Monotonic Shear Tests Research Articles

Pore water pressure developed by a soft fine-grained soil subjected to combined monotonic and cyclic shear stresses

The magnitude of the pore water pressure developed by a soft fine-grained soil subjected to shear stresses is what determines the occurrence of sudden failure. As part of an extensive testing programme, besides identification and classification tests, 20 monotonic simple shear tests and 139 cyclic simple shear tests with different levels of static stresses (τo ) before the cyclic phase were performed. In this research, reference is made to pore water pressure (u) developed by a soft fine-grained soil subjected to combined monotonic (τo ) and cyclic shear (τc ) stresses. Laboratory tests were carried out on undisturbed samples from the Port of Barcelona, Spain. The samples were obtained at depths between 29 m and 52 m, that correspond to an effective vertical stress of (σ’ov ) between 277 kPa and 413 kPa, respectively. In general, the results indicate that: (a) the combination of τo and τc controls the generation of pore water pressure (u); (b) it is not always true that, as a consequence of increased pore water pressure when τo/σ’ov + τc/σ’ov ≈ Su/σ’ov , the soil reaches the failure cyclically (with Su being the undrained shear strength); (c) in the monotonic case, it is possible to establish empirical functions between u/σ’ov and the monotonic shear strain (γm ); and (d) in the combined monotonic and cyclic case, it is possible to establish empirical functions among u/σ’ov , the cyclic shear strain (γc ) and the number of cycles (N), for different (τo/σ’ov ) ratios. In addition, an application was made to study the seabed stability based on the knowledge of the material under cyclic load.

Liquefaction resistance of Christchurch sandy soils from direct simple shear tests

A series of cyclic direct simple shear tests are performed to investigate the liquefaction resistance of sandy soils from Christchurch, New Zealand. The study focuses on the combined effects of soil density and fines content on liquefaction resistance of sandy soils. Two sands, a non-plastic silt, and their mixtures prepared at different fines contents are tested at two sets of relative densities. Test specimens are reconstituted using a procedure for water sedimentation, yielding soil fabric and soil structure resembling those of fluvial soil deposits. Differences are observed between the two host sands in the sensitivity of the cyclic liquefaction resistance to changes in relative density. Differences are also seen in the effects of the addition of fines to the liquefaction resistances of the two sands. Monotonic undrained direct simple shear tests are employed to explore an interpretation of the liquefaction resistance of the tested soils within the critical state framework. The state-concept interpretation provides more consistent quantification of the effects of initial state on the liquefaction resistance; however, the relationship between the state parameter and liquefaction resistance is soil dependent. The results of cyclic direct simple shear tests for soils containing up to 30% fines show reasonably consistent liquefaction resistances relative to estimates from empirical CPT-based liquefaction triggering relationships.

Coupling effects of particle shape and cyclic shear history on shear properties of coarse-grained soil–geogrid interface

Accurate measurement of particle shape is necessary to properly investigate the cyclic shear behavior of a soil–geogrid interface. In this study, the particle shapes of crushed limestone, quartz sand, round gravel and spherical granular media were described and quantified in terms of particle regularity. Cyclic direct shear tests were then performed with particles of various shapes for different relative densities and shear displacement amplitudes. Monotonic direct shear tests and post cyclic direct shear tests were performed to compare the relationship between particle regularity and interface friction angle. The test results showed that the interface underwent cyclic shear hardening for relative densities between 0.453 and 0.672. The area of the hysteresis loop, cyclic peak shear strength and maximum vertical displacement increased with increasing shear displacement amplitude, and decreased with increasing particle regularity. In monotonic and post cyclic direct tests, the friction angle decreased linearly as particle regularity increased. Comparing to the monotonic direct shear tests, the slope and intercept of the regression lines increased in the post cyclic direct shear tests after undergoing cyclic shearing. This study shows that it is important to analyze the interface shear behavior in the monotonic, cyclic and post cyclic direct shear tests using parameterized particle shapes.

