Ring Shear Research Articles

Hydrothermal clays and their influence on slope stability in residual soils of Southern Brazil

Clayey veins may develop in areas of hydrothermal activity by seepage of hot water through the soil and rock. In a slope located in Southern Brazil, such veins occur along the slip surface of some landslides. This paper aims to characterize this kind of clay and investigate its influence over the stability of that slope. An electro resistivity survey had been conducted to characterize the soil profile, and SPT boreholes were used to confirm geophysical data. An experimental laboratory campaign included the investigation of chemical, mineralogical, and geotechnical properties of these clay veins and the embedding residual soil. Drained shear strength of residual soil was measured through triaxial tests, and residual strength was measured for both materials using a ring shear apparatus. A series of limit equilibrium analyses were conducted to assess the role of clay veins on the slope stability. The geometrical discretization of veins is a difficult task, and an approach based on the assumption of overall shear strength is required to establish the operational shear strength parameters for this slope. A series of back analyses showed that failure surfaces consistent with those observed in the field were only reproduced when the adopted shear strength parameters of the soil mass were c’ = 5 kPa and φ’ = 20°. Considering the measured shear strength parameters of the hydrothermal clay c’ = (0 and φ’ = 8.5°) and the pattern in which the clay veins occur, it seems that they do not fully control the slope stability, but they significantly reduce the global cohesion and the average mass friction angle.

Discrete element modeling of switchgrass particles under compression and rotational shear

Switchgrass is a perennial herbaceous plant regarded as a biomass energy crop in the United States for its high adaptability and yield potential. Processing and handling of switchgrass particles are challenging due to the erratic mechanical and flow behavior originating from their intrinsic particulate properties. In this work, we present a bonded-sphere discrete element model designed specifically for switchgrass particles. The model simultaneously captures three key particulate features, i.e., fibrous particle shapes, a wide range of particle sizes, and particle deformability. Realistic yet computationally efficient particle shape templates are created based on the image analysis data of switchgrass specimens. A fitting procedure is proposed to ensure both the particle width and length distributions are captured, a unique requirement for fibrous particles. Two full-scale numerical models, i.e., a uniaxial compression model and a Schulze ring shear model, are developed using information from physical experiments. The model is calibrated using experimental data of chopped-small switchgrass specimens, and then, is validated using data of chopped-large specimens in both compression and ring-shear tests. Numerical results show that the numerical models capture bulk densities accurately (with an error of 3%) while slightly underestimate the bulk friction angle. Furthermore, an extensive sensitivity analysis reveals that (1) switchgrass particles with rougher edges (due to different processing techniques) exhibit a higher shear strength and a lower flowability; (2) stiffer particles yield a lower bulk density (up to 21% lower) compared to more deformable particles, indicating particle deformability should be incorporated when modeling biomass flow in a preprocessing system.

Analysis of the Possible Reactivation of the Krbavčići Landslide in Northern Istria, Croatia

The Krbavčići landslide occurred in January 1979 near the town of Buzet, Croatia, after a long period of heavy rainfall. It is located in Northern Istria in the area built of flysch rock mass where numerous mass movements in the past and recent history have been recorded. A flysch rock mass is highly susceptible to weathering, which leads to material disintegration, changes in geotechnical properties, and shear strength decrease, finally resulting in instability processes in flysch slopes. This paper describes existing information about the Krbavčići landslide occurrence, laboratory testing of siltstone samples from a flysch rock mass, and numerical slope stability analyses of a possible landslide reactivation caused by possible long rainy periods and further weathering of the flysch rock mass. Slope stability analysis using the Rocscience, Slide software, as well as landslide numerical simulations using the LS-Rapid simulation software were performed on the basis of the digital elevation model (DEM) and laboratory test results of siltstones with different weathering grades. A DEM of the Krbavčići landslide was obtained on the basis of the unmanned aerial vehicle (UAV) survey conducted in March 2016. The residual shear strength of siltstones to predict a reactivation of landslides is of highest importance and was determined by ring shear and direct shear tests on siltstone samples with different weathering grades. The results of the numerical simulations show that an increase of the groundwater level in the landslide body in combination with the further weathering of the flysch rock material at the sliding surface would have the main influence on a possible landslide reactivation and the further development of the landslide displacement.

