Showing papers on "Direct shear test published in 2019"

Shear Rate Effects on the Post-peak Shear Behaviour and Acoustic Emission Characteristics of Artificially Split Granite Joints

Abstract: Rock joints may be sheared at different rates under quasi-static or dynamic loading. Understanding the mechanical response of rock joints at different shear rates is of great importance for the mitigation of dynamic geo-hazards such as earthquakes, fault slip rockbursts and landslides. In this study, direct shear tests at various shear rates (0.001–0.1 mm/s) under different normal stresses (3–40 MPa) are conducted on split granite joints, and the influences of shear rates on the shear strength, post-peak shear behaviour and acoustic emission (AE) characteristics are analysed and discussed. The research findings suggest that both peak and residual shear strengths tend to decrease with increasing shear rate. Stick–slip occurs on all the joints, during which stress drop values increase with increasing shear displacement and normal stress. The stress drop magnitudes during stick–slip decrease with shear rate, while the time intervals between stress drops during stick–slip increase with shear rate. Further, the energy rate tends to increase while the AE events decrease with increasing shear rate, which is caused by the time-dependent deformation behaviour. The AE b value decreases linearly with the shear rate on a logarithmic scale, and the influence is more significant under high normal stress conditions. The variations in the b value can reflect the evolution process (first loading at lower and then higher shear rates) of dynamic geo-hazards and can be used as an effective indicator to predict the dynamic shear failure of granite joints in a temporal sequence. The results of this study will encourage better understanding of the rate-dependent shear behaviour of rough granite joints, particularly under high normal stress, and will provide some references for the monitoring and prediction of dynamic geo-hazards with respect to the AE (or micro-seismic) technique.

Direct shear test for the assessment of rheological parameters of concrete for 3D printing applications

Abstract: Rheology of concrete plays a major role in concrete 3D printing applications, where the concrete is pumped at high pressure and extruded through a nozzle at low speed to build the structural component. The 3D printable concrete should be stiff and is different to normal or self-compacting concrete. Hence, the common testing methods used to estimate the rheological parameters are not suitable for 3D printable concrete. In this study, the direct shear test is trialled as a potential method to measure the rheological parameters of different mixes of concrete. The effects of water–cement ratio and shear rates on rheological parameters were examined. The tests were carried out with varying shear rates, ranging from 0.5 to 15 min−1, and normal stresses, ranging from 2 to 15 kPa, for mixes with water–cement ratios of 0.3, 0.4 and 0.6. Further testing was carried out on mixes with varying aggregate to cement and fine to total aggregates ratios to study the effect of binder and aggregate proportions on the rheology of mortar. It was found that the shear rates, 0.5 to 15 min−1, have little effect on the cohesion values and friction angles. Further, the behaviour of the mixes was found to be following the Mohr–Coulomb model.

Analytical Prediction of the Shear Behaviour of Rock Joints with Quantified Waviness and Unevenness Through Wavelet Analysis

Abstract: We present an analytical model for the shear behaviour of a rock joint with waviness and unevenness. The waviness and unevenness of a natural joint profile are quantitatively separated through wavelet analysis. The critical waviness and critical unevenness of a joint profile are subsequently determined. The degradation process of each-order asperity is predicted by considering the role of plastic tangential work in shear, by which the sheared-off asperity area and the dilation angle are quantified. Both the dilation angles of critical waviness and critical unevenness decay, as plastic tangential work accumulates. The analytical predictions are compared with the experimental data from direct shear tests on both regular- and irregular-shaped joints. Good agreement between analytical predictions and laboratory-measured curves demonstrates the capability of the developed model. Therefore, the model is capable of assessing the stability of rock-engineering structures with ubiquitous joints.

Shear strength components of adjustable hybrid bonded CFRP shear-strengthened RC beams

Abstract: This paper presents an experimental study on shear behavior of full-scale rectangular reinforced concrete (RC) beams under shear-strengthening of carbon fiber-reinforced polymer (CFRP) U-strips. The test involved five RC beams with transverse reinforcement, including one control beam, one shear-strengthened by externally bonded (EB) CFRP U-strips only, and the remaining three strengthened in shear by a hybrid bonded (HB) CFRP system under different normal pressures applied on the CFRP U-strips. The transverse reinforcing steel bars and CFRP U-strips were extensively instrumented with strain gauges, which enabled the accurate quantitative assessment of shear strength contributions from CFRP (Vf), stirrups (Vs) and concrete (Vc) during the whole loading process of a shear test. Further, by comparing Vs and Vc for beams with and without CFRP strengthening, the interactions can be evaluated and clarified. In addition, key features of shear behaviors, e.g. failure mode, load-deflection curves, shear capacity and ductility, shear crack angle, and strain development and distribution of stirrups as well as CFRP strips, are presented, compared and analyzed.

