Shear Dilation Research Articles

Mechanical properties of multi-recycled aggregate concrete under combined compression-shear loading

The mechanical properties and strength failure criteria of multi-recycled aggregate concrete (multi-RAC) under combined compression and shear loading states are investigated in this paper. The peak shear strength, peak shear displacement, and failure patterns are compared under different regeneration cycles and normal compressive stress ratios. The results reveal that both the peak shear strength and peak shear displacement increase with the increased normal stress ratio. The shear failure pattern with higher severity corresponds to more spalling powder and debris deposited on the shear fracture surface. When the regeneration cycles of coarse aggregate increase, the peak shear strength decreases and the descending trend become more evident with the higher vertical compressive stress ratio. Under the normal compressive stress, contact friction strength is the dominant component of peak shear strength among the cohesive strength, contact friction strength, and shear dilation strength. Based on different stress expressions, three compression-shear failure criterion models considering the regeneration cycles of coarse aggregate under planar stress state were established for RAC. The stress invariance failure criterion model and octahedral stress failure criterion model in quadratic parabolic functional form can provide high prediction accuracies.

Triaxial Testing and Numerical Simulation on High Fill Slopes of Gobi Gravel Soils in Urumchi

Abstract Gobi gravel soils (GGS), widely distributed in northwestern China, are used as main filling materials in construction of high filling slopes in Urumqi airport. To analyze the stability of slopes, the shear strength of the granular materials should be investigated. To this end, a series of large-scale triaxial tests were performed on GGS to investigate the influencing factors on their mechanical behavior, including coarse content, degree of compaction, and confining pressure. The results show that the stress-strain curves show strain-hardening behavior within 90 % coarse content. The specimens show a transition from shear contraction to shear dilation with increasing coarse fraction and degree of compaction; they show contraction with an increase of confining pressure. The interlocking strength is significantly affected by the degree of compaction and coarse content, whereas the frictional angle changes within a narrow range from 35° to 40°. Based on testing results, the stability of the filling slopes was simulated using limit equilibrium method. The effect of the slope angle and geotextiles on the safety factor is discussed, through which the optimum slope angle and optimum geotextiles reinforcement are determined.

Microscopic mechanism analysis of influence of high effective confining pressure on mechanical properties of hydrate-bearing sediments

It was discovered in tests that the increase in effective confining pressure could have a great impact on the mechanical behavior of hydrate-bearing sediment, which was not only decreased the internal friction angle and dilatancy, but also enhanced cohesion and strength. Significant crushing of the hydrate-host sand was founded after shearing under high pressure. In this study, a discrete element method (DEM) model considering the effects of particle breakage and hydrate repolymerization was proposed, and a series of numerical triaxial tests were performed to shed light on the mechanism of mechanical properties evolution observed in the physical tests. The numerical results revealed that the breakage of host particles was a leading cause of the decrease in the internal friction angle and transformation from shear dilation to contraction. As the hydrate saturation, axial strain, and effective confining pressure increased, the force chain network of the model became denser and the force magnitude on the chains became increasingly uniform, which leads to the increase in model strength; the number and force of bonded contact also increased gradually, resulting in increased cohesion on the macro scale.

Shear strength deterioration effect of rock mass joint surface under cyclic shear load

Understanding the shear strength degradation mechanism of a rock mass joint surface under cyclic shear load and determining a corresponding analytical model is an important foundation for accurately evaluating the safety of rock mass engineering under seismic loads. It is worth noting that, to date, there has been a dearth of studies on the strength characteristics of joint surfaces that consider the number of loading cycles, normal load, and initial undulant angle of the structural plane. In this study, focused on the behaviour of sandstone, the particle flow code (PFC) modelling framework was used to simulate a joint surface cyclic shear test considering first- and second-order undulations. Based on the experimental results, the comprehensive effects of the number of cyclic shear cycles, the normal stress, first- and second-order undulation and the dilatancy angle on shear stress during cyclic shear were analysed. Formulas for the joint surface shear basic friction angle and dilatancy angle under cyclic shear were proposed, and a method for calculating the joint surface peak shear strength under cyclic shear considering the deterioration of the dilatancy angle and basic friction angle was established. The peak shear strength of a sample after five cycles of shearing was calculated using the proposed formula and compared with the results of numerical simulations, the Barton method, and the Homand method. The results showed that the calculated values have good consistency with the results of the numerical simulations, demonstrating the effectiveness and accuracy of the proposed formula. However, under a low normal stress, there could be errors in estimating the cyclic shear strength of the joint surface.

