Simple Shear Tests Research Articles

Comparison of sand liquefaction in cyclic triaxial and simple shear tests

The liquefaction resistance and correction factors Kσ and Kα of Nakdong River sand obtained from cyclic triaxial (CTX) tests were compared with those determined by cyclic simple shear (CSS) tests to ascertain the importance of the reduction factor Cr and correction factors Kσ and Kα in liquefaction evaluations, especially in view of the lack of comparative liquefaction assessments based on different laboratory test apparatuses. All samples used for the comparisons were obtained from the same type of sand by using similar preparation methods and they were subjected to similar stress states to minimize the number of factors influencing the comparison results; moreover, the apparatuses used in the two tests were manufactured by the same company and all tests were conducted by a single operator. It was found that the liquefaction resistance in CTX tests was always greater than that in CSS tests. Furthermore, Cr varied from 0.63 to 0.36, and it depended on the relative density Dr and initial static shear ratio α. Kσ, which increased with the normal effective stress σ′nc in CTX tests, was identical to Kσ observed in CSS tests when α was increased up to 0.1. By contrast, Kα in the CSS tests was 58%–97% of Kα measured in the CTX tests, and it depended on the combined effect of Dr, σ′nc, and α. The relationship between Kα and α in both CTX and CSS tests was well represented by a parabolic function. Moreover, the differences in Kα values between the CTX and CSS tests were also found to be a parabolic function of α. This information can be used for converting CTX (or CSS) values into equivalent CSS (or CTX) values.

Evaluation of Simple Shear Test Geometries for Constitutive Characterization using Virtual Experiments

In-plane simple shear tests have become commonplace in the fracture characterization of automotive sheet metals but have received less attention for constitutive characterization. Unlike tensile tests, simple shear tests do not have any tensile instability and remain in a state of plane stress until fracture. From plastic work equivalence, an isotropic hardening model can be readily constructed from the tensile and shear test data without inverse finite-element analysis. The success of the methodology hinges upon the shear specimen geometry and how the local strains in the gage region are measured using digital image correlation (DIC). In this study, finite-element simulations of seven shear test geometries were evaluated for an isotropic material in a series of virtual experiments by varying the input hardening response. The data from the simulations was extracted from the surface as if DIC was employed and used to determine the hardening behavior in comparison with the exact solution. Shear geometries without a notch eccentricity in the gage region appear to be best suited for characterizing low hardening materials with an error of less than 1% in the stress response for an n-value of 0.02. Conversely, for higher hardening materials corresponding to an n-value of 0.20 or greater, the geometries with a notch eccentricity performed best.

Behavior of discrete fiber-reinforced sandy soil in large-scale simple shear tests

This study presents results from a series of large-scale simple shear tests performed on sandy soil reinforced with randomly distributed fibers. The soil was mixed with polypropylene, glass and basalt fiber, respectively, with fiber content varying from 0% to 2% and sheared under a normal stress ranging from 100 kPa to 300 kPa. The effects of fiber type, content and normal stress on shear strength and the dilatancy behavior were examined. The friction state theory was employed to interpret the influence of fiber reinforcement on the dilatancy behavior. The results indicate that the additions of polypropylene and basalt fibers are beneficial to enhancing ultimate shear strength, whereas the glass fiber makes little contribution. It is shown that the fiber inclusion hinders the close packing and interlocking of soil particles and hence produces relatively looser soil fabric in the as-compacted state, leading to increased volume contraction accompanied by a negligible improvement or even reduction in shearing stiffness over a small to moderate strain range. The most obvious increase in shear strength generally occurs at a shear strain exceeding 10%, where the fibers and particles are in tight contact with each other, allowing full interface interaction and the mobilization of fiber tensile strength.

Modelling of Flexible Adhesives in Simple Mechanical States with the Use of the Darijani-Naghdabadi Strain Tensors and Kirchhoff-de Saint-Venant Elastic Potential.

