. The included model parameters are easy to calibrate and the new functions may also be useful in developing enhanced constitutive models for the simulation of the cyclic interface shear behavior between other geosynthetics and soils.     

Change pattern of geomembrane surface roughness for geotextile/textured geomembrane interfaces

Applying textured geomembrane improves the frictional performance of geotextile and geomembrane interfaces. However, very limited research has been conducted to analyze the variation in textured geomembrane roughness during geotextile/geomembrane interface shear processes. In this study, a geomembrane surface roughness measurement method for measuring asperity height data with fixed intervals was presented. Normalized profile length and fractal dimension were used to quantitatively describe the geomembrane surface deformation during the geotextile/textured geomembrane interface shear process. It was found that applying normal stress led to a reduction of the roughness parameters. After the mobilization of the peak shear stress during the shear process, the chosen roughness parameters decreased with the shear displacement. And, increasing the normal stress made the shear-induced reduction of roughness parameters more obvious. The hyperbolic model can be used to describe the quantitative relationship between the geomembrane roughness parameters and the shear displacement. This study can help explain the displacement-softening post-peak behavior of the geotextile/textured geomembrane interfaces.     

Microscale investigation into mechanical behaviors of heat-bonded nonwoven geotextile using DEM

Heat-bonded nonwoven geotextiles (HBNGs) made from synthetic fibers are widely used in engineering practices. One of the challenges on the way is to link the properties of fibers and the fabric's microstructure to the deformation and failure mechanisms of HBNGs. In this study, a random distribution geometry method was developed to reproduce the complex fibrous structure of HBNG. A piecewise linear model was adopted to reproduce the nonlinear stress-strain relationships of single fibers. The present method has been successfully applied in the simulation of uniaxial and biaxial tensile tests and puncture test. The orientation distribution of fibers and the mechanical behaviors (e.g., deformation, strain localization, force-strain relationship) of HBNG specimen were reasonably simulated. Specifically, the hourglass shape during uniaxial tensile test, the axisymmetric deformation pattern during biaxial tensile test and the trumpet shape during puncture test were all well reproduced. The present method provides an applicable tool to study the complicated mechanical behaviors of HBNG and is also helpful to obtain a better understanding of its deformation and failure mechanisms.     

Dynamic friction and the seismic performance of geosynthetic interfaces

The stability of geotechnical structures which contain geosynthetic interfaces is closely linked to the shear strength between the geosynthetics themselves, both in static and dynamic conditions. Static friction is the maximum interface shear strength mobilised before displacement, whereas dynamic friction is related to the kinematics of the displacement itself. In polymer materials, dynamic friction may be widely variable, depending on the type, geometry and integrity of the surfaces in contact, as well as on the intensity and time-history of the seismic signal. This means that predicting interface shear strength is not simple. This paper focuses on the evaluation of dynamic interface shear strength between geosynthetics, using the results of both inclined plane tests and shaking table tests; this latter test also provided a means to analyse interface behaviour under the conditions of real seismic records. To this purpose, two common geosynthetic interfaces, which exhibit different behaviour under dynamic loading, were tested. One interface was a smooth HDPE geomembrane in contact with a nonwoven polypropylene geotextile, while the second was a textured HDPE geomembrane in contact with a different type of nonwoven polypropylene geotextile.The test results shows that dynamic friction mobilised during seismic events depends on the relative speed according to the same law outlined by the free sliding tests and by the shaking table tests carried out with sinusoidal base motions. Moreover, for the two different types of studied interfaces dynamic friction may be greater, lesser or equal to the static friction and the assumption of a constant value of dynamic friction does not lead to an accurate prediction of the seismic displacements under various earthquakes.     

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.     

Load-settlement response of shallow square footings on geogrid-reinforced sand under cyclic loading

