Simple Shear Apparatus Research Articles

Influence of shear stress level on cyclic deformation behaviour of intact Wenzhou soft clay under traffic loading

Under traffic loading, the soil elements in subgrade are subjected to a cyclic cardioid-shaped stress path in the 2τvh~(σv−σh) stress plane. This significantly differs from the conventional stress paths that have been simulated using the cyclic triaxial and simple shear apparatuses, as widely reported in the literature. To date, the influence of shear stress level on cyclic deformation behaviour of intact soft clay subjected to cyclic traffic loading were rarely considered in the existing experimental investigation. For this reason, a series of cyclic hollow cylinder experiments that mimic cardioid-shaped stress paths are carried out on intact samples of Wenzhou soft clay. A wide range of vertical stress levels combined with different shear stress levels (i.e., different cardioid-shaped stress paths) are simulated in the experiments. The results show that the shear stress level significantly influences the cyclic straining and exacerbates the potential of cyclic shear failure. The difference of cyclic tests with variable shear stress level increases as the vertical cyclic stress ratio value increases. Based on the test results, an allowable cyclic stress ratio is determined to be about 0.22, which may serve as the upper bound limit of cyclic traffic loadings on soft clayey deposit. For the hollow cylinder specimens used in this study, a resilient axial strain of 3% could be a proper failure criteria value. A simplified permanent strain model that considers shear stress is proposed for the deformation behaviour of soft clay under cyclic traffic loading.

Wetting tests of partially saturated soils under simple shear conditions

The paper provides some remarks on a new suction-controlled simple shear apparatus. The features of the testing device are first outlined and the material used in the experimental programme is described. As a fundamental field of application, wetting tests under simple shear conditions are performed, being representative of the initial stress conditions and stress paths related to the typical triggering mechanisms of rainfall-induced shallow landslides. The benefits of this new device are illustrated, as an alternative or additional tool to other existing equipment. This paper presents a short selection of the measurements obtained using a partially saturated volcanic soil, which is prone to static liquefaction and volumetric behaviour on shearing. The challenges associated with simple shear tests are also discussed using a few examples from the ongoing work.

Cyclic Direct Simple Shear Test to Measure Strain-Dependent Dynamic Properties of Unsaturated Sand

The dynamic properties of a clean sand under different degrees of saturation were investigated using a modified custom built Direct Simple Shear (DSS) apparatus at the University of New Hampshire. The specific characteristics of the DSS were presented, and the testing procedures were discussed. The device used the axis translation and tensiometric techniques to control the matric suction in the soil specimen. The investigation on F75 Ottawa Sand showed a decrease in shear modulus and an increase in damping by increasing the shear strain over the tested range of strains for various degrees of saturation: dry, saturated, and partially saturated. The modulus reduction in the applied range of medium shear strains regardless of the degree of saturation demonstrated the capability of the DSS in consistently capturing the changes of dynamic properties. Experimental results indicated that the matric suction can have a substantial effect on the stiffness of the soil. However, the extent of this effect may depend on the induced strain level of the effective stress in unsaturated soil. In addition, partially saturated specimens resulted in lower dynamic compression.

Predicting long term performance of offshore wind turbines using cyclic simple shear apparatus

Offshore wind turbine (OWT) foundations are subjected to a combination of cyclic and dynamic loading arising from wind, wave, 1P (rotor frequency) and 2P/3P (blade passing frequency) loads. Under cyclic/dynamic loading, most soils change their characteristics. Cyclic behaviour (in terms of change of shear modulus change and accumulation of strain) of a typical silica sand (RedHill 110) was investigated by a series of cyclic simple shear tests. The effects of application of 50000 cycles of shear loading having different shear strain amplitude, cyclic stress ratio (ratio of shear to vertical stress), and vertical stress were investigated. Test results were reported in terms of change in shear modulus against the number of loading cycles. The results correlated quite well with the observations from scaled model tests of different types of offshore wind turbine foundations and limited field observations. Specifically, the test results showed that; (a) Vertical and permanent strain (accumulated strain) is proportional to shear strain amplitude but inversely proportional to the vertical stress and relative density; (b) Shear modulus increases rapidly in the initial cycles of loading and then the rate of increase diminishes and the shear modulus remains below an asymptote. Discussion is carried out on the use of these results for long term performance prediction of OWT foundations.