The effect of water content on the shear strength characteristics of granitic soils in South China

Abstract: Shear strength is crucial for slope stability and soil erosion measurement, and shearing deformation is one of the most harmful processes for land and environment degradation. The heterogeneity and anisotropy of shear behavior for granitic soils have been scarcely investigated in previous studies. The pedogenic differentiation of an intact granite weathering profile was characterized by soil properties from surface (0 ∼ 45 cm), lateritic (45 ∼ 110 cm and 110 ∼ 170 cm), sandy (170 ∼ 430 cm) and detritus layers (> 430 cm). Shear strength with response to water content variation (7% to saturation) for undisturbed soils from these layers was determined by direct shear test and its anisotropic characteristics were explored by shearing samples selected from the vertical and horizontal directions of the profile. Soils in the clay loam layer (depth 0.75, RMSE 0.05) and showed a less apparent anisotropy. The unsaturated shear strength of granitic soils determined by the cement materials and the porosity of granitic soils could be predicted by the constitutive model considering the variation of matric suction contribution to shear strength (Adj-R2>0.47; RMSE

Utilization of Cementitious Material from Residual Rice Husk Ash and Lime in Stabilization of Expansive Soil

Abstract: Geological disasters often occur due to expansion and shrinkage properties of expansive soil. This paper presents a cementitious material combined with rice husk ash (RHA) obtained from biomass power plants and lime to stabilize expansive soil. Based on compressive and flexural strength of RHA-lime mortars, blending ratio of RHA/lime was adopted as 4 : 1 by weight for soil stabilization. When mix proportion of RHA-lime mixture varied from 0% to 20%, specific surface area of stabilized expansive soil decreased dramatically and medium particle size increased. The deformation and strength properties of stabilized expansive soil were investigated through swelling test, consolidation test, unconfined compression test, direct shear test, and so on. With increase in RHA-lime content and curing time, deformation properties including swelling potential, swelling pressure, compression index, crack quantity, and fineness of expansive soil lowered remarkably; meanwhile, strength properties involving unconfined compressive strength, cohesion, and internal friction angle improved significantly. Considering engineering performance and cost, mix proportion of 15% and initial water content of 1.2 times optimum moisture content were recommended for stabilizing expansive soil. In addition, effectiveness of RHA-lime to stabilize expansive soil was achieved by replacement efficiency, coagulation reaction, and ion exchange.

Characterisation of nano magnesia–cement-reinforced seashore soft soil by direct-shear test

Abstract: In this study, the mechanical behaviour of nano magnesia–cement-reinforced seashore soft soil (NmC3S) was evaluated and characterised by the direct-shear testing of seashore soft soil (3S),...

Probabilistic approach for open pit bench slope stability analysis – A mine case study

Abstract: The geotechnical slope design of an open pit wall starts at the bench scale configuration. At this scale, the rock slope stability is governed primarily by the geological discontinuities within the rock mass and as a result, structurally-controlled failures (e.g. planar, wedge or toppling) are most likely to occur. The probabilistic approach offers a major advantage over the traditional deterministic method in that it accounts for the different degrees of variability and uncertainty often encountered in rock properties. This paper presents a bench slope stability assessment for an open pit mine in Peru using a probabilistic-based approach by coupling a kinematic analysis based on stereographic projection techniques followed by a kinetic analysis by means of the limit equilibrium method. Finally, these two probabilities are combined to provide an overall measure of the probability of failure (PoF) of the bench slope system. The case study is characterized by significant scatter in the geometrical and mechanical properties of the joints. Extensive surface mapping was conducted at 36 different sites following the ISRM suggested procedures. Several direct shear tests were carried out. It is shown that by combining field and laboratory measurements and engineering judgment, the probability density functions (PDF) of the discontinuity parameters can be obtained. These are then used in a Monte Carlo simulation process to compute both kinematic and kinetic probabilities of failure. The overall probability of failure aims to provide the design engineer with a tool to critically evaluate the bench performance from a geotechnical risk perspective and to provide a basis for future bench design optimization.

Effect of Grain Size Distribution of Sandy Soil on Shearing Behaviors at Soil–Structure Interface

Abstract: For geotechnical construction and maintenance, assessing the shearing behavior at the soil–structure interface is significant. This study presents an experimental investigation into the effect of the grain size distribution of a sandy soil on the shearing behaviors at the soil–structure interface, using a modified direct shear apparatus. Five soil samples with different coefficients of uniformity were prepared. The normalized roughness of the structure surface (the ratio between the maximum roughness of the structure plate and the mean grain size of the soil); relative density; and maximum, mean, and minimum grain sizes of all samples were controlled in the same way. During the tests, the shear force, shear displacement, and vertical displacement were monitored. The results show that at a given shear displacement and normal stress, the sample with lower coefficient of uniformity C u Cu presented higher shear stress and more pronounced dilative behavior. The increase of C u Cu led to a decrease of the friction angle for the soil–structure interface (at both peak and ultimate states) and a decrease of the maximum vertical deformation of the soil sample during the shearing process. As C u Cu increased, the main force chain at the soil–structure interface turned from contact between the coarser grains to that mainly formed by the finer grains, resulting in a decrease of the shearing resistance. In comparison with the previous relevant studies, the decreasing trend of the friction angle with the increase of C u Cu is strongly supported.