Interface shear strength and shear fatigue resistance of fibreglass grid-reinforced asphalt concrete test specimens

ABSTRACT This research was undertaken to study the effects of grid opening size and tack coat type on both the shear strength and shear fatigue resistance at the interface of fibreglass grid-reinforced asphalt concrete specimens and, more importantly, to correlate these effects with the interfacial fatigue performance under four-point bending beam fatigue loading. The specimens were fabricated using a 12.5-mm nominal maximum aggregate size surface mixture. Two types of fibreglass grid, differing only in the grid opening size, and two tack coats, a PG 64-22 asphalt binder and a slow-setting anionic asphalt emulsion with low viscosity, were used to reinforce the specimen interfaces. The digital image correlation technique was utilised to measure displacements, strains, and crack lengths. The test results indicate that a strong tack coat and a grid with large openings can potentially improve the shear resistance of a grid-reinforced interface but the effects of these two parameters might differ under monotonic and cyclic loading conditions. The cyclic shear, beam fatigue, and monotonic displacement-controlled shear test results suggest that the grid opening size plays a key role in the interface shear fatigue performance, however tack coat quality plays a more important role on the interface shear strength of the specimens.

Large Scale Direct Shear Experiments to Study Monotonic and Cyclic Behavior of Sand Treated By Polyethylene Terephthalate Strips

The continuously increasing production of plastics in the last few decades has taken a heavy toll on the environment. Soil reinforcement with the use of randomly distributed fibers, plastics and glass has recently received a lot of attention. The use of polyethylene terephthalate (PET) strips as soil reinforcement has been observed to result in the increase in the shear strength. In this paper, the monotonic, cyclic, and post-cyclic properties of sand reinforced with different contents and aspect ratios of PET strips are investigated by carrying out a set of monotonic and cyclic direct shear tests. Accordingly, the influence of various parameters on the direct shear response of sand-PET mixture is examined. Results show that in the monotonic tests, for mixtures containing 0%, 0.5%, 0.75% and 1% PET with AR = 1, the peak friction angle is 37.5°, 41.8°, 42.7°, and 44.6°, repectively, while these values increase to 41.8°, 45.4°, 48.2°, and 50.4°, respectively, for post-cyclic monotonic tests. In addition, in the monotonic tests, for mixtures containing 0.5%, 0.75% and 1% PET with AR = 5, the peak friction angle is 48.8°, 52.2°, 54.0°, respectively, while these values increase to 50.9°, 54.8°, 55.9°, respectively, for post-cyclic monotonic tests. Results clearly imply that normal stress and cyclic shear strain amplitude contribute jointly to the contractive behavior of the reinforced mixtures. Moreover, the hardening behavior becomes less eminent when the sand is reinforced with PET strips due to the resistance mobilized against particle rearrangement. The post-cyclic shear strength of the PET-sand mixture is higher than their monotonic strength due to the densification occurring during the cyclic loading. In summary, the favorable impact of PET inclusion on the monotonic and cyclic performance of granular materials is deduced.

Micromechanical behaviour in shearing of reproduced flat LBS grains with strong and weak artificial bonds

The shearing behaviour of reproduced flat LBS grains artificially bonded with ordinary Portland cement (OPC) and plaster of Paris (PP) was examined using micromechanical experiments. Monotonic shearing tests showed a distinct variation in the load–displacement relationship at low, medium and high normal loads, and a nonlinear shear strength envelope was proposed. For OPC-bonded sand grains, a brittle–ductile transition at 20–30 N normal load was observed and three breakage mechanisms in shearing (chipping, shear cracks and crushing) were distinguished in accordance with the changes in the load–displacement curves. OPC-bonded sands showed a predominant dilation at lower normal loads, whereas PP-bonded sands were highly compressive. Based on previously published works using element-scale tests, a new mechanism for dilation under micromechanical testing was proposed in the study. Cyclic shearing tests were conducted on OPC-bonded sands, and the effects of increased displacement amplitude and normal load were highlighted.