Large simple shear testing of soft Bangkok clay stabilized with soil–cement-columns and its application

In practice, the slip circle analysis is still commonly used to determine internal stability of soil–cement-columns (SCCs) -stabilized ground subjected to lateral load. Conventionally, the weighted average shear strength of stabilized soil columns and soft soil is calculated, with a correction factor applied to reduce the strength of unstabilized soil. In this study, a new large simple shear apparatus has been developed and used to test soft Bangkok clay stabilized with SCCs of different patterns, namely, Square Group Columns (SGC), Longitudinal Tangent Wall (LTW), and Transverse Tangent Wall (TTW). From the experimental results, the LTW pattern yielded the highest strength gain from SCCs, while the TTW specimen had very minimal gain in strength, as compared to the original reconstituted sample. It was also observed that the SCCs had failed predominantly as a result of tilting and bending, and the columns would not have fully mobilized its shear strength as in unconfined compression. A modified weighted average shear strength equation was thus proposed whereby the reduction factor is applied to the SCC strength, and not to the strength of unstabilized soil. A slip circle analysis of an actual failure case was performed based on SCC shear strength estimated using the modified approach, which gave much more satisfactory results. The concept that the correction factor be applied to the stabilized soil is recommended for the slip circle analysis of the cases in which tilting and bending of SCCs would govern the failure. This would be cases of soft ground stabilized with SCCs with a relatively low improvement area ratio of 0.1–0.3.

Cyclic Shear Response of Cemented Paste Backfill

AbstractA constant-volume direct simple shear (DSS) apparatus was used to characterize the liquefaction resistance of cemented paste backfill (CPB) prepared using unclassified fine-grained tailings. The effects of curing age, cement content, and initial void ratio (also expressed through initial solids content of the mixture) on the resistance to liquefaction were investigated. Higher cement contents, longer curing periods, or higher initial solids contents were found to increase CPB liquefaction resistance in a consistent manner. The commonly used “100-kPa unconfined compressive strength (UCS) rule of thumb” for CPB liquefaction susceptibility, as originally adopted from lightly cemented sands, was found to be conservative under a large earthquake-induced cyclic stress ratio (CSR). For two different mixes, a UCS of approximately 70 kPa was found adequate to resist liquefaction under a CSR typically generated by a maximum ground acceleration of 0.3 g. For the samples tested, a UCS of 70 kPa was found to c...

Effects of confining stress, density and initial static shear stress on the cyclic shear response of fine-grained unclassified tailings

A constant-volume direct simple shear apparatus was used to investigate the effects of confining stress, initial static shear stress and void ratio on the liquefaction resistance of fine-grained tailings. The cyclic resistance ratio (also expressed as the overburden stress correction factor) was found to decrease with increasing confining stresses in the range 100–400 kPa and to gradually increase from 400 kPa onwards, when samples were tested at the same initial void ratio. The presence of an initial static shear stress decreased the liquefaction resistance. A key outcome of the work was to reveal the critical importance of density on liquefaction susceptibility of compressible tailings. Thus, a combined correction factor to account for overburden stress and density on cyclic resistance was proposed.

Fundamental investigation of constant stress simple shear test using DEM

The discrete element method (DEM) is employed to explore the responses of three major versions of direct simple shear (DSS) apparatus, i.e., stacked-ring, Cambridge, and NGI types, during shearing. In this regard, results of a stacked-ring simple shear test on glass bead specimens are employed to validate the 3D-DEM model parameters. It is shown that different boundary conditions do not influence internal responses of the specimens, significantly. The uncertainties associated with the behavior of DSS specimens, such as stress and strain non-uniformity, principal stress and strain rotations, the orientation of the failure surface, anisotropy, and non-coaxiality, are investigated. Suggestions are presented to interpret the DSS test results and estimate the stress state inside the specimen during shearing using conventional boundary measurements in the laboratory testing.