Practical aspects of modelling of flexible adhesives with the energy conjugate measures of stress and strain of the Darijani–Naghdabadi (D-N) family are discussed. A possibility of description of materials exhibiting non-linear physical characteristics with the use of non-linear geometric relationships and linear elastic constitutive law is considered. Nominal stress vs. stretch relations are specified in cases of simple tension and simple shear with the use of the Kirchhoff–de Saint-Venant elastic potential and D-N energy conjugate stress and strain measures. Obtained theoretical estimates were compared with experimental results of simple tension and simple shear tests performed on Sika PM polyurethane (Cracow, Sika Poland). The deformation rate was fixed in order to minimize the influence of viscosity. Values of parameters in the definition of the D-N strain tensor were optimized in order to provide good agreement between model predictions and experimental results. Observed discrepancies indicate that the proposed approach is not appropriate for constitutive modelling of the PM polymer. The presented approach is proposed to be used as a simple design model providing practical formulas describing the behavior of materials of non-linear characteristics in chosen mechanical states. Admissible values of exponents are discussed regarding its bijectivity in a limited range of variation of principal stretches.

Analysis of the stress distribution in a laminar direct simple shear device and implications for test data interpretation

Direct simple shear (DSS) testing allows observation of load-deformation response under rotation of the major principal stress plane, which is descriptive of many actual field problems. While the simplicity of the test configuration makes its use popular in research and industry, key uncertainties still remain regarding the interpretation of the laboratory data. This study uses laboratory validated discrete element method (DEM) models to examine the stress transmission in laminar-type direct simple shear devices under drained constant effective stress conditions. The DEM models (comprised of spheres) closely replicate physical specimens of precision chrome steel ball bearings for which the properties (e.g., shape, surface friction, and stiffness) were measured directly. The DEM models were also validated using experimental tests, so that conclusions regarding the system response can be derived with confidence from the available DEM data. The testing program included both loose and dense specimens, allowing for a comparison of the influence of density on stress state which has not been examined in previous simple shear DEM studies. Differences were observed between vertical effective stresses and shear stresses derived from boundary measurements (as commonly carried out in experimental programs) and those derived from force measurements within the DEM specimens. The failure state of the material in simple shear was also examined through Mohr’s circles of stress. The evolution of stresses on both the horizontally and vertically oriented planes were considered so that established methods of direct simple shear interpretation could be critically assessed. For the loose specimens, the angle of shearing resistance can be confidently estimated considering the maximum shear stress acting on the horizontal plane, which is easily inferred from measurements of the shear force during the physical test. This was true considering both internal and boundary calculated stresses. This approach, however, is inaccurate for the dense specimens. Analysis of the particle-scale kinematics of the response illustrates that the deformation field within the central portion of the specimen is in simple shear, although the magnitude of this shearing was significantly larger than what was measured on the boundary. This study and the conclusions derived focus on smooth spherical particle specimens; the objective was to examine the stress distribution within DSS devices and the implications for test interpretation using DEM models that more closely matched the physical laboratory specimens tested than in previous studies. When considered alongside the existing studies, the findings show that there is no broad conclusion that can be applied for all materials and all conditions in simple shear and that interpretation should be carefully tied to the physical conditions simulated.

Dynamic Properties of Granulated Rubber Using Different Laboratory Tests

Due to the socio-environmental hazards arising from the stockpiling of disposed scrap tires, the necessity to utilize such material in civil construction and other applications is deemed mandatory. The lightweight of rubber and its high damping capacity are excellent properties of a geomaterial that could be used successfully in seismic isolation and vibration damping applications in civil construction. Scrap tires could be shredded into specific sizes, and their category and application depend on their particle size range. Thus, understanding the dynamic properties and behavior of shredded scrap tires under cyclic loading is of paramount importance. In this study, the dynamic characteristics of granulated rubbers (<12 mm) are investigated using cyclic triaxial and cyclic direct simple shear tests. The effect of using different testing techniques, i.e., cyclic triaxial test (CTT) and cyclic simple shear test (CSST), on the dynamic properties of granulated rubber material is further addressed. Undrained cyclic triaxial and constant-volume direct simple shear tests are conducted on granulated rubber samples under vertical consolidation stresses of 25, 50, 100 and 200 kPa at a frequency of 0.5 Hz. The shear strain amplitude is varied from 0.01% to 10%. Furthermore, the variations of shear modulus and damping ratio with shear strain amplitude are presented. In addition, the obtained dynamic properties from this study are compared with existing experimental data from the literature. It was found that the ranges of shear moduli of granulated rubber from the CTT and CSST are 278 to 2647 kPa and 85 to 2270 kPa, respectively. Moreover, the damping ratios obtained from CTT were higher than those from CSST at shear strains of less than 1%. The damping ratio of granulated rubber was also found to be independent of the vertical consolidation stress.