Monotonic behavior of interface shear between carbonate sands and steel

The characteristic of interface shear between carbonate sands and steel is important for the design of onshore/offshore foundations in coral reef deposit. By using an interface ring shear apparatus, the influences of particle size, normal stress, steel roughness and shear displacement on the interface shear strength were studied in this paper. During the tests, the particle movement localized in the deformation band was observed and the degree of particle breakage was evaluated. The test results showed that under the effect of large displacement interface shear (≥ 1 m), a significant particle breakage occurred in the thin zone near the interface, and the roughness of steel interface decreased accordingly. The relative smooth steel surface shows small interface friction angle without dilation phenomenon during shearing, while higher roughness leads to higher interface strength due to stress dilation. It was also found that there was a kind of particle size discontinuity in the shear zone. Particle breakage is main reason for the increase in interface friction angle in large displacement shear. A new dimensionless parameter was proposed to consider the influences of particle breakage and interface roughness on the mobilization of interface strength between sands and steel. It is recommended that the influence of fine particles embedded into steel interface should be considered in large displacement shear for carbonate sands.

A hybrid particle-geometric scaling approach for elasto-plastic adhesive DEM contact models

The computation time of Discrete Element Method (DEM) simulations increases exponentially when particle size is reduced or the number of particles increased. This critical challenge limits the use of DEM simulation for industrial applications, such as powder flow in silos. Scaling techniques can offer a solution to reduce computation time. In this paper, we have developed a hybrid particle-geometric scaling approach with a focus on Elasto-Plastic Adhesive contact models. It established relationships between particle scaling factors and DEM contact input parameters. The isolated effects of varying particle size and geometric dimensions on bulk properties were also evaluated using uniaxial consolidation, static angle of repose, and ring shear tests. This paper shows how the particle scaling can be applied together with geometric scaling to incorporate two important aspects of bulk materials, their Elasto-Plastic behaviour and their cohesive forces.

The effects of flocculation on the entrainment of fluid mud layer

Flocculation is an integral part for cohesive sediment transport processes as it will further influence other processes such as settling, deposition as well as erosion processes. To understand the impacts of flocculation on entrainment process of fluid mud with different salinities, a series of entrainment and sedimentation experiments were conducted. Entrainment experiments were conducted under constant hydrodynamic conditions driven by a shear ring on the top of an annular flume. While the sedimentation experiments were conducted within cylinders with different diameters. Results show that fluid mud samples with different salinities can form different distributions of sediment concentration in the vertical direction, which will significantly influence the entrainment rate of fluid mud. Results also show that entrainment rate of fluid mud with higher salinity (E18) is smaller than that of fluid mud with lower salinity (E4) in the upper layer of fluid mud. Sedimentation tests show that the settling rate of fluid mud sample with higher salinity is enhanced and the gelling concentration is reached at a lower sediment concentration. These two mechanisms cause the different distributions of sediment concentration for fluid mud under different flocculation states. Accordingly, the mud characteristics such as density, viscosity, yield strength, buoyancy effects and eventually the entrainment rate vary with different fluid mud samples (the initial sediment concentrations are the same). An effort has also been made to predict the entrainment rate with existing equation. The good agreement between from equation and experimental results further demonstrates that the flocculation process should be considered when calculating the entrainment rate, as accurate prediction of the entrainment rate of fluid mud requires the details of sediment concentration and rheological properties of fluid mud in the vertical direction.