Simple Shear Tests on Unsaturated Soils

Rainfall induces landslides in several geo-environmental contexts through a variety of triggering mechanisms. In shallow covers of unsaturated silts or loose sands, static liquefaction may occur upon shearing or wetting. The paper proposes simple shear and wetting tests performed on remoulded specimens of air-fall volcanic pyroclastic silty sands from Sarno site (Italy), where huge flowslides occurred. Simple shear wetting tests reproduce the initial stress conditions in the field as well as the stress path of the most common triggering mechanisms which take place during critical rainfalls. The tests are carried out through the Unsaturated Simple Shear apparatus designed at the University of Salerno. First, simple shear tests are performed in dry and saturated conditions and the results compared in terms of shear strength and volumetric response. Then, wetting tests – under constant vertical stress or constant shear stress – are performed. The experimental results are presented and the future research is outlined.

Suction-controlled dynamic simple shear apparatus for measurement of dynamic properties of unsaturated soils

State variables such as confining pressures and the degree of saturation can have a profound effect on the dynamic properties of a soil. These dynamic properties are essential when performing seismic response analysis of geotechnical systems in the phreatic zone. In order to accurately obtain these properties, laboratory testing using a Dynamic Simple Shear system is often implemented. This paper concentrates on the advancements and modifications of a custom-built dynamic simple shear system at the University of New Hampshire to accommodate soils with unsaturated conditions by employing axis translation technique. Tests could be performed under drained (constant suction) or undrained (constant water content) conditions. The procedure for preparing an unsaturated soil sample and testing is discussed, followed by the methods for interpreting the data and the challenges involved. Preliminary data confirms the ability of the system to control and track suction during the cyclic simple shear test. Suction in unsaturated soil increased the shear modulus and decreased the damping ratio comparing with those in dry and saturated conditions.

Cyclic plastic deformation characteristics of subgrade under moving train wheel load

Cyclic plastic deformation of subgrade and other engineered layers is generally not taken into account in the design of railway bridge transition zones, although the plastic deformation is the governing factor of frequent track deterioration. Actual stress behavior of fine grained subgrade/embankment layers under train traffic is, however, difficult to replicate using the conventional laboratory test apparatus and techniques. A new type of torsional simple shear apparatus, known as multi-ring shear apparatus, was therefore developed to evaluate the actual stress state and the corresponding cyclic plastic deformation characteristics of subgrade materials under moving wheel load conditions. Multi-ring shear test results has been validated using a theoretical model test results; the capability of the multi-ring shear apparatus for replicating the cyclic plastic deformation characteristics of subgrade under moving train wheel load conditions is thus established. This paper describes the effects of principal stress rotation (PSR) of the subgrade materials to the cyclic plastic deformation in a railroad and impacts of testing methods in evaluating the influence of principal stress rotation to the track deterioration of rail track.

Measurement and modeling of simple shear deformation under load reversal: Application to advanced high strength steels

In this paper, the stress–strain behavior under load reversal of advanced high strength steel (AHSS) sheet samples was measured using a modified simple shear (SS) apparatus. The forward–reverse loading behavior was characterized for three different grades of AHSS; namely, DP, TRIP and TWIP steels. For comparison purpose, compression–tension (CT) tests were also carried out for the same materials. For all the cases, a typical complex anisotropic hardening behavior, including the Bauschinger effect, transient strain hardening with high rate and permanent softening, was observed during load reversal. No premature localization and sheet buckling occurred in these experiments, which have been major technical hurdles in CT tests. For example, an engineering shear strain of over 40%, which corresponds to an effective strain of roughly 0.2, at reversal was achieved for DP980 although the uniform elongation of this material in uniaxial tension is only 5%. A recently developed distortional hardening model (HAH) was employed to reproduce the SS stress–strain curves. Using the coefficients determined with these SS data, the CT behavior was predicted with the HAH model and compared with experimental results. This complementary study indicated that the constitutive model determined from the SS flow curves satisfactorily reproduced the CT hardening behavior. As an application, finite element simulations of springback were carried out for 2D draw-bending of a strip sheet.