Effect of aperture ratio on the cyclic shear behaviour of aggregate-geogrid interfaces

In order to study the effect of aperture ratio (geogrid aperture size (AS)/average particle size (D50)) on the shear behaviour of aggregate–geogrid interfaces, a series of monotonic direct shear (MDS) tests, cyclic direct shear (CDS) tests and post-cyclic monotonic direct shear (PCDS) tests were carried out on two particle-shaped reinforced interfaces at five values of AS/D50. The cyclic shear behaviour of the two interfaces under three displacement amplitudes is discussed, and the interface shear characteristics before and after cyclic shear are analysed. The test results indicate that four possible cyclic shear behaviours can occur for two reinforced interfaces, including shear hardening (H), cyclic cumulative softening (SI), direct softening (SII) and wave softening (SIII). With increasing amplitude, the cyclic shear response of both interfaces changes from shear hardening to shear softening. The vertical displacements of both interfaces indicate an overall shear contraction trend in the CDS tests. The stiffness of two reinforced interfaces was significantly affected by the displacement amplitude and the ratio AS/D50. Compared with the maximum dilation of two reinforced interfaces in the MDS tests, the corresponding value has increased in the PCDS tests; the increase is more significant at rounded aggregate-geogrid interfaces.

Environmental impact evaluation and long-term rutting resistance performance of warm mix asphalt technologies

The adoption of warm mix asphalt (WMA) technologies is well accepted due to its promising environmental benefits, however, their long-term field performance is not well understood. This study focused on the long-term rutting resistance of WMA and their companion hot mix asphalt (HMA) based on 83 field samples collected from 28 field projects across the United States covering four climate zones, and quantified environmental impacts for various WMA technologies as compared to HMA. Field cores were evaluated by dynamic modulus, creep compliance, and Hamburg wheel-tracking device tests. Asphalt binders extracted from field cores were evaluated by continuous performance grading (PG), multiple stress creep recovery (MSCR), and monotonic shear tests. The results showed that WMA binders exhibited similar rutting resistance compared with the HMA binders after long-term field aging. Existence of polymer in the asphalt binder was displayed through the plot of non-recoverable creep compliance and percent recovery. The long-term field aged polymer modified asphalt binders for WMA and HMA were comparable. The results also showed that the CO2 reduction by using WMA ranged from 89.09% to 42.79% depending on WMA type. On average, the CO2 emission was reduced to 66.89% by using WMA technologies. Overall, environmental impact analysis and long-term field performance results validated the confident use of WMA technologies.

DEM analysis of monotonic and cyclic behaviors of sand based on critical state soil mechanics framework

A series of monotonic and cyclic triaxial shearing tests on Toyoura sand are simulated by discrete element method (DEM) incorporating rolling resistance. The input parameters are calibrated against laboratory triaxial test data considering different initial states and monotonic loading paths. The calibrated DEM model successfully captures the density and stress dependency of sand behavior under both monotonic and cyclic conditions. It is shown that when approaching liquefaction, contact yielding alters from sliding-dominant to rolling-dominant. The expression of mechanical redundancy index (IR) is derived considering both effects of contact sliding and rolling. The IR = 1.0 condition is shown to be a unified criterion distinguishing between solid state and liquid state upon both monotonic and cyclic liquefactions. A unique micro critical state line (CSL) is obtained in MCN-p′ plane. The cyclic resistance ratio is exponentially correlated with both the macro state parameter (ψe0) derived from the e-p′ CSL and the micro state parameter (ψMCN0) determined from the MCN-p′ CSL. The specimens with the same ψe0 may have distinct cyclic behaviors, while cyclic behaviors of specimens with the same ψMCN0 are close to each other, indicating that ψMCN0 is a more plausible state variable for characterizing the behavior of sand than ψe0.

Effect of Temperature on the Cyclic Behavior of Clay–Structure Interface

The shaft capacity of foundations highly depends on the monotonic and cyclic loads applied to the soil–structure interface. In energy geostructures that exploit the heat of soil using earth-contact elements, the interface is subject to cyclic thermomechanical loads. Monotonic and cyclic constant-volume equivalent-undrained (CVEU) direct shear tests were performed on clay–clay and clay–structure interfaces at different temperatures (22°C and 60°C). An effective vertical stress of 300 kPa was applied to the samples and the cyclic and average shear stress ratios (τcy/SuDs and τa/SuDs, respectively) were varied between 0.35 and 0.57. The tested soil was a kaolin clay (plasticity index=24) prepared in a normally consolidated state. The results showed that the number of cycles to failure for the clay–structure interface test was lower than that for the clay–clay case in the same range of cyclic and average shear stress ratios. In cyclic clay–structure tests, decreasing the cyclic stress ratio increased the number of cycles to failure; however, decreasing the average shear stress ratio decreased the number of cycles to failure. Increasing the temperature decreased the rate of strain accumulation, and the number of cycles to failure increased by two to three times. The rate of degradation (degradation parameter, t) decreased by 16% with heating from 22°C to 60°C for the different cyclic stress ratios tested.