A density dependent Drucker-Prager/Cap model for ring shear simulation of ground loblolly pine

Integrated biorefineries suffer from equipment down-time due to poor understanding of the flowability of biomass-derived feedstock. This work presents the characterization of the mechanical behavior of ground loblolly pine by combining physical experiments and numerical simulations. A modified Drucker-Prager/Cap (MDPC) model enhanced with density dependence is used for finite element modeling. Cyclic oedometer tests with increased magnitude of compressive stress and Schulze ring shear tests are performed to characterize material compressibility and shear behavior. The laboratory tests yield stress-strain curves, density-stress curves and Mohr-Coulomb shear envelopes, which are then used directly and indirectly to calibrate elasticity, as well as cap and shear failure parameters of the MDPC model. Full size 3D Schulze ring shear tests are modeled in Abaqus with calibrated model parameters, and the simulated Mohr-Coulomb envelops match well against experimental results, indicating the stress state within the shear band of the ring shear test is close to triaxial compression. The modeling results also show the MDPC model does not predict dilation followed by constant volume as observed by experiments. The mechanical behavior derived from the experiments and the methodology proposed for model calibration and stress state characterization provide a foundation for studying biomass flow behavior in material feeding and handling equipment.

Mechanical Behaviour of Atrazine-Contaminated Clay

Atrazine (ATZ) is one of the most heavily used types of herbicide that is currently applied in the agricultural industry all around the world, especially Australia and the United States. This study investigates the effect of atrazine contamination on the mechanical characteristics of two Western Australian natural clays and one commercial type of clay. A series of the Atterberg limit, compaction, and torsional ring shear tests were performed on the clays contaminated with 2, 4, and 6% atrazine content. The results showed that increasing the atrazine content led to a reduction in both liquid limit (LL) and plastic (PL) of the tested soils. Similarly, the optimum moisture content (OMC) and maximum dry density (MDD) decreased by increasing the atrazine in all tested clays. The ring shear results showed that the peak shear strength and residual stress ratio of the clays decreased by increasing the contamination. Also, the results showed that atrazine contamination caused an increase in cohesion and a decrease in the friction angle of the tested soils. Also, longer periods of contamination caused a reduction in strength characteristics of the tested soils.

Investigation of calcium hydroxide powder for thermochemical storage modified with nanostructured flow agents

Thermal energy storage is a key component for power supply based on concentrating solar power. A promising thermochemical storage material in terms of cost-efficiency, storage density and reaction temperatures is based on the chemical reaction of calcium oxide with water. Handling of the solid materials, however, has proven to be difficult due to their powdery state particularly in dynamic storage systems. It is therefore proposed in the presented work to modify calcium hydroxide powder with nanostructured flow agents to improve the powder's flowability. Therefore, five additives consisting of nanostructured silicon and/or aluminium oxide were mixed systematically with calcium hydroxide powder and the flowability of the mixtures was determined using a ring shear cell. Four additives caused an improved flowability of the powder with additive weight fractions of 6–12%. After thermochemical cycling of the mixtures, however, flowability of the mixtures was clearly decreased while that of the pure powder was increased. Analysis of the samples showed a correlation between growth in particle size and increased flowability. Additionally, formation of calcium silicate and/or aluminate phases led to a decrease in absolute heat release of up to 50% as measured by thermal flux calorimetry, although some of the side products seemingly add to the measured heat release by hydrating exothermally. Based on these results, stabilisation of a moderately increased particle size preferably without the addition of additives is recommended, as a sole prevention of agglomeration by means of flow agents does not result in a manageable storage material.

Stretching the limits of dynamic and quasi-static flow testing on cohesive limestone powders

Powders are a special class of granular matter due to the important role of cohesive forces. Because the flow behavior of powders depends on both their flow states and confining stresses, it is difficult to measure/quantify with only one experiment. In this study, the most commonly used characterization tests that cover a wide range of states are compared: (static, free surface) angle of repose, the (quasi-static, confined) ring shear steady state angle of internal friction, and the (dynamic, free surface) rotating drum flow angle are considered for free flowing, moderately and strongly cohesive limestone powders.The free flowing powder gives good agreement of angles among all different situations (devices), while the moderately and strongly cohesive powders behave more interestingly. Starting from the flow angle in the rotating drum and going slower, one can extrapolate to the limit of zero rotation rate, but then observes that the angle of repose measured from the static heap is considerably larger, possibly due to its special history. When we stretch the ring shear test to its lowest confining stress limit, the steady state angle of internal friction of the cohesive powder becomes comparable with the flow angle (at free surface) in the zero rotation rate limit of the rotating drum test, by defining an appropriate effective stress.