On Empirical Correlations for Normalised Shear Strengths from Fall Cone and Direct Simple Shear Tests in Soft Swedish Clays

Empirical correlations provide valuable information in early design stages, and they help to validate or discard single values from site investigations. This paper presents a multivariate database from commercial projects consisting of evaluated shear strengths obtained from direct simple shear tests and fall cone tests (which are calibrated to the field vane test), including index tests. The multivariate database is used to investigate the performance of common transformation models and to test the recommended correction for fall cone tests. It is found that the measured normalised shear strength evaluated from direct simple shear tests and fall cone tests is correlated to the liquid limit and that the results conform to Swedish and Norwegian recommendations. However, the scatter is large, more for fall cone tests than for direct simple shear tests, which is thought to depend mainly on sample disturbance. It can however be concluded that the trend of normalised shear strengths increases with increasing plasticity.

Dynamic, in situ, nonlinear-inelastic response and post-cyclic strength of a plastic silt deposit

This study presents the use of controlled blasting as a source of seismic energy to obtain the coupled, dynamic, linear-elastic to nonlinear-inelastic response of a plastic silt deposit. Characterization of blast-induced ground motions indicates that the shear strain and corresponding residual excess pore pressures (EPPs) are associated with low-frequency near- and far-field shear waves that are within the range of earthquake frequencies, whereas the effect of high-frequency P-waves is negligible. Three blasting programs were used to develop the initial and pre-strained relationships between shear strain, EPP, and nonlinear shear modulus degradation. The initial threshold shear strain to initiate soil nonlinearity and to trigger generation of residual EPP ranged from 0.002% to 0.003% and 0.008% to 0.012%, respectively, where the latter corresponded to ∼70% of the maximum shear modulus, Gmax. Following pre-straining and dissipation of EPPs within the silt deposit, the shear strain necessary to trigger residual excess pore pressure increased two-fold. Greater excess pore pressures were observed in situ compared to those of intact direct simple shear (DSS) test specimens at a given shear strain amplitude. The reduction of in situ undrained shear strength within the blast-induced EPP field measured using vane shear tests compared favorably with that of DSS test specimens.

Cyclic Behaviour of Palu Sandy Soils in Cyclic Simple Shear Test

A 7.5-moment magnitude earthquake hit Palu, Central Sulawesi, Indonesia, in September 2018. This earthquake caused many damages and casualties, including tsunami and liquefaction. Following these events, Palu sand’s cyclic properties were investigated under cyclic simple shear testing conditions, which could convincingly portray the real seismic ground shaking under repeated shear force. All the tests were performed under the frequency of 1.0Hz with varied relative densities and cyclic stress ratio (CSR). The results show that grain size distribution plays a significant role in liquefaction resistance of sandy soils tested-the soils tested under higher CSR need fewer cycles to liquefy. Furthermore, soils which in loose conditions tend to have lower liquefaction resistance.

Constant Volume Ring Shear Specimen Trimming and Testing

The constant volume ring shear (CVRS) apparatus can use intact/undisturbed specimens to measure the peak and residual undrained shear strengths to estimate the post-peak strength reduction for seismic stability and permanent deformation analyses. A trimming apparatus and specimen container were fabricated to allow the trimming of an intact annular specimen directly into a container that is used for the shearing phase of the test. This article describes the specimen trimming, recommendations for handling and converting the trimming apparatus to the shearing container, which can be done either by hand or with a specially prepared trimmer holder, and suggestions for maintaining an undrained or constant volume shear condition. CVRS undrained peak and residual shear strengths for soils with different plasticity and clay size fractions are presented, and they are in agreement with peak undrained strength ratios from direct simple shear tests. This article also introduces correlations for peak and residual undrained shear strength ratios as a function of plasticity index for verification of future test results and planning level design.

Evaluation of Thawing and Stress Restoration Method for Artificial Frozen Sandy Soils Using Sensors.

