Simple Shear Tests Research Articles

Cyclic simple shear behaviour of saturated and moist sandy soils

ABSTRACT Liquefaction and settlement of sandy soils cause significant damage during earthquakes. In this research, cohesionless soils of Kutch region with history of large earthquakes are explored. Twenty-six soil samples were collected from different locations of Kutch region and tested under saturated and moist conditions by performing two series of Electromechanical Dynamic Cyclic Simple Shear (EMDCSS) tests. In series I, the liquefaction characteristics of saturated samples were observed to be dependent on fines content and plasticity index. Kutch soils with non-plastic fines exhibited low liquefaction resistance and showed increased resistance at higher FC; however, for sands with plastic fines, PI rather than FC controlled the cyclic degradation. Saturated sandy soils with non-plastic fines exhibited lower shear stress mobilisation, higher pore pressure and larger degradation as compared to sands with clay content. In series II, the settlement characteristics of moist samples at constant normal stress revealed decreased axial deformation and increased mobilised shear stress with increased FC and PI. Silty-sands exhibited higher deformations and larger strength as compared to clayey-sands. The EMDCSS tests at saturated and moist conditions revealed Kutch soils to be prone to liquefaction and large settlements, respectively.

The Influence of Apparatus Stiffness on the Results of Cyclic Direct Simple Shear Tests on Dense Sand

Abstract A series of undrained cyclic direct simple shear (CDSS) tests on dense Toyoura sand has been performed with the aim to investigate the influence of the stiffness of the DSS device on test results. To this end, springs were installed to reduce deliberately the stiffness of the apparatus. It is shown that the cyclic resistance of the sand depends strongly on the rigidity of the apparatus frame. In particular, as the stiffness of the DSS device increases, the number of loading cycles required to reach liquefaction decreases. This pronounced apparatus–stiffness dependence is of great practical concern in geotechnical engineering because it directly implies that the CDSS response of a soil sample can be predominantly controlled by the stiffness of the apparatus and not by the soil behavior alone. In addition, the test results indicate that the effect of equipment compliance in cyclic undrained DSS testing can be minimized when the ratio of the stiffness of the tested sand sample to the stiffness of the apparatus has a significantly low value.

Large-Strain Damping of Sands: Parameter Effects

A recently developed damping calculation method for undrained load-controlled cyclic tests was implemented on some 70 cyclic triaxial and direct simple shear tests conducted by various researchers on a range of different sands for a correct understanding of the high-strain (0.1%–4%) damping behavior of sands. The effect of various controlling parameters was investigated, including the shear strain, confining stress, number of cycles, void ratio, gradation, nonplastic fines content, and overconsolidation ratio. The data analysis shows that general trends at high strains are in good agreement with the damping behavior of sands at small strains. The results also indicate a direct correlation between the damping behavior of sands and cyclic strength. The recently proposed guideline on load-controlled damping ratio bounds for clean sands at high strains was also in good agreement with the damping behavior of sands with nonplastic fines content up to 25%.

The effect of tamping conditions on undrained shear strengths of a non-plastic sandy silt tailings

A series of direct simple shear (DSS) tests were carried out on a non-plastic sandy silt lead−zinc−silver tailings to develop a relationship between undrained shearing behaviour and density for contractive states. The critical state line was also obtained through triaxial compression tests to enable the DSS tests to be viewed in a critical state framework and allow comparison with in situ testing. It was found that the gravimetric water content (GWC) used to tamp the specimens had a significant effect on the resulting undrained strengths when attempting to achieve dense states — with higher GWC giving lower strength at a given density than a lower GWC. Intact and slurry deposited (SD) samples were also tested to access denser states without inducing tamping-related stresses. These showed a more consistent trend with the loose-tamped specimens, and with other data from the literature. Plausible explanations as to the causes of the increased strength of dense-tamped samples were obtained through estimating potential preconsolidation stresses and “locked in” horizontal stresses that may occur from dense tamping. The importance of these observations on the development of density−strength profiles in engineering practice was outlined.