Effect of moisture content and shearing speed on shear zone structure in fine-grained soils at large displacement

Fine-grained soils are the main component of slip zones of landslides in the Three Gorges area, China. Studying the shear zone structure in fine-grained soils is helpful to evaluate landslides. In this study, the effect of moisture content and shearing speed on the shear zone structure in fine-grained soils subjected to large displacement was investigated through a series of drained ring shear tests, grain size analyses, and microscopic observations. The concentration of clay fraction around the shear planes and corresponding variations in the shear zone structure were highlighted. The results indicate that with increasing moisture content and shearing speed, the residual strength of the soil samples gradually decreased. The soil grains within the samples were redistributed during extended shearing. The clays tended to concentrate around the shear planes and decrease in number away from the shear planes, which could be caused by grain crushing and the diffusion of silty water. The most obvious concentration of clay fraction was observed at high moisture content and high shearing speed. Moreover, the effect of moisture content and shearing speed on the shear zone structure depended on the grain size. Increasing moisture content and shearing speed resulted in a smaller median grain size and ratio of the amount of sand fraction after shearing to before shearing, while a larger ratio of clay fraction. The corresponding change in the shear zone structure caused significant deterioration in the shear strength and porosity of shear zone soils, to the detriment of landslide stability.

Flow Function of Pharmaceutical Powders at Low-Stress Conditions Can Be Inferred Using a Simple Flow-Through-Orifice Device

Multiple pharmaceutical powder processes operate at stresses lower than utilized in typical lab-scale shear cell testing. To bridge this gap, we developed a method to determine intrinsic powder flow properties, in particular, flow function (FFc), under such low stresses. A simple, commercially-available flow-through-orifice device (Flodex™ apparatus) was selected. By developing a theoretical framework using Jenike’s radial stress field analysis, the major principal stress and FFc of the tested powder can be derived from the otherwise empirical “Flodex” experiment. This method was applied to 10 distinct pharmaceutical powders. The major principal stresses associated with the test were estimated to be in the order of 100 Pa, significantly lower than what is achievable using shear cell-based methods. The resulting FFc values are generally consistent with the data extrapolated from ring shear testing. We showed that for pharmaceutical powders, FFc decreases with decreasing consolidation stress, but the values are always greater than 1. Therefore, the threshold for poor/acceptable flowability (by FFc) should be used with caution at low-stress conditions. Through this work, we showed that by integrating the radial stress field theory with a simple flow-through-orifice experiment, intrinsic powder flow properties under low stresses could be reliably determined.

DEM calibration of cohesive material in the ring shear test by applying a genetic algorithm framework

This research demonstrates capturing different stress states and history dependency in a cohesive bulk material by DEM simulations. An automated calibration procedure, based on the Non-dominated Sorting Genetic Algorithm, is applied. It searches for the appropriate simulation parameters of an Elasto-Plastic Adhesive contact model such that its response is best fitted to the shear stress measured in experiments. Using this calibration procedure, the optimal set of DEM input parameters are successfully found to reproduce the measured shear stresses of the cohesive coal sample in two different pre-consolidation levels. The calibrated simulation resembles the stress history dependent values of shear stress, bulk density and wall friction. Through the case study of the ring shear tester, this research demonstrates the robustness and accuracy of the calibration framework using multi-objective optimization on multi-variable calibration problems irrespective of the chosen contact model.