Undisturbed frozen samples can be efficiently obtained using the artificial ground freezing method. Thereafter, the restoration of in situ conditions, such as stress and density after thawing, is critical for laboratory testing. This study aims to experimentally explore the effects of thawing and the in situ stress restoration process on the geomechanical properties of sandy soils. Specimens were prepared at a relative density of 60% and frozen at −20 °C under the vertical stress of 100 kPa. After freezing, the specimens placed in the triaxial cell underwent thawing and consolidation phases with various drainage and confining stress conditions, followed by the shear phase. The elastic wave signals and axial deformation were measured during the entire protocol; the shear strength was evaluated from the triaxial compression test. Monotonic and cyclic simple shear tests were conducted to determine the packing density effect on liquefaction resistance. The results show that axial deformation, stiffness, and strength are minimized for a specimen undergoing drained thawing, restoring the initial stress during the consolidation phase, and that denser specimens are less susceptible to liquefaction. Results highlight that the thawing and stress restoration process should be considered to prevent the overestimation of stiffness, strength, and liquefaction resistance of sandy soils.

Seismic Compression of Unsaturated Silty Sands: A Strain-Based Approach

The vast majority of surface structures are located on or surrounded by unsaturated soil deposits and may suffer excessive settlements during earthquakes. However, the fundamental understanding of the mechanisms by which the degree of saturation impacts the volumetric deformation of soils during seismic loading is still not mature. Consequently, it is critical to develop and calibrate seismic compression models while considering these mechanisms. The objective of this paper is to experimentally investigate the impacts of the degree of saturation, fines content, and desaturation technique on the seismic compression of sand and silty sands. The experimental program involved undrained cyclic direct simple shear tests on specimens prepared using suction control and wet-compaction techniques with similar relative densities but different levels of saturation. A strain-based predictive model was adapted and modified to capture the observed trends in the seismic compression of soils with different degrees of saturation. The suitability and applicability of the model were verified by comparing the measured and estimated compression values in this study with ones reported in the literature for other soils and desaturation approaches.

Episodic direct simple shear tests to measure changing strength for whole-life geotechnical design

This technical note describes a set of direct simple shear (DSS) tests that characterise the evolving geotechnical properties of a soft soil, through a loading history that represents episodic load...

Methodology for calculate bearing capacity of soft soils under cyclic loading

The bearing capacity in soft soils under cyclic loading has not been widely studied as the static bearing capacity. In soft saturated soils under cyclic load, the phenomenon of cyclic softening occurs, this consists of an increase in the generation of pore-water pressure due to the cyclic load and a decrease in effective soil stresses. The purpose of this work was to propose a methodology for calculating the bearing capacity in soft soils under cyclic loading. The research is based on the results of monotonic and cyclic simple shear tests carried out with samples from the PRAT port of Barcelona. From an analysis carried out on all the experimental data, an equation for the generation of pore-water pressure is proposed. This formulation was implemented in the FLAC2D finite difference software and is part of the methodology. The methodology of the proposed calculation process consists of 6 stages and verification under two criteria. This process is iterative in case of not meeting any of the two verification criteria. Under this methodology of the calculation process, charts are proposed to calculate the bearing capacity of soft soils under cyclic loading. This application takes into account the properties of the soil, the static bearing capacity (P he), and the effective load outside the foundation (q). Also, an analysis of the parameters that most influence the bearing capacity of soft soils under cyclic loading was carried out.

Effect of partial drainage on the pullout behaviour of a suction bucket foundation

Suction-bucket-supported jacket foundations are being used increasingly for offshore wind turbines, the advantages being large bearing capacity and efficient installation. For a large-capacity wind turbine, the pullout capacity and displacement of the bucket in the tripod or tetrapod jacket can be critical to the safety of the structure. The loading rate, which is commonly dominated by the offshore environmental conditions, has a significant effect on the ultimate capacity and stiffness of the bucket foundation. In this paper, a numerical model is developed to investigate the response of a bucket foundation under vertical pullout loading at different rates. A bounding-surface model based on critical state soil mechanics is introduced to account for the soil behaviour under different stress states and drainage conditions. The model is modified and calibrated against a comprehensive set of laboratory tests, including both triaxial compression, triaxial extension and direct simple shear tests in drained and undrained conditions. How partial drainage affects the load–displacement response and pore pressure field are thus presented. A parametric study on the selection of soil parameters is also conducted to shed light on practical foundation design.