Monotonic and cyclic behaviour of sand in direct simple shear test conditions considering low stresses

Many geotechnical problems, such as buried pipeline–soil interaction and wave loading on sand seabeds, require the soil behaviour at relatively low effective stresses as compared to that in typical geotechnical practice. Monotonic and cyclic direct simple shear (DSS) tests were conducted on dry sand under constant normal stress, ranging from 12.5 kPa to 400 kPa, to investigate the effects of confining pressure on stress–strain behaviour. The tests were conducted using a combined advanced dynamic cyclic simple shear apparatus, which has a high-precision feedback system for controlling and measuring the forces and displacements while maintaining a high level of accuracy, which is essential, especially for tests at low normal stresses. The monotonic test results with this advanced system show an increased shear to normal stress ratio, thereby the mobilized friction angle, at the low-stress level compared to that of high normal stresses, as observed in some previous studies. The cyclic test results show that the load–displacement behaviour and cyclic compaction are governed by the normal stress. The sand becomes densified when the applied shear strain amplitude is greater than its threshold value. In constant strain amplitude cyclic loading, the lower normal stresses result in higher compaction; however, an opposite trend exists in constant stress amplitude cyclic tests.

Constitutive modelling of rubbers: Mullins effect, residual strain, time-temperature dependence

Filled rubbers are an important part of engineering materials because they have unique properties such as large deformability, increased stiffness and energy dissipation ability. The complexity of the material model parameter identification and the consideration of essential behaviours such as temperature dependence or residual strain may be a great challenge during the constitutive modelling. In this paper, a hyper-visco-pseudo-elastic constitutive equation is introduced and applied to model the complex behaviour of filled rubbers by considering (i) the Mullins effect, (ii) the temperature effect, and (iii) the effect of residual strains. The time-independent material response is provided by the modified Dorfmann–Ogden pseudo-elastic material model, while the time and temperature dependence is taken into consideration by the Prony series-based linear viscoelastic theory and the time-temperature superposition principle. Furthermore, by assuming homogeneous deformations, numerical stress solutions are presented to perform the material model calibration simple and fast, even when considering uniaxial tension/compression and simple shear simultaneously. The material model can be adapted to handle any hyperelastic model, arbitrary damage and residual strain variable, user-defined time-temperature superposition function, and any number of Maxwell elements (spring-dashpot terms). Here, the model parameters are determined from uniaxial stress relaxation-recovery tests, as well as cyclic uniaxial tensile and cyclic simple shear tests of different strain rates. Finally, the theoretical results are compared with experimental data for an EPDM rubber filled with 50 phr of carbon black.

Influence of linear coupling between volumetric and shear strains on instability and post-peak softening of sand in direct simple shear tests

This paper focuses on the influence of volume change boundary condition on the instability and post-peak softening of sand. The laboratory program comprises an extensive series of Direct Simple Shear (DSS) tests with different degrees of linear coupling between volumetric and shear strains. It is observed that progressive loosening of sand associated with the volumetric expansion in a coupled strain path intensifies instability susceptibility, whereas volumetric contractive strains reduce vulnerability to instability and post-peak softening. Analysis of the experimental data using Hill’s second-order work criterion designates a family of state-dependent relational trends for instability lines, normalized peak shear strengths, and maximum pore-water pressure ratios depending on the degree of coupling between the volumetric and shear strain rates. It is found that a simple state-dependent constitutive model can predict relational trends in sand specimens suffering from instability and post-peak softening under different degrees of the linear coupling between the volumetric and shear strains.

Cyclic shearing behavior and dynamic characteristics of a fibrous peat

Cyclic shearing behavior, dynamic characteristics, and post-cyclic volume change of a peat sublayer from the Port Lands area of Toronto (Ontario, Canada) are investigated in this study. Laboratory specimens are trimmed from block samples collected from a depth of about 4.0 to 4.5 m. Constant-volume cyclic direct simple shear tests indicate an initial reduction of effective stress with number of stress cycles. However, the corresponding excess pore pressure ratios do not exceed 60%, indicating a cyclic mobility behavior in the peat specimens. Maximum shear moduli of the peat samples are also determined from shear wave velocity measurements. Post-cyclic volumetric strain, as well as the variations of secant modulus, modulus reduction, and damping ratio of the peat, are presented in terms of cyclic shear strain and compared with other studies. Empirical relationships are proposed for characterizing the shear modulus and damping ratio of Toronto peat.

Long-term dynamic bearing capacity of shallow foundations on a contractive cohesive soil

The calculation of the long-term dynamic bearing capacity arises from the need to consider the generation of maximum pore-water pressure developed from a cyclic load. Under suitable conditions, a long-term equilibrium situation would be reached, when pore-water pressures stabilized. However, excess pore-water pressure generation can lead to cyclic softening. Consequently, it is necessary to define both the cohesion and the internal friction angle to calculate the dynamic bearing capacity of a foundation in the long term, being necessary to incorporate the influence of the self-weight of soil and therefore the width of the foundation. The present work is based on an analysis of the results of cyclic simple shear tests on soil samples from the port of El Prat in Barcelona. From these experimental data, a pore-water pressure generation formulation was obtained that was implemented in FLAC2D finite difference software. A methodology was developed for the calculation of the maximum cyclic load that a footing can resist before the occurrence of the cyclic softening. The type of soil studied is a contractive cohesive soil, which generates positive pore-water pressures. As a numerical result, design charts have been developed for long-term dynamic bearing capacity calculation and the charts were validated with the application of a real case study.