To study the settlement and dynamic response characteristics of shallow square footings on geogrid-reinforced sand under cyclic loading, 7 sets of large scale laboratory tests are performed on a 0.5?m wide square footing resting on unreinforced and geogrid reinforced sand contained in a 3?m?×?1.6?m?×?2?m (length?×?width?×?height) steel tank. Different reinforcing schemes are considered in the tests: one layer of reinforcement at the depth of 0.3B, 0.6B and 0.9B, where B is the width of the footing; two and three layers of reinforcement at the depth and spacing both at 0.3B. In one of the two double layered reinforcing systems, the reinforcements are wrapped around at the ends. The footings are loaded to 160?kPa under static loading before applying cyclic loading. The cyclic loadings are applied at 40?kPa amplitude increments. Each loading stage lasts for 10?min at the frequency of 2?Hz, or until failure, whichever occurs first. The settlement of the footing, strain in the reinforcement and acceleration rate in the soil have been monitored during the tests. The results showed that the ultimate bearing capacity of the footings was affected by the number and layout of the reinforcements, and the increment of bearing capacity does not always increase with the number of reinforcement layers. The layout of the reinforcement layers affected the failure mechanisms of the footings. Including more layers of reinforcement could greatly reduce the dynamic response of the foundations under cyclic loading. In terms of bearing capacity improvement, including one layer of reinforcement at the depth of 0.6B was the optimum based on the test results. It is found that fracture of geogrid could occur under cyclic loading if the reinforcement is too shallow, i.e. for the cases with the first layer of reinforcement at 0.3B depth.     

Dynamic stress accumulation effects on soil strength under cyclic loading

To further understand the influence of dynamic stress accumulation effects on soil strength under cyclic loading, a series of dynamic–static coupling tests and stepwise cyclic loading tests were carried out using typical granite residual soil. The mechanism of dynamic stress accumulation was analyzed from the macro and micro perspectives, and the effect thereof on soil strength was discussed from the perspectives of energy and mechanics. The test results showed that a dynamic stress accumulation effect would occur in the soil under cyclic loading, which was one of the main reasons for the failure of the soil structure and the change of the strength. When the accumulation degree was small, the effect facilitated the compaction of soil, and improved the static load strength of the soil. When the accumulation degree exceeded a certain threshold, the effect resulted in rapid the soil damage. The greater the dynamic stress accumulation in the soil, the more obvious the weakening degree of the progressive cyclic loading strength. At the same time, the more prone the soil to dynamic stress accumulation, the greater the internal accumulation of variable situation energy. Failure occurred when the dynamic stress accumulation in the soil exceeded the bearing capacity of the structure, providing further insight for solving the negative impact of dynamic stress accumulation on subgrade soil.     

The determination of interface friction by means of vibrating table tests

This paper concerns the laboratory evaluation of dynamic friction in geosynthetic interfaces subjected to sinusoidal base motions. Tests were performed with a sliding block over a vibrating table with both a horizontal plane and an inclinable plane. The horizontal configuration is widely used because it is easier to interpret, whereas the inclined plane set-up is more complicated due to the variation in time of the normal component of the acceleration. An analytical method for interpreting the vibrating table test with the inclined plane configuration is described: for the purpose of comparison two geosynthetic interfaces were chosen, which exhibit very different behaviour from each other; one interface had a constant value of dynamic friction, whereas the second exhibited a relationship between dynamic friction and the relative speed of sliding. The tests, carried out with both the horizontal and the inclined plane configuration, showed how the mobilised friction was influenced by the kinematics of the block: at the same relative speed, the mobilised interface friction during tests with the horizontal plane was greater than that resulting from tests with the inclined plane. This difference may be ascribed to the patterns of relative motion at the interface, occurring in a single direction in the case of the inclined plane, and with a cyclic reversal of direction in the case of the horizontal plane.     

Morphological insights into the liquefaction and post-liquefaction response of sands with geotextile inclusions using drained constant volume simple shear tests

The present study aims to explore and bring out morphological insights into the prior-liquefaction, liquefaction, and post-liquefaction response of sands with geotextile inclusions. For this, a series of multi-stage drained constant volume simple shear tests with different cyclic stress ratios (CSR ranging from 0.1125 to 0.225) and different frequencies (f of 0.2 and 1.0 Hz) were carried out on completely dry specimens constituted with granular materials of three distinct grain morphologies (rounded, subrounded, and angular) reinforced with a nonwoven geotextile. The study also consists of morphological quantifications through image analysis algorithms and direct shear tests on sand-geotextile interfaces. Test results revealed that the inclusion of geotextile increased the liquefaction resistance and post-liquefaction shear strength of all the materials, irrespective of their particle morphology. However, the beneficial effects are more in the case of specimens constituted with angular particles. The effect of loading frequency on the response is also established. The interlocking and ploughing tendency of the angular particles leads to the mobilization of the maximum tensile strength of geotextile, which enhances the additional confinement and prevents the lateral movement of particles, thereby providing the maximum benefit.     