Anti-liquefaction Performance of Stacked Sandbags: Shaking Table Tests and Numerical Simulations

Stacked sandbags are popularly used as building foundations and reinforcing slopes because of their high compression bearing capacity. However, their anti-liquefaction performance under seismic vibrations has not been well studied before. This paper carefully explored the anti-liquefaction performance of stacked sandbags by shaking table tests and numerical simulations. The testing results prove that the pore-water pressures and excess pore pressure ratios inside stacked sandbags were much lower than those of a traditional sand foundation. The deeper the sandbag is, the lower the pore-water pressure is. Therefore, stacked sandbags have evident anti-liquefaction performance. The simulated results prove that the effective stresses of the sand inside the sandbags were substantially higher, and the tensile stresses of the sandbag packages play an essential role in the anti-liquefaction performance. The greater tensile stress inside the sandbag results in better anti-liquefaction performance, and the anti-liquefaction performances at the bag edges are better than those at the center places. The elastic modulus and thickness of the bags are also beneficial for anti-liquefaction performances. Furthermore, because of the tensions of the packages, the sand shows shear dilation, not a shear contraction, under horizontal vibrations, especially at the edges of sandbags. The simulated results show that the settlements of the stacked sandbags are small and evenly distributed during the vibration. Therefore, stacked sandbags are appropriate for buildings and slopes located in seismic areas.

Shear Strength and Dilatancy of Calcareous Sand in the South China Sea

Shear Strength and Dilatancy of Calcareous Sand in the South China Sea

The Brittle–Ductile Transition and the Formation of Compaction Bands in the Savonnières Limestone: Impact of the Stress and Pore Fluid

Carbonate sediments play a prominent role on the global geological stage as they store more than 60% of world’s oil and 40% of world’s gas reserves. Prediction of the deformation and failure of porous carbonates is, therefore, essential to minimise reservoir compaction, fault reactivation, or wellbore instability. This relies on our understanding of the mechanisms underlying the observed inelastic response to fluid injection or deviatoric stress perturbations. Understanding the impact of deformation/failure on the hydraulic properties of the rock is also essential as injection/production rates will be affected. In this work, we present new experimental results from triaxial deformation experiments carried out to elucidate the behaviour of a porous limestone reservoir analogue (Savonnières limestone). Drained triaxial and isotropic compression tests were conducted at five different confining pressures in dry and water-saturated conditions. Stress–strain data and X-ray tomography images of the rock indicate two distinct types of deformation and failure regimes: at low confinement (10 MPa) brittle failure in the form of dilatant shear banding was dominant; whereas at higher confinement compaction bands orthogonal to the maximum principal stress formed. In addition to the pore pressure effect, the presence of water in the pore space significantly weakened the rock, thereby shrinking the yield envelope compared to the dry conditions, and shifted the brittle–ductile transition to lower effective confining pressures (from 35 MPa to 29 MPa). Finally, permeability measurements during deformation show a reduction of an order of magnitude in the ductile regime due to the formation of the compaction bands. These results highlight the importance of considering the role of the saturating fluid in the brittle–ductile response of porous rocks and elucidate some of the microstructural processes taking place during this transition.

Discrete Element Simulation of the Macro-Meso Mechanical Behaviors of Gas-Hydrate-Bearing Sediments under Dynamic Loading

Under the action of dynamic loadings such as earthquakes and volcanic activities, the mechanical properties of gas-hydrate-bearing sediments will deteriorate, leading to a decrease in the stability of hydrate reservoirs and even inducing geological disasters such as submarine landslides. In order to study the effect of dynamic loading on the mechanical properties of hydrate sediments, triaxial compression tests of numerical specimens were carried out by using particle flow code (PFC2D), and the macro-meso mechanical behaviors of specimens were investigated. The results show that the loading frequency has a small effect on the stiffness of the hydrate sediment, while it has a large effect on the peak strength. The peak strength increases and then decreases with the increase in loading frequency. Under the same loading frequency, the peak strength of the hydrate sediment increases with the increase in loading amplitude, and the stiffness of the specimen decreases with the increase in loading amplitude. The maximum shear expansion of the specimen changes with the movement of the phase change point and the rearrangement of the particles. The maximum shear expansion of the specimen changes with the movement of the phase change point and the change of the bearing capacity of the particles after the rearrangement, and the more forward the phase change point is, the stronger the bearing capacity of the specimen in the plastic stage. The shear dilatancy angle and the shear dilatancy amount both increase linearly with the increase in loading amplitude. The influence of loading frequency and amplitude on the contact force chain, displacement, crack expansion, and the number of cementation damage inside the sediment is mainly related to the average axial stress to which the specimen is subjected, and the number of cracks and cementation damage of the sediment specimen increases with the increase in the average axial stress to which the sediment specimen is subjected. As the rate of cementation damage increases, the distribution of shear zones becomes more obvious.