Investigation of Initial Static Shear Stress Effects on Liquefaction Resistance Using Discrete Element Method Simulations

Prior laboratory and in situ investigations show that monotonic preshearing can have a significant effect on the cyclic response of granular materials. The underlying mechanics that are responsible for these changes in behavior in response to preshearing are not well characterized, however. Herein, we use the discrete-element method (DEM) to simulate undrained monotonic and cyclic simple shear tests with the constant volume method. Through published comparisons to laboratory data, DEM simulations have been shown to reasonably predict the cyclic response of granular materials, making them an appropriate tool for studying the effects of initial static shear stress on liquefaction initiation. Mechanical coordination number and the normal force-weighted fabric tensor are used to describe the evolution of fabric after monotonic preshearing and during cyclic loading. We find that higher initial static shear stress induces smaller cyclic strength in stress-controlled cyclic simple shear tests, but that there is no such effect in strain-controlled cyclic simple shear tests in which the normal force-weighted anisotropy is removed in the first three loading cycles. In addition, the stability of specimens is found to be closely related to pore-pressure increases. Finally, we show that the effect of initial static shear stress on liquefaction resistance is a combination of reduced stability before cyclic loading and accumulated shear strain during cyclic loading.

Seismic Retrofitting of Traditional Masonry with Pultruded FRP Profiles

This paper presents a study on the potentiality of seismic retrofitting solutions with pultruded Fiber Reinforced Polymer (FRP) profiles. This material can be used in connected frames providing lightweight, corrosion-free and reversible retrofitting of masonry buildings with the moderate requirements of surface preservation. In a hypothetical case study, an experimental program was designed; monotonic shear tests on a half-size physical model of the sample wall were performed to assess the structural performance before and after retrofitting with a basic frame of pultruded Glass Fiber Reinforced Polymer (GFRP) C-shaped profiles, connected to the masonry by steel threaded bar connections. During the tests, the drift, the diagonal displacements in the masonry and the micro-strain in the profiles were measured. The retrofitted system has proven very effective in delaying crack appearance, increasing the maximum load (+85% to +93%) and ultimate displacement (up to +303%). The failure mode switches from rocking to a combination of diagonal cracking and bed joint sliding. The gauge recordings show a very limited mechanical exploitation of the GFRP material, despite the noticeable effectiveness of the retrofit. The application seems thus promising and worth a deeper research focus. Finally, a finite element modelling approach has been developed and validated, and it will be useful to envisage the effects of the proposed solution in future research.

Experimental study on the monotonic shear strength of GM/CCL composite liner interface

The interface shear strength between a geomembrane (GM) and a compacted clay liner (CCL) or a geosynthetic clay liner (GCL) is of great importance for landfill stability analysis. An experimental study of the interface shear behaviour between a smooth GM (GMS) or a textured GM (GMX) and a CCL is presented in this paper. A series of monotonic shear tests was conducted using a large direct-shear apparatus. Peak and residual shear strength parameters were obtained by linear fitting of the experimental data. The shear strength of both GMS/CCL and GMX/CCL interfaces was mainly driven by the normal stress level, while the displacement rate had very little influence on the interface shear strength. The residual shear strength of the GMX/CCL interface was even higher than the peak shear strength of the GMS/CCL interface at all normal stress levels. The GM/CCL interface showed considerably higher shear strength over the GM/GCL interface; the residual strength of the GM/CCL interface was even higher than the peak strength of the GM/GCL interface at all normal stress levels. The results presented in this paper are useful for the preliminary design of landfill composite liner systems.

Shearing resistance of tailing sand waste pollutants mixed with different contents of fly ash.