Sand-sand and sand-steel interface grain-scale behavior under shearing

The behavior of a sand-steel interface can be different than the shearing behavior of a sand. This study focuses on the effect of shear loading on a sand-to-sand and a sand-steel interface as well as on particle breakage. A series of ring shear and simple shear tests were conducted to measure the effects of particle shape and surface roughness characteristics before and after shearing at the shear zones within a sand-to-sand and at a sand-steel plate interface. It was found that sand density had a large effect on the interface shearing behavior. While interface sliding began at a shear displacement of 0.9 mm (corresponding to 22% of the total shear displacement) in dense specimens, this occurred at a much higher displacement of 5.6 mm (62% of total displacement) in loose sand at rough steel interface. Sliding displacement prevailed at the sand-steel plate interface in simple shear tests as the sand sample became denser. Critical states were observed in the sand-to-sand ring shear tests after shear displacements of 65–79 mm and 87–98 mm in medium-dense and loose samples, respectively. Comparing sand-to-sand and sand-steel interface results indicated that sand-steel interface behaves less dilative with the critical state occurring at much smaller shear displacements of about 12–14 mm at the interface compared to that occurring within the sand at 65–79 mm. Both loose and dense sands reached a residual strength of 98 kPa at a shear displacement of 7 mm in simple shear tests. The inter-particle shear resistance and particle crushing, combined with particle rearrangement/rotation were the mechanisms which developed shear resistance. An interface friction angle of 27° was subsequently mobilized at the critical states of the simple shear tests, while particle crushing and abrasion resulted in a slightly elevated (by 2°) interface friction angle in the ring shear tests. The difference between sand-steel interface and sand-to-sand interaction mechanism highly influenced particle damage induced by particle fracturing and abrasion. As a result of particle crushing, roughness and aspect ratio of sand particles sheared at the interface with a steel plate reduced by respectively 39.7% and 12–14% sheared at normal stresses of 50 to 200 kPa. In particular, particles' circularity increased from 0.685 to 0.817, indicating symmetrically rounded particles. The maximum size of particles in all tests reduced to about 16.9–18.1% of the original maximum particle size (Dmax = 0.85 mm). While the skewness of particles sheared within the sand increased from −0.231 to 0.656 (indicating increased surface roughness), those of particles collected from the sand-steel interface decreased to −1.31 reflecting smoother particle surfaces.

Frictional properties between geocells filled with granular material

The parameters concerning the interaction between geocell and granular materials is required for the design of many geotechnical structures. With this in mind, a series of experiments using simple direct shear tests are conducted in order to understand the frictional properties between geocells filled with granular materials. The 54 test samples are prepared by filling the geocell with granular materials having three different gradations. These samples are tested at three different relative densities under three different normal stress levels. As a result, it was observed that interface resistance between the geocells filled with granular material is found to be generally greater than in the samples without geocells. Additionally, these samples with geocells are found to be stiffer; this is due to the fact that the samples with geocell gained more cohesion because geocells confined the grains within a restricted volume.

Undrained Failure Criteria for Isotropic and Anisotropic Soils

A new total stress failure criterion applicable to both isotropic and anisotropic soils under undrained conditions is proposed. The new criterion, which is motivated by elastic anisotropy in the underlying effective stress-strain behavior, is shown to be consistent with reported data for undrained shear strengths obtained in triaxial compression, triaxial extension, and direct simple shear tests.

Strength and durability of biocemented sands: Wetting-drying cycles, ageing effects, and liquefaction resistance