Deformation and Strength Characteristics of Structured Clay under Different Stress Paths

In geotechnical applications such as deep foundation excavation, slope cutting, and tunnel construction, the stress state of the soil subjected to loading or unloading will change and undergo different stress paths with corresponding changes in its mechanical properties. Therefore, a series of comprehensive laboratory tests were conducted on the natural strong structured clay to evaluate the effects of stress paths on the stress-strain, pore water pressure-strain, deformation characteristics, shear strength behaviors, and the microscopic mechanism of the natural strong structured clay. The results show that the mechanical properties of structured clay under different stress paths are quite different and are closely related to its structure. The peak deviatoric stresses of the soil under both the compression path and the tensile path are affected by the stress path, and the order of its strength is INP > KCP > DEP. The variations of the pore water pressure reflect the shear dilation and the shear contraction characteristics of the soil under different stress paths; the pore pressure of the specimen is particularly complicated in the case of both axial stress and lateral stress unloading; therefore, the pore water pressure in the soil under unloading conditions should be monitored in real time in engineering practice. The stress path affects the total stress strength indices of the structured clay to a significant extent and the effect on soil cohesion is greater than that on the internal friction angle, and the microstructure of the specimens has been changed after shearing tests with different stress paths. Therefore, design calculations should be conducted according to the experimental mechanical parameters under the stress path experienced by the soil under the actual engineering conditions to ensure safety and stability during construction.

Coupled sliding–decohesion–compression model for a consistent description of monotonic and fatigue behavior of material interfaces

A thermodynamically consistent model proposed in this work introduces a generative symbolic framework for computational components, which can capture the inelastic behavior of interfaces with 3D kinematics in response to monotonic, cyclic, and fatigue loading histories. It contributes to a long list of existing cohesive zone models by introducing novel forms of potentials for free energy, threshold function, and dissipation to deliver the following model properties: (i) the evolution equations derived from the potentials are linked in such a way that damage develops along with the cumulative measure of plastic sliding, (ii) the convex, continuous, and smooth shape of the threshold function introduced in the effective stress domain enables a general and efficient time integration that automatically handles unloading and reloading, and (iii) the non-associative flow potential includes a scalable degree of coupling between normal and tangential damage evolution. The studies performed in the paper demonstrate that these model ingredients lead to a consistent coverage of arbitrary non-proportional loading scenarios in tangential and normal directions. For monotonic loading, the presented examples illustrate the ability of the model to reproduce the shear dilatancy, pressure sensitivity including the vertex effect, stiffness recovery in compression, and abrasion due to sliding. For cyclic fatigue loading, the studies show that the model reflects the link between the hysteretic behavior and the fatigue damage evolution. This link constitutes the basis for a physically sound description of fatigue degradation along material interfaces. It manifests itself in the presented energy breakdown with distinguished fractions of energy release due to damage and plasticity ascribed to normal and tangential directions. Two calibration and validation examples using pull-out tests exposed to monotonic and cyclic loading are provided to show the applicability of the model for the characterization of the bond between steel bars and concrete. A publicly available implementation using a computer algebra system (CAS) is provided in the form of an interactive Jupyter notebook to document the reproducibility of the results.

Study on rheological properties and polishing performance of viscoelastic material for dilatancy pad

Shear dilatancy polishing (SDP) is an emerging flat polishing method for hard and brittle materials, which can achieve high-efficiency and high-precision polishing of workpieces under high-pressure and high-speed conditions. In our research, a novel viscoelastic material (VM) with non-Newtonian fluid properties was prepared. The dilatancy pad was subsequently obtained based on VM, abrasives, and a polyurethane pad. The effects of abrasive size and abrasive concentration on the rheological properties of VM were analyzed. The mechanism of abrasives in VM was investigated, and the principle of SDP was revealed. VM-30 wt%Diamond (0.5 μm) with excellent shear hardening performance was selected in the polishing experiment to study the effects of polishing time, polishing speed, and polishing pressure on polishing performance, and the optimal polishing parameters of SDP for tungsten were determined. Compared with traditional abrasive free polishing (AFP), SDP can realize a higher material removal rate (MRR increased by 33.4%) and better surface quality (Sa reduced by 32.2%) due to the shear dilatancy effect under high-pressure and high-speed conditions. After 90 min of SDP, the surface roughness (Sa) of tungsten workpiece was reduced from 307.9 nm to 12.1 nm. Results show that SDP is a promising and feasible polishing method for hard and brittle materials.