The co-placement of mine tailings and fly ash (CMF) can reduce acid mine drainage (AMD) production and decrease metal mobilization. This aids in waste management construction. However, few people have studied a large number of tailing sand-fly ash mixtures under the condition of neutral saturated solution in tailing ponds, wherein the pozzolanic reaction is highly gradual. In this study, a series of tests were conducted to determine the monotonic and cyclic shear characteristics of a mixture of fly ash and tailings. In particular, the effects of the fly ash content on the monotonic shear peak, shear strength parameters, dynamic modulus, and damping ratio of the mixture were analyzed. The results reveal that in a monotonic shear test, the peak shear strength of the saturated CMF mixture decreases as the fly ash content increases. The shear strength parameters (cohesion c and internal friction angle φ) were observed to increase and decrease linearly, respectively, as the fly ash content increased. Furthermore, the maximum dynamic shear modulus was observed to decrease by 41.4% as the fly ash content increased from 0 to 50%, during the cyclic cutting process. Moreover, the experimental results fit well with the fitting formula for the variation in shear modulus in the cyclic shear process of the saturated CMF mixture with varying fly ash content. Meanwhile, the initial damping ratio of the cyclic shear was observed to increase from 10.3 to 13.6% as the fly ash content increased. Therefore, when the CMF method is used to treat AMD waste, it is necessary to consider the extent to which the design stability of the tailing pond may be reduced. These experimental results can be used as a reference for similar CMF projects.

Thermal effects on one-way cyclic behaviour of clay-structure interface

In energy geostructures, which exploit the heat in soil using earth contact elements, the interface is subjected to cyclic thermo-mechanical loads. Monotonic and cyclic constant-volume equivalent-undrained (CVEU) direct shear tests were performed on clay-clay and clay-structure interface at different temperatures (22 and 60 °C). Different cyclic and average stress ratios (CSR and ASR) were applied to the kaolin clay-structure interface under 300 kPa of normal stress. The results showed that, the number of cycles to failure for the clay-structure interface test was lower than that for the clay-clay case in the same range of cyclic and average shear stress ratios. In cyclic clay-structure tests, decreasing the cyclic stress ratio, increased the number of cycles to failure; however, decreasing the average shear stress ratio decreased the number of cycles to failure. Increasing the temperature, decreased the rate of strain accumulation and the number of cycles to failure increased by 2-3 times. The rate of degradation (degradation parameter, t) decreased by 16% with heating from 22 to 60 °C for the different cyclic stress ratios tested.

Analysis of shear fatigue properties of iron single crystals regarding possible effects of the resolved normal stresses on the slip planes

Crystal orientation effects on the shear fatigue limits of iron single-crystals were investigated by considering the influence of normal stress of the slip plane on the critical resolved shear stress (CRSS). The shear fatigue limit with the (123) plane in the shear loading position was experimentally confirmed to exceed that with shear loading on the (110) plane. On the contrary, the shear yield stress of the (123) plane, evaluated by monotonic shear tests, was lower than that of the (110) plane. To solve this seeming contradiction, it was postulated that body-centred cubic crystals possessed 24 slip systems and that the CRSS was sensitive to the resolved normal stress (RNS) on the slip plane. That is, it was assumed that the CRSS decreased under the tensile RNS according to its magnitude, but increased under the compressive RNS. The mathematical model of the CRSS dependence on the RNS was simply introduced to the work-hardening law proposed by Peirce et al. allowing formulation of the algebraic equations yielding the crystalline slip rates. This crystal plasticity model was applied to analyse monotonic shear and shear fatigue tests. By considering the effect of the RNS sensitivity, the shear yield stress with the (123) plane in the shear loading position was calculated to be lower than that with the (110) plane, showing agreement with the experimental results of the monotonic shear tests. Furthermore, in the application of cyclic shear loads corresponding to the fatigue limits, it was confirmed that the plastic strain range of the primary slip plane converged to similar values despite the differences in the crystal orientations of the shear plane and the normal stresses to it. These results imply that the present method allows the prediction of fatigue limits under different crystal orientations and loading conditions.