Particle breakage of uniformly graded carbonate sands in dry/wet condition subjected to compression/shear tests

The behaviour of a granular material is primarily affected by its particle size distribution (PSD), which is not necessarily a soil constant as assumed in traditional soil mechanics. The PSD may change over time due to mechanical as well as environmental actions. In this study, a series of ring shear tests and one-dimensional compression tests were completed on carbonate sand, in both dry and saturated conditions. Samples were prepared with different initial uniform gradings, to investigate: (1) the influence of the saturation state and initial grading on mechanical and deformational behaviour of carbonate sands and (2) the evolution of the PSD as a result of breakage. The ring shear tests show that the residual friction angle remains almost constant, but dilatancy reduces with increasing saturation degree. In the one-dimensional compression test, the yield stress decreases with increasing saturation degree, but the compressibility (as defined by Cc) remains almost constant, irrespective of the saturation state. Moreover, saturated samples suffer more breakage than dry samples during ring shear tests, while there is no obvious effect of saturation state on particle breakage in one-dimensional compression. A recently proposed PSD model with only two parameters (λp and κp) is employed to model the evolution of PSD, as it can more broadly capture the whole PSD throughout the breakage process than existing breakage indices. Test results demonstrate that parameter λp is linearly related to Einav’s breakage index $$ B_{\text{r}}^{*} $$ and is dependent on initial grading, but independent of test mode. Parameter κp is in power relationship with $$ B_{\text{r}}^{*} $$ and is independent of initial grading or test mode. The evolution of parameters λp and κp is related to the input work for both ring shear and compression tests, with λp being hyperbolically related to input work and κp in power relationship with input work. Using such an evolution law provides an alternative approach to capture the effects of particle breakage in constitutive models.

Frictional properties of actinolite-chlorite gouge at hydrothermal conditions

Exhumed subduction zones frequently show widespread actinolite-chlorite (Act-Chl) dominated compositions, suggesting that this may play an important role in controlling megathrust fault slip. We investigate the frictional behavior of simulated Act-Chl (85:15) gouge mixtures derived from natural mafic metamorphic rocks, under hydrothermal conditions using a ring shear deformation apparatus. Experiments were performed at effective normal stresses (σneff) of 50–200 MPa, pore fluid pressures (Pf) of 50–200 MPa, at temperatures (T) of 23–600 °C. In each experiment we applied a shear displacement (x) of ~10 mm at a constant sliding velocity (v) of 10 μm/s, followed by v-stepping in the range 0.3–100 μm/s, and slide-hold-slide (SHS) tests with hold times (t) ranging from 3 s to 3000 s. We quantified the rate- and state-dependent friction parameter (a-b), and investigated the effect of t on fault healing (Δμpk). The results showed no effects of temperature on the coefficient of friction (μ ≈ 0.6–0.7), or on (a-b), with some experiments showing persistent, displacement-hardening or -weakening trends. Nonetheless, effects of v and of normal stress (σn = σneff + Pf) on (a-b) fall into three temperature regimes: (1) T = 23–100 °C, (2) T = 200–400 °C, and (3) T = 500–600 °C. In Regimes (1) and (3), (a-b) > 0 for all conditions tested, whereas in Regime (2), (a-b) ≤ 0, at σneff = Pf = 50 MPa and v = 0.3–3 μm/s. We discuss on the origin of persistent displacement-hardening or -weakening trends observed, and assess the implications of our data for subduction zone seismogenesis. Extrapolation of (a-b)-data using multiple linear regression suggests that high pore pressure ratios (>0.9) are needed to promote seismogenesis in faults cutting actinolite-chlorite compositions.

Soil-solid interface shear strength review and its possibility on interlayer slope stability analysis

A landslide has been one of the many problems in geotechnical engineering, whereas, from one of the cases, a failure happened at the interface layer between the overburden and its underlying soil. As interface shear strength has become one of the growing topics in research, this paper summarizes and discusses the research development of interface shear strength and its possibility to explain the interface shear behavior at a slope. The discussion was limited to cohesive soils and experiment-based behavior of the interface shear. Some research has been selected in order to understand the development of interface shear strength through time and two examples of slope interlayer shear behavior were selected. It was known that there are three common tests of interface shear behavior used: simple shear, direct shear, and ring shear test. The development of interface shear strength started at 1960s between soil and construction materials where the four major components were defined. As the research grows, many other types of soils, interfaces, and the effect of the tests became the topic of the research. In the end, the examples given from modeling the interface shear behavior from a slope gives a new perspective of cases for interface shear strength in slope analysis.