Microbially induced calcite precipitation (MICP), an emanating, sustainable and potentially sound technique for biocementation of granular soils. The method is based on the metabolic activity of urease-producing microbes. Bioaugmentation through individual microorganisms has been investigated till now, and synergistic effect of bacteria mixtures (hybrids) for bioaugmentation is under-explored yet. Durability aspect of biocemented soils in terms of wetting–drying resistance and ageing also requires special attention and thorough investigation before adopting in field application. Thus, considering these limitations of MICP, present study has been carried out to aim multiple objectives, including optimization of cementation solution injection strategy. Poorly graded liquefiable sand was treated in polyvinyl chloride (PVC) tubes by augmenting it with Sporosarcina pasteurii, Bacillus sphaericus, and their mixed culture for hybridized activity. The biotreatment was continued by using 0.50 M cementation solution using injection frequency 12 and 24 h, and injection pore volumes (PV) 1, 0.75, and 0.5 up to 18 days. Permeability and calcite content of biocemented specimens were determined after treatment. For durability analysis, biocemented specimens were subjected to 0, 5, 10, 15, and 20 wetting–drying (W-D) cycles and kept in an uncontrolled temperature environment for 0, 1.5, 3, 4.5, and 6 months to investigate ageing effects. After W-D cycles and ageing time, samples were tested for ultrasonic pulse velocity (UPV), shear modulus, unconfined compressive strength (UCS), and split tensile strength (STS). The mass-loss rate of biocemented specimens was also determined after cyclic W-D actions. The liquefaction resistance of biotreated sand was compared with untreated dense sand specimen using dynamic cyclic simple shear (DCSS) tests. The results showed W-D resistance of biocemented sand even after 20 W-D cycles, and ageing effects showed more than two times increase in mechanical properties. The DCSS test results showed that excess pore pressure ratio was not reached to 1 in biocemented sands up to 100 cycles.

Quality of the strain state in simple shear testing using field measurement techniques

Shear testing holds a peculiar place in sheet metal characterization for multiple reasons. Indeed, as opposed to tension or bi-axial expansion, the principal strains of such a deformation change their direction, non-linearly, as a test proceeds. It is, in general, also impossible for an experimental setup to reproduce theoretical conditions. The kinematics assumptions are therefore not verified in absolute, but satisfied up to a certain level that often remains subjective. Fortunately, field measurement methods provide comprehensive data that can be used for assessing and improving experimental processing. In this work, digital image correlation is applied to different types of shear testing performed on a dual phase steel. Results are used to construct quantitative estimators to assess the quality of measurement as well as to decrease the number and weight of assumptions. A method is also proposed to separate the measured kinematics into the expected theoretical ones and the unwanted experimental deviations. It is quantitatively shown that, for the considered material and sample types, translational simple shear testing achieves kinematics that are closer to theory than pseudo-simple shear testing. The other major finding of this work is a method to identify shear strains independently of experimental biases.

The critical state behaviour of granular material in triaxial and direct simple shear condition: A DEM approach

The critical state (CS), the anchor concept in the critical state soil mechanics (CSSM) framework, has been comprehensively evaluated in triaxial stress conditions, where all the principal stresses are known but the pure shear stresses in the shear directions are absent. Therefore, the evolution of first and second invariants of the stress tensor (σ′) i.e. the mean effective stress (p′) and deviatoric stress (q) along with volumetric strain (εv) is often used to identify the CS. However, the same does not apply for a direct simple shear (DSS) test, which is more representative of the real ground condition, as in most cases the minor principal stresses are not measured. This leads to the challenge of determining the CS line in the e-log(p') space and thus the concept of state parameter (ψ) cannot be used to characterise soil behaviour. Using DEM, this study evaluates CS for DSS, using evolution σ′ and εv, in the e-σ'N space, compare its location with triaxial CS lines in the e-log(p') space. The CS line for DSS in e-σ'N space was then used to define a modified state parameter, ψ, (ψm). It was found that both ψ and ψm showed good correlations with characteristic behaviour e.g. onset of liquefaction, phase transformation state, fabric anisotropy etc. Therefore, ψm maybe a good alternative of ψ for DSS test where the minor principal effective stresses are not measured to define a CSL in e-log(p') space.

Predictive Continuum Constitutive Modeling of Unfilled and Filled Rubbers

The general CSE model fits Treloar’s uniaxial extension test and predicts unfitted uniaxial compression, equibiaxial extension, biaxial extension, pure shear, and simple shear tests. As a newly proposed method, the general CSE model, along with the stress-softening ratio, the residualstretch ratio, and the weighted piecewise two-point interpolation function, fits the Cheng–Chen’s test and the Diani–Fayolle–Gilormini’s test in cyclic uniaxial extension at different pre-stretches and predicts corresponding responses at untested pre-stretches. Physical mechanisms of the Mullins effect have also been predicted based on the evolution of constitutive parameters.