The mechanical behaviour of Bolders Bank till

Advanced sampling and testing techniques are applied to assess the mechanical behaviour of a stiff, silt-and-clay dominate, stony, low plasticity UK till whose genesis and post-depositional history imparted complex profiles of yielding pressures and shear strengths that differ considerably from those of K0 consolidated waterborne sediments. It is shown that the till reaches the limits of its elastic behaviour and undergoes Y1 yielding at very small strains, after which stiffness is nonlinear. Tests reach stable critical state failures at strains exceeding 25%. The Y1 yield surface is kinematic and, once engaged, travels with the effective stress path, as does a second Y2 surface. Stiffnesses vary with mean effective stress raised to fractional exponent ≈0.5. The till displays far more marked sample size dependency and anisotropy in stiffness than in shear strength. It also shows significantly strain-rate dependent behaviour and displays an isotach response. The main features of the till’s shear strength and dilatancy behaviour can be synthesised within a critical state based interpretive framework that brings field and in situ testing outcomes together with laboratory experiments conducted over a wide range of stress levels and overconsolidation ratios, considering both natural and reconstituted specimens.

Experimental Investigation on Seepage Characteristics of Clay-Structure Interface after Shear Deformation.

There exist shear and seepage behaviors on the interface between clay core and concrete slab in clay core dams. In order to investigate the seepage characteristics of the clay–structure interface after shear deformation, a shear-seepage test system is proposed, in which the seepage direction is perpendicular to the shear direction. The shear test and shear-seepage test are performed on clay–metal and clay–mortar interfaces under different normal stresses (100, 200, 400, 800, and 1600 kPa). The shear stress-deformation curves of two clay–structure interfaces exhibit softening behavior and residual friction behavior. The interface roughness can enhance peak and residual shear strength and increase peak displacement. The shear-seepage test results show that specimen permeability decreases first and then increases to a stable value as shear deformation increases under low normal stress, while it decreases continuously and then retains stability under high normal stress. The interface roughness enhances specimen permeability under low normal stress, whereas it has a weak effect on specimen permeability under high normal stress. Compared with initial permeability, shear deformation reduces specimen permeability rather than raise it. The ratio of stabilized permeability coefficient to initial value ranges from 0.6 to 0.8. The clay–structure interface still has a good resistance to seepage failure after shear deformation. The shear dilation features and interface pore decrease caused by shear behavior are the internal attributions of clay–structure specimen permeability evolution.

Numerically Coupled Thermo-Hydro-Mechanical Analyses of Ultra-Heavy Oil Reservoirs during the Micro-Fracturing Stage

Water injection in SAGD well pairs has rapidly developed to improve the circulation preheating efficiency and the steam chamber propagation speed. However, a key problem is to evaluate the coupled thermo-geomechanical effects of water injection under complex operations. In this paper, a coupled thermo-hydro-mechanical model considering skeleton shear dilation and phase change of bitumen was established. Major conclusions were drawn that there were large temperature changes only in two separate areas around wells. The pore pressures in the reservoir around wells and the base rock right below the production well increased significantly. The void ratio was improved in the reservoir around wells, especially in the inter-well region. The reservoir was lifted, and the largest uplift was right above the wells. This information can guide engineers in properly evaluating the field operations.

Effects of pH Values and H2O2 Concentrations on the Chemical Enhanced Shear Dilatancy Polishing of Tungsten.

In order to obtain tungsten with great surface qualities and high polishing efficiency, a novel method of chemical enhanced shear dilatancy polishing (C-SDP) was proposed. The effects of pH values and H2O2 concentrations on the polishing performance of tungsten C-SDP were studied. In addition, the corrosion behaviors of tungsten in solutions with different pH values and H2O2 concentrations were analyzed by electrochemical experiments, and the valence states of elements on the tungsten surface were analyzed by XPS. The results showed that both pH values and H2O2 concentrations had significant effects on tungsten C-SDP. With the pH values increasing from 7 to 12, the MRR increased from 6.69 µm/h to 13.67 µm/h. The optimal surface quality was obtained at pH = 9, the surface roughness (Ra) reached 2.35 nm, and the corresponding MRR was 9.71 µm/h. The MRR increased from 9.71 µm/h to 34.95 µm/h with the H2O2 concentrations increasing from 0 to 2 vol.%. When the concentration of H2O2 was 1 vol.%, the Ra of tungsten reached the lowest value, which was 1.87 nm, and the MRR was 26.46 µm/h. This reveals that C-SDP technology is a novel ultra-precision machining method that can achieve great surface qualities and polishing efficiency of tungsten.