Physicochemical effect on shear strength characteristics of clayey soils based on ring-shear experiment

Pore fluid chemistry can significantly influence the shear strength characteristics of a clayey soil. To explore the underlying mechanisms, a series of ring shear experiments are performed on two natural clays, which represent two typical types of clayey minerals, i.e., expansive montmorillonitic clay and low-plasticity kaolinitic clay. The effects of pore solution concentrations on the shear strength of the two clays are experimentally characterized. It is shown that the shear strength of the expansive clay can be significantly influenced by the pore solution chemistry, whereas that of the low-plasticity clay proves to be relatively insensitive to it. To capture the main features of the shear strength behavior of clayey soils, the concept of intergranular stress, which is an extension of the Terzaghi’s effective stress to incorporate physicochemical effect, is introduced to interpret the experimental data. It is found that the evolution of residual shear strength can be very well characterized by using the intergranular stress, showing that the proposed intergranular stress formulation can be used alternatively to describe the stress state of clayey soils saturated with various pore solutions.

Calibration of the discrete element method: Strategies for spherical and non-spherical particles

The discrete element method (DEM) is a numerical tool used for the modelling of granular materials. However, accurate predictions of material behaviour can only be made if the input parameter values are carefully selected. Several laboratory experiments were used to measure the bulk behaviour of corn grains and then used to calibrate the parameter values using a number of different strategies for spherical (with rolling resistance) and non-spherical particles (with and without rolling resistance). The contact stiffness was calibrated using a confined compression test, and the coefficients of sliding and rolling friction were calibrated using either the individual or combined results from a draw down, rotating drum and annular ring shear test. The results showed that the draw down test alone was sufficient to calibrate the coefficients of sliding and rolling friction, this was confirmed by the results from the rotating drum and shear tests.

Creep properties of clastic soil in a reactivated slow-moving landslide in the Three Gorges Reservoir Region, China

Most slow-moving landslides in the Three Gorges Reservoir (TGR) region of China are characterized by pre-existing shear surfaces. The large deformation within the shear zones usually gives rise to clastic soil formation. The creep properties have large influence on the kinematic feature of landslides. In this paper, we report an in-situ direct shear creep test carried out in the shear zone of a reactivated slow-moving landslide in the TGR region. Correspondingly, some laboratory ring shear creep tests are carried out to interpret the movement pattern of this landslide. The shear zone soil exhibits similar non-attenuating creep responses in both the in-situ direct shear and laboratory ring shear creep tests. At the same stress level, however, the in-situ direct shear creep test yields a larger rate of creep displacement due to shearing along the landslide direction. In the ring shear creep tests, at the prepeak stage, the critical creep stress that triggers creep failure is slightly lower than the peak shear strength but much larger than the residual strength; at the postfailure stage, the critical creep stress of the shear-zone soil is equal to the residual shear strength. The rate-dependent residual shear strength may account for the stepwise movement pattern of the landslide.

Variation in Shearing Rate Effect on Residual Strength of Slip Zone Soils Due to Test Conditions

Shearing rate effect on the residual strength of slip zone soils is considerable for sliding or reactivated landslides. It is complicated and variably affected by external inducing factors, and its variation has not attracted much attention. Using drained ring shear tests on the slip zone soils from two selected landslides with the shearing rates of 0.1–20 mm/min, this study investigated the variation in the shearing rate effect on the residual strength under the influences of the clay fraction, water content, and normal stress. In addition, the shear zone structures were analysed using microscopic observations. The results show that the residual strength decreases with increasing shearing rate (i.e., negative rate effect) and exists a good linear relationship with the logarithmic shearing rates. The slope of the fitting lines of the residual stress ratios versus shearing rates is proposed to assess the shearing rate effects. Furthermore, the effect is significantly affected by the clay fraction, water content, and normal stress. The samples with the low clay fraction exhibit a more significant negative rate effect, and the effect weakens with decreasing water content and normal stress, which are associated with the variations in the shear zone structures due to particle breakage. Additionally, increasing shearing rates and shear displacement change the shear zone structures and thus induce a negative rate effect on the residual strength of the soils.