Mechanical properties and associated seismic isolation effects of foamed concrete layer in rock tunnel

Abstract: Foamed concrete has a good energy absorption capability and can be used as seismic isolation material for tunnels. This study aims to investigate the mechanical properties and associated seismic isolation effects of foamed concrete layer in rock tunnel. For this, a series of uniaxial/triaxial compression tests was conducted to understand the effects of concrete density, confining stress and strain rate on the mechanical properties of foamed concrete. The direct shear tests were also performed to investigate the effects of concrete density and normal stress on the nonlinear behaviors of foamed concrete layer-lining interface. The test results showed that the mechanical properties of foamed concrete are significantly influenced by the concrete density. The foamed concrete also has high volumetric compressibility and strain-rate dependence. The peak stress, residual stress, shear stiffness and residual friction coefficient of the foamed concrete layer-lining interface are influenced by the foamed concrete density and normal stress applied. Then, a crushable foam constitutive model was constructed using ABAQUS software and a composite exponential model was also established to study the relationship between shear stress and shear displacement of the interface, in which their parameters were fitted based on the experimental results. Finally, a parametric analysis using the finite element method (FEM) was conducted to understand the influence of foamed concrete layer properties on the seismic isolation effect, including the density and thickness of the layer as well as the shear stiffness and residual friction coefficient of the interface. It was revealed that lower density and greater thickness in addition to smaller shear stiffness or residual friction coefficient of the foamed concrete layer could yield better seismic isolation effect, and the influences of the first two tend to be more significant.

Crystallographic preferred orientations of ice deformed in direct-shear experiments at low temperatures

Abstract: . Synthetic polycrystalline ice was sheared at temperatures of −5 , −20 and −30 ∘ C, to different shear strains, up to γ=2.6 , equivalent to a maximum stretch of 2.94 (final line length is 2.94 times the original length). Cryo-electron backscatter diffraction (EBSD) analysis shows that basal intracrystalline slip planes become preferentially oriented parallel to the shear plane in all experiments, with a primary cluster of crystal c axes (the c axis is perpendicular to the basal plane) perpendicular to the shear plane. In all except the two highest-strain experiments at −30 ∘ C, a secondary cluster of c axes is observed, at an angle to the primary cluster. With increasing strain, the primary c -axis cluster strengthens. With increasing temperature, both clusters strengthen. In the −5 ∘ C experiments, the angle between the two clusters reduces with strain. The c -axis clusters are elongated perpendicular to the shear direction. This elongation increases with increasing shear strain and with decreasing temperature. Highly curved grain boundaries are more prevalent in samples sheared at higher temperatures. At each temperature, the proportion of curved boundaries decreases with increasing shear strain. Subgrains are observed in all samples. Microstructural interpretations and comparisons of the data from experimentally sheared samples with numerical models suggest that the observed crystallographic orientation patterns result from a balance of the rates of lattice rotation (during dislocation creep) and growth of grains by strain-induced grain boundary migration (GBM). GBM is faster at higher temperatures and becomes less important as shear strain increases. These observations and interpretations provide a hypothesis to be tested in further experiments and using numerical models, with the ultimate goal of aiding the interpretation of crystallographic preferred orientations in naturally deformed ice.

Shear strength of an unsaturated loam soil as affected by vetiver and polyacrylamide

Abstract: Shear strength is an important soil mechanical property that its improvement is one of the rehabilitation program for the reduction of soil erosion and degradation. The objective of this study was to investigate the effect of polyacrylamide (PAM) and the vetiver system as a cheap and long-term bioengineering method and their combination on unsaturated shear strength parameters (effective cohesion, c' (kPa), angle of effective internal friction, φ' (°) and angle of internal friction related to matric suction, φb (°)) of a loam soil. The experimental treatments included vetiver plant (VP0), two concentrations of PAM dissolved in water [0.2% (V0P2) and 0.4% (V0P4)], and simultaneous presence of vetiver and two concentrations of PAM (VP2 and VP4). Direct shear tests were performed at combinations of three normal stresses of 25, 50 and 100 kPa and four matric suctions of 0, 10, 30 and 50 kPa (12 tests per each treatment) to determine the shear strength parameters. It was found that vetiver and PAM decreased apparent angle of internal friction (φ) and φ', and increased c', total cohesion (c), φb, and as a result increased unsaturated shear strength. However, the positive effect of vetiver on shear strength was greater than that of PAM. It seems that PAM and vetiver enhanced the contact area, inter-particles bonds, aggregation, and inter-aggregate porosity, and as a consequence, increased the soil effective saturation, effective stress, cohesion and shear strength. Also, when matric suction increased the c and shear strength increased although the φ increased slightly. Simultaneous application of vetiver and 0.4% of PAM resulted in maximum shear strength, indicated although vetiver can increase the cohesive strength, it would be more pronounced when PAM was simultaneously applied. Simultaneous application of vetiver and PAM through increasing soil shear strength can be suggested as the rehabilitation program to reduce soil erosion and degradation.