An experimental study on the micromechanical behavior of pumice

Pumice has a large potential to be used in structural and geotechnical engineering projects as a lightweight aggregate, and it is a material of unique characteristics. Gravel-sized pumice from Yunnan, China, was examined in the present study with a custom-built micromechanical apparatus investigating the properties of pairs of grains at their interfaces subjected to monotonic and cyclic tests under different saturation states. Based on interferometer measurements, the vesicular surface of the pumice was characterized to be very rough. From the interface micromechanical experiments, it was shown that the virgin surfaces appeared to be very soft under normal loading tests, during which micro-asperity breakage was caused. After preloading and preshearing, the normal load–displacement curves became much smoother and the Young’s moduli, which remained relatively low in magnitude compared with other geo-materials previously studied, could be quantified applying the Hertzian model. Large hysteretic loops were observed from cyclic normal load tests. Plastic displacements dominated the first normal loading cycle, while elastic displacements prevailed in the subsequent cycles. High values of the apparent coefficient of friction were observed from the monotonic shearing tests. A modification of the power value of the Mindlin and Deresiewicz model was necessary to be applied during the fitting of the experimental data so that theoretical and laboratory curves could match. It was shown that this power had a direct correlation with the coefficient of friction.

Engineering properties of sand–rubber tire shred mixtures

ABSTRACT This study deals with the determination of the engineering properties of sand–rubber tire shred mixtures. The rubber tire shreds of fine size are mixed with river sand in various controlled proportions and tested under dense, fully saturated conditions. The monotonic direct shear tests, one-dimensional compression, and the permeability tests are conducted on these mixtures to determine their shear behaviour, compressibility, and drainage characteristics. The increase in rubber content results in the reduction of shear strength, constrained modulus and the increase of compressibility of the mixtures. The brittleness index which is a measure of ductility and the energy absorption capacity is also discussed. Intriguingly, the permeability of the mixtures reduces with the increase in rubber content. The contact angles of sand-water and the rubber–water interfaces determined using Goniometer explains this behaviour.

Cyclic and postcyclic simple shear behavior of binary sand-gravel mixtures with various gravel contents

The effect of gravel contents on the cyclic and post-cyclic monotonic simple shear behavior of gravelly soils during seismic events has significant implications for civil infrastructure. Large-scale cyclic simple shear tests and post-cyclic monotonic simple shear tests were performed on gravelly soils with different levels of gravel content (GC) and loading conditions. The test results indicated that the GC of a sand-gravel mixture (SGM) profoundly influences the mixture's cyclic simple shear characteristics. The SGM specimen with the lowest GC had the broadest hysteresis loops. The shear modulus of the SGM specimens differing in GC was found to increase with increasing cyclic number. The increase in shear modulus was more pronounced during the first 10 cycles. The damping ratio of the SGM decreased to some extent as the cyclic loadings increased. Overall, the results suggested a modified relationship between the damping ratio and shear modulus. The modified function accurately represented the relationship between the normalized damping ratio and the normalized secant shear modulus for SGMs varying in GC. Shear strain obviously decreased with the number of cycles during the initial cyclic loadings, when loading frequency was low. However, at a higher loading frequency, shear strain was less affected by the number of cycles. In general, the inclusion of gravel particles slightly reduced the post-cyclic monotonic shear resistance.

Mechanical behavior and texture evolution of aluminum alloys subjected to strain path changes: Experiments and modeling

The mechanical responses and crystallographic texture evolution of four representative aluminum alloy sheets (2xxx, 5xxx, 6xxx and 7xxx series), commonly used in the transportation industry, are studied under reverse shear loading in this work. The monotonic and forward-reverse simple shear tests are carried out to obtain the stress-strain response. The initial and after-deformation crystallographic texture are experimentally measured. The deformations are simulated using a dislocation theory-related hardening model incorporated in the Visco-Plastic Self-Consistent (VPSC) framework. The simulation results are in good agreement with the experiments in both the mechanical response and texture evolution. Furthermore, the changes of texture components during shear and reverse shear deformation are contrastively discussed as well as the influence of initial texture on the evolution. The cube component, as the primary initial orientation, transforms into other orientations as the applied shear deformation progresses. The rate of change is dependent on the initial texture intensity. Although the loading direction reverses once, the intensity variation of the orientation components showed two inflection points.