Simple shear tests with a varying shearing direction during cyclic shearing

Tests with cyclic lateral loading on a pile have shown that the pile reacts to a change of loading direction with increasing accumulation of lateral displacements. This phenomenon is known as ‘pile drift’. To investigate this behaviour at small scale under laboratory conditions, a specifically designed simple shear apparatus with two shearing axes was developed. This paper presents the first results from tests with this apparatus. The first test series consisted of monotonic tests with different densities and cyclic tests on dense sand with varying load level and varying angle of loading ψ. The cyclic tests showed an increased accumulation of shear strain if the shearing direction varied during cyclic shearing.

A New Simple Shear Apparatus and Testing Method for Unsaturated Sands

This paper deals with a new simple shear apparatus for the investigation of mechanical soil behavior of unsaturated coarse-grained soils under monotonous and cyclic loading conditions. Non-cohesive soils, such as sands, typically encounter low capillary effects, and little research has been dedicated to them so far. This holds true especially for effects of repeated loading. The presented test setup allows control of small matric suction up to 85 kPa within the soil specimens with the help of a vacuum control method. Although the pore water pressures are negative as occurring in nature, the pore air pressure is kept at an atmospheric level. Depending on the pore water drainage condition of the specimen boundaries, either drained constant suction tests (CS-tests) or undrained constant water content tests (CW-tests) can be performed. The focus of the research is placed on the interaction of loading and volume change with matric suction and degree of saturation, i.e., the hydraulic-mechanical coupling, during monotonous and cyclic shear. Central questions are the impact of matric suction and degree of saturation on the densification behavior as well as, vice versa, the effect of cyclic loading on a change of matric suction and degree of saturation. The new simple shear apparatus and testing method will be explained, their benefits and shortcomings will be discussed, and selected results of a series of tests will be presented.

An improved simple shear apparatus for GCL internal and interface stress-displacement measurements

Geosynthetic clay liners (GCLs) have been widely used as a hydraulic barrier along with HDPE geomembrane (GM) in landfills. Both the internal and interface strengths of GCLs are very important for evaluating landfill stability. An improved simple shear apparatus used in this study does not force the shear failure to occur along a pre-determined plane. The composite specimens can fail along any interface or through shearing of the GCL internally. The displacement of each component of the specimen can also be measured during the tests. When non-woven geotextile side contacts textured GM (NWGT/GMX) and woven geotextile side contacts compacted clay layer (WGT/CCL) for a dry GCL, the failure interface can change from NWGT/GMX interface to WGT/CCL interface with the increases of the normal stresses. Conversely, when woven geotextile side contacts textured GM (WGT/GMX) and non-woven geotextile side contacts compacted clay layer (NWGT/CCL) for a dry GCL, the failure always occurs at the WGT/GMX interface for all applied normal stresses. The internal failure of the GCL did not occur when the normal stress increased up to 1,500 kPa for the GCL in dry condition.

Some Observations on the State of Stress in the Direct Simple Shear Test Using 3D Discrete Element Analysis

Abstract Assessment of the mobilized friction angle in a soil specimen tested in a given shear apparatus requires adequate information to establish the stress state at the instance of interest. In the most commonly used version of the direct simple shear (DSS) apparatus, where a cylindrical soil specimen is confined by wire-reinforced membrane, only the normal and shear stresses on the horizontal plane are measured. The knowledge of these stresses alone does not provide adequate information to construct the Mohr circle defining the state of stress. In this context, drained and constant volume (i.e., equivalent to undrained) discrete element simulations of a cylindrical DSS specimen were performed and the results are presented with emphasis on the DSS mobilized friction angle during shearing. It was found that planes of maximum stress obliquity rotate with the development of shear strain. This finding implies that it is not possible to calculate the friction angle accurately from typical DSS laboratory tests with unknown normal stress on the vertical plane. However, it seems that at large shear strains, the horizontal plane becomes a plane of maximum stress obliquity, and the friction angle calculated using the stress state on the horizontal plane is a good approximation to the mobilized friction angle at such shear strain levels.