Limit load instabilities of anisotropic tubes under combined tension and torsion

Under tensile loadings, thin-walled structures such as sheet metal and tubes develop load maxima, or limit loads, beyond which deformation localizes leading to rupture. For uniform stress states, Considère-type estimates of limit loads can serve as forming limits in applications. The paper explores the effect of anisotropy on such estimates for thin-walled Al-alloy tubes under combined tension and torsion. Anisotropy is modeled using Yld04-3D with an exponent of 8. Material hardening originates from a simple shear test using this yield function, taking into account material axes rotation caused by the shearing. A Considère formulation is developed for the problem, which also incorporates the effect of material frame rotation. The analysis is used to establish limit loads for a set of circumferentially constrained tension–torsion experiments tested under radial nominal tension-shear stress paths. The predictions reproduce the strains measured at the limit loads for the range of biaxiality ratios considered. By contrast, corresponding results produced using the isotropic yield functions of von Mises and Hosford(8) increasingly deviate from the measured results as the shear stress increases. Considère-type formulation is also developed for the same tension–torsion loadings for a uniform thickness tube. The results exhibit a similar trend but the limit strains for shear dominant paths are significantly lower, demonstrating the stabilizing effect of the circumferential constraint used in the experiments.

Numerical study of random material properties within a soil specimen

Material properties derived from laboratory soil tests often assume that the property is uniform throughout the specimen. For some exceptional soils, this may hold true, but for many others it is obviously false. We have been performing cyclic and irregular torsional simple shear (TOSS) tests on hollow cylinder samples for decades and were intrigued by the idea of how to model inherently non-uniform specimens. As an added corollary, we wanted to understand the influence of imperfections (voids, inclusions) on the measured stress-strain behaviour in these tests. This paper examines two general classes of problems: (a) uniform specimens with inclusions of voids or imperfections, and (b) non-uniform specimens with random distributions of material properties within the specimen. Finite element modelling was performed on a TOSS specimen (ID = 4cm, OD = 6cm, L = 14cm) using a set of over 500 different elastoplastic material properties within the specimen. Various distributions (Normal, Log-normal, Bimodal) of stiffness and strength properties were examined. The results were examined as torque vs. twist curves since those values are typically measured in the TOSS laboratory test before being converted (with assumptions of uniformity) to shear stress-shear strain hysteresis. The Bimodal distributions were used to represent soils with distinct hard and soft zones. Additionally, distributions with some degree of spatial correlation were also examined.

Cyclic and monotonic simple shear testing of native Christchurch silty soil

The first cyclic simple shear tests on high-quality retrieved specimens of Christchurch silty and sandy soil were performed to evaluate their liquefaction potential. The test specimens originate from stratified silty soil deposits in southwestern Christchurch which did not typically have surface manifestations of liquefaction after the intense shaking of the 2011 Christchurch earthquake. The soil's undrained cyclic stress-strain responses indicate cyclic mobility. The differences in the cyclic responses of the soil with different amounts of non-plastic silt are subtle. Specimens with greater cyclic stiffness degradation typically had lower post-cyclic stiffness. The soil stiffness was significantly lower in post-cyclic tests than in monotonic tests due to the large differences in the initial effective stresses between the two types of tests, loss of strength, age, and fabric effects. The field-adjusted laboratory-based cyclic resistances agree with CPT-based cyclic resistances from simplified liquefaction triggering procedures. Both assessments indicate the silty soil deposits generated high excess pore water pressures and liquefied during the Christchurch earthquake even though surface manifestations of liquefaction were not typically observed at the considered sites. The absence of liquefaction manifestation at the stratified silty soil sites highlights the importance of their system response.

Experimental Study on the Monotonic and Cyclic Behavior of Carbonate Sand in the South China Sea

Carbonate soils have specific properties that differ from those of siliceous sand. The monotonic and cyclic behavior of carbonate sand from Jinqing Island in the South China Sea was examined by undrained monotonic and cyclic simple shear tests in this paper. The monotonic test results show that the phase transformation state friction angle is 25.7°, and the critical state friction angle decreases from 38.7° to 37.3° due to particle crushing. By comparing the experimental data from the monotonic and cyclic tests, we found that the trend of the peak shear stress ratio with shear strain in each cycle of cyclic tests was close to that of monotonic tests. If the cyclic stress ratio of symmetrical cyclic tests is normalized with the phase transformation strengths obtained by the corresponding monotonic tests, it has a good relationship with the number of cycles at failure and is not affected by the relative density and vertical consolidation stress. The effects of the relative density, vertical consolidation stress and initial static shear stress on the undrained cyclic strength are discussed. Finally, a comparison of the undrained cyclic strength of carbonate sand from different regions shows that different carbonate sands have almost identical undrained cyclic strength under the medium dense state, but significant quantitative differences are observed between these carbonate sands under the dense state.