Experimental determination of deformation homogeneity and shear states using the digital image correlation method

One of the great challenges in the pure shear and simple shear tests is to maintain the deformation state and deformation homogeneity of a specimen under large deformations. For this reason, the aim of this work is to investigate the state of shear deformation and degree of deformation homogeneity of three different shear specimens. A simple and easily replicable methodology was proposed to estimate the degree of deformation homogeneity of specimens, while a time-independent kinematic vorticity parameter was employed to identify pure shear and simple shear deformation states. All data were obtained from full-field displacements that were extracted with the two-dimensional digital image correlation (2D-DIC) method. Specimens of polytetrafluoroethylene (PTFE) were tested using the modified slotted shear test and the V-notched rail shear test, which are used to induce a simple shear state. The planar tensile test was used to produce a pure shear state in a thin sheet of rubber-like material. For small deformations, both slotted and V-notched shear tests induced a good degree of deformation homogeneity along the shear path between the notches of specimens, mainly in the central region. However, the V-notched specimens buckled at large deformation, producing a decrease in the degree of deformation homogeneity. In this case, the results were qualitatively assessed due to the errors associated with out-of-plane motions. High degree of deformation homogeneity was also observed in pure shear specimens. All tests demonstrated to be effective for inducing a state of pure shear or simple shear. Results show that the two approaches can be used as supplementary tools for determining the state of shear deformation and degree of deformation homogeneity.

Cyclic behavior of sand under non-uniform shear stress waves

During an earthquake, soil deposits are often subjected to complex stress/strain time histories that are quite different from the simplified ones used in laboratory tests for evaluation of the stress-strain-strength of these soils. To mimic the stress/strain time histories experienced at different depths of a mildly sloping soil deposit, a series of direct simple shear tests were conducted on Ottawa F65 sand specimens. The trends observed in these experiments were then used to estimate the lateral spreading of mildly sloping saturated deposits of the same sand subjected to various base motion time histories measured in the centrifuge tests conducted as part of an international research project (Liquefaction Experiment and Analysis Project, LEAP-2015 and LEAP-2017).In each CDSS test, after imposing the initial stress state, the specimen was subjected to non-uniform stress waves that resembled the shear stress time histories likely to develop in the LEAP-2015 and LEAP-2017 centrifuge specimens. A total of 17 CDSS tests were performed on specimens with relative densities of approximately 66%. Initial vertical and shear stresses were imposed on each soil specimen to correspond to the state of stress in a mildly sloping soil layer at two depths of about 4 m and 3 m. A ramped sinusoidal shear stress wave similar to the ramped sinusoidal base motion used in the LEAP centrifuge experiments was employed. The test results displayed a consistent trend between the peak cyclic stress ratio and the permanent shear strain that developed in the soil. The effects of overburden stress on the magnitude of permanent deformations were observed. Smaller initial vertical stress resulted in a more dilative response and smaller permanent shear strain. The observed permanent shear strain vs. peak cyclic stress ratio curves were then used to predict the lateral spreading of mildly sloping deposits tested in the LEAP-2015 and LEAP-2017 centrifuge experiments. Reasonably close correlation between the predicted and measured displacements were obtained. The developed dataset, made publicly available on DesignSafe-CI, can be used for calibration, evaluation, and further development of constitutive models for liquefiable soils.

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.

Test method for minimum and maximum densities of small quantities of soil

The minimum and maximum void ratios are required to establish the relative density of sandy soil. Undisturbed simple shear test specimens are often too small to provide enough material to employ the Japanese Standard JIS A 1224:2009 Test Method. An alternative test method for evaluating the dry minimum and maximum densities of soil is proposed for those cases when a test specimen does not provide enough material to use the standard-size mold. The alternative test method was applied to soil from 42 small test specimens. Soil from test specimens of the same material were subsequently mixed to produce eight soil composites for which their minimum and maximum densities could be determined using both the standard and alternative methods. The minimum and maximum void ratios determined by both methods for the composite soil are in good agreement. Moreover, trends in the dataset are consistent with those from previous studies on sand and non-plastic fine-grained soil.

Penetration and retrieval forces during sampling in a very soft clay

Three series of sampling with thin-walled samplers, with and without inside clearance and without a piston, have been performed in a very soft organic clay deposit. The penetration and retrieval forces were measured throughout the operation, thus contributing to a clearer understanding of the sampling process. The measured forces show the importance of proper borehole cleaning conditions, and also identify when samples were lost during retrieval. This occurs when the underpressure at the sampler bottom does not appear in the retrieval force versus time chart. The obtained values have been compared to the sleeve friction measured by piezocone tests. Direct simple shear (DSS) tests and vane shear tests have been used as references to back-calculate the dimensionless skin friction factor from both sample penetration and retrieval. The measurement of force during sampling proved useful for controlling sampling operation, also providing further information with respect to the regular procedure.