Experimental Study on Behavior of Interface between Coarse-Grained Soil and Gravel-Clay under Seismic Loading

The behavior of interfaces between coarse-grained soil and gravel-clay introduced by earthquake plays an important role in analyzing the stability of the impermeable structure in rock filling dams. Using the NGI-type simple shear apparatus, cyclic simple shear tests were performed to study the mechanical behaviors of the interface; shear deformations both along the interface and in the coarse-grained soil were analyzed. The test results indicate that: the cyclic shear strength of the interface was found approximately linearly proportional to the normal stress applied on the interface; meanwhile, the shear deformations reached their peak values in the first cycle, respectively; and the direction of initial shear stress significantly influenced the mode of relative displacement along the interface. In addition, shear shrinkage and shear dilation occurred alternately in each cycle. It also revealed that shear strength of the interface increases as the shear cycle increased and is approximately linear proportional to the normal stress level on the interface.

Monotonic and Cyclic Behavior of Chiang Mai Sand Under Simple Shear Mode

Record of damages and casualties caused by earthquakes in Thailand reveals that Chiang Mai, the second largest province of Thailand, has faced the great threat among other provinces. Triaxial tests, standard penetration tests, and multichannel analysis of surface wave have been recently performed to understand the dynamic response of Chiang Mai ground. However, the cyclic simple shear test, which could convincingly represent the real seismic ground shaking under repeated horizontal shear force, has not been conducted yet. Therefore, this paper aims to characterize the behaviors of Chiang Mai sand under monotonic and cyclic loadings using a direct simple shear apparatus. Sand specimens taken from the Ping River were prepared by dry deposition technique. The mobilized friction angle at the critical state measured by the direct simple shear test is equal to 37.3° which is 9 % less than that measured by direct shear box test. This paper also provides the appropriate dynamic properties of Chiang Mai sand with the shear strain level <2 % for any practical purpose. Test results showed that the normalized shear modulus agrees well with other investigators while the damping ratio is noticeably smaller than the lower-bound range for sand reported in the past studies.

Static and dynamic properties of a lightly cemented silicate-grouted sand

This paper describes the results of an experimental investigation on the behaviour of a lightly grouted loose medium sand under both monotonic and cyclic loading. The experimental programme in the present study was carried out on both untreated and treated sand specimens stabilized with a silica-based grout, and comprises isotropic triaxial compression tests equipped with bender elements, drained triaxial monotonic shearing tests, and undrained cyclic tests in a simple shear (SS) apparatus. The results highlight that the weak cementation level induced by chemical treatment was sufficient to moderately increase small-strain stiffness and stress–dilatancy of the grouted sand during drained monotonic shear. A small cohesion intercept was observed in the analyzed failure envelope, while cyclic liquefaction resistance exhibited a much more significant increase, with a different pattern of behaviour between low and high stress levels. Finally, undrained cyclic SS test results evidenced that silicate grout employed in this research improves undrained cyclic shear strength in a manner similar to densification of the same material up to a density index of approximately 75%.

Evolution of microstructure and texture during planar simple shear of magnesium alloy

A rolled magnesium alloy was deformed using a planar simple shear apparatus, in which prismatic slip is expected to be a major deformation mode. Examination of the microstructure and texture indicated considerable tension twinning activity. Simulation of flow stress and texture using the viscoplastic self-consistent model was able to reproduce this twinning activity. Although the deformation twinning had a large influence on the microstructure and texture, the flow stress was mostly controlled by prismatic slip.