Effect of subgrade on leakage through a defective geomembrane seam below saturated tailings

Experiments are conducted to quantify leakage through saturated tailings (with 30% fines) underlain by a geomembrane with either a 100-mm-length knife cut slit defect or a 110-mm-length defective extrusion seam. Various subgrades, a poorly graded gravel (GP) with or without nonwoven geotextile (450 or 1420 g/m²) above, a well-graded gravel (GW), and a poorly graded sand (SP) are evaluated. Test results show that leakage through the slit defect and defective seam increase with the subgrade coarseness and subgrade unevenness. The inferred upper and lower bounds opening width of a slit based on the measured leakage increase with the overburden pressure and are categorized for each subgrade. Indentations in the overlapped defective extrusion seam area arise from both the entrapped materials inside the overlap and the materials underlying/overlying the geomembrane, incrementally inducing a greater interface transmissivity with the increasing stress. Overlain by tailings at total overburden pressure of 510 kPa and water head of 35m, leakage through the slit defect is 1.1L/day for SP, 2.2L/day for GW, 2.6–3.3L/day for both GP and uneven GW; leakage drops to 0.8L/day with a geotextile layer directly above the GP; leakage through the defective extrusion seam is 0.4L/day for SP and 0.8L/day for GW.     

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.     

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.     

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。     

Influence of micro and macroroughness of geomembrane surfaces on soil-geomembrane and geotextile-geomembrane interface strength

The interface shear strength involving geosynthetics and other materials can be influenced by various parameters, such as the material type and the normal stress on the interface. Although several investigations have been conducted over the years on this topic, the large variation of interfaces that can be used has led researchers to develop other sources of information to improve design methods. This paper investigates how roughness parameters can influence the shear strength developed between different interfaces based on many inclined plane tests and microscopic analyses of the surface roughness. One smooth and three textured geomembranes were used to simulate a barrier layer and sand, and two nonwoven geotextiles were installed on them to simulate drainage or protective layers. A powerful optical apparatus provided fifteen bidimensional and tridimensional surface parameters for the two faces of the analyzed geomembranes. The results showed that the mean height of profile elements (R_c) and the core material volume (V_mc) parameters presented stronger correlations with the interface shear strength. The concept of interface roughness factor was introduced to estimate the interface friction between different materials based on the materials properties for interfaces with geotextiles.     

循环荷载下铁精矿动力特性试验研究

利用空心圆柱扭剪仪（HCA）施加竖向#x02014;环向耦合循环荷载模拟波浪荷载的作用,对船载散装铁精矿进行了动力特性试验。通过对动应变、动强度和孔隙水压力变化曲线进行分析,探讨了含水率、动应力和振动周数等因素对铁精矿动力特性的影响。试验表明：含水率是影响铁精矿动力特性的关键因素。铁精矿存在临界含水率,如果超过这一临界含水率,铁精矿即使在较小的动应力、足够长的振动周数下,也会缓慢发展达到破坏状态。而高含水率的铁精矿试样即使在较小的动应力作用下也会迅速发生破坏。对比常规动三轴试验,可以发现竖向-扭向循环耦合剪切试验得出的铁精矿动强度随含水率的增加而迅速降低。研究可为实际船载铁精矿提供安全的含水率范围。     

Performance of geosynthetic-encased stone column-improved soft clay under vertical cyclic loading

This paper presents the results of a laboratory investigation into the performance of geosynthetic-encased stone column-improved (GESC-improved) soft clay under vertical cyclic loading. A reduced-scale model is adopted to perform a series of tests considering the principal parameters, such as the cyclic loading characteristics, including the loading frequency and amplitude, and the encasement length. The results indicate that, among other things, the overall benefit of the geosynthetic encasement of stone columns installed in soft clay is greater under cyclic loading than under static loading, and that the cyclic effect tends to lead to a stress concentration ratio that is smaller than that under static loading. The effectiveness of this encasement in improving the performance of GESCs becomes greater when subjected to cyclic loading with a lower loading frequency and/or a smaller amplitude. The settlement and pore pressure variations with the encasement length, together with the exhumed GESCs taken after the tests, suggest that full encasement is necessary to maximize the performance of GESCs under cyclic loading.