Experimental study on nonlinear friction behavior of granular flow lubrication

Solid granules have good lubrication performance and load-bearing capacity, which has been concerned by researchers. However, the correlation mechanism between lubrication performance and granular medium dynamics is hardly focused on. In this study, the characteristics of two parallel friction pairs lubricated by the granular medium are experimentally investigated to study the nonlinear friction behavior during this process. The experimental results showed that the fluctuation behavior of friction force and friction coefficient with time shows a more flat nonlinear trend with the increase of the shear velocity and the pressure load. The discrete element simulation results corresponding to the experiment revealed that the change of nonlinear behavior of measured friction and friction coefficient is closely related to the evolution of force chains and dynamic characteristics of this granular flow lubrication system. The shear dilatancy amplitude of this granular flow lubrication system decreases obviously with the increase of the shear velocity and the pressure load. The flow states of granular medium is the main reason influencing the nonlinearity of shear gap. This study reveals the key factors affecting the friction nonlinearity and provides an evidence for the formation of granular shear dilatancy between two friction pairs lubricated by granular medium.

Transient conduit permeability controlled by a shift between compactant shear and dilatant rupture at Unzen volcano (Japan)

Abstract. The permeability of magma in volcanic conduits controls the fluid flow and pore pressure development that regulates gas emissions and the style of volcanic eruptions. The architecture of the permeable porous structure is subject to changes as magma deforms and outgasses during ascent. Here, we present a high-resolution study of the permeability distribution across two conduit shear zones (marginal and central) developed in the dacitic spine that extruded towards the closing stages of the 1991–1995 eruption at Unzen volcano, Japan. The marginal shear zone is approximately 3.2 m wide and exhibits a 2 m wide, moderate shear zone with porosity and permeability similar to the conduit core, transitioning into a ∼ 1 m wide, highly sheared region with relatively low porosity and permeability, as well as an outer 20 cm wide cataclastic fault zone. The low-porosity, highly sheared rock further exhibits an anisotropic permeability network, with slightly higher permeability along the shear plane (parallel to the conduit margin), and is locally overprinted by oblique dilational Riedel fractures. The central shear zone is defined by a 3 m long by ∼ 9 cm wide fracture ending bluntly and bordered by a 15–40 cm wide damage zone with permeability enhanced by ∼ 3 orders of magnitude; directional permeability and resultant anisotropy could not be measured from this exposure. We interpret the permeability and porosity of the marginal shear zone to reflect the evolution of compactional (i.e. ductile) shear during ascent up to the point of rupture, which was estimated by Umakoshi et al. (2008) at ∼ 500 m depth. At this point the compactional shear zone would have been locally overprinted by brittle rupture, promoting the development of a shear fault and dilational Riedel fractures during repeating phases of increased magma ascent rate, enhancing anisotropic permeability that channels fluid flow into and along the conduit margin. In contrast, we interpret the central shear zone as a shallow, late-stage dilational structure, which partially tore the core of the spine, leaving a slight permanent displacement. We explore constraints from monitored seismicity and stick-slip behaviour to evaluate the rheological controls, which accompanied the shift from compactional toward dilational shear as magma approached the surface, and discuss their importance in controlling the permeability development of magma evolving from overall ductile to increasingly brittle behaviour during ascent and eruption.

Effect of Loading Conditions on the Shear Behaviors of Rock-like Materials Containing Circular Holes, with the CZM Method

The mechanical behaviors of rock masses are significantly affected by the distribution and shape of the holes in it. In this research, the fracture mechanism and the shear properties of rock masses containing holes were investigated by the cohesive zone model (CZM) method. At first, a uniaxial compression laboratory test was carried out, and some mechanical parameters were obtained, and then the obtained mechanical parameters were used to build a mathematical model with zero-thickness elements. Subsequently, the numerical shear test was performed under a mixed-mode test, using the obtained mechanical parameters. Finally, the mechanical characteristics and crack behaviors were discussed separately. The results exhibit that the shear process in this research could be determined and identified as the elastic, strengthening, plastic, and residual stress stages, respectively. Note that the shear rate and normal stress of rock masses containing holes are significantly and positively correlated with their shear strength and dilatancy. In addition, the cracking behavior and mechanical properties of the specimens are closely related to the shear rate and normal stress. Overall, the results of this study have great significance in guiding future, in-depth research of rock masses containing holes in rock slopes.

Deformation, strength and water variation characteristics of unsaturated compacted loess

The bearing capacity and safety of geotechnical structures are determined by the shear strength of soil. Soil is usually unsaturated in nature, and variation in unsaturated strength is very complicated. To investigate the unsaturated shear strength characteristics of compacted loess, a series of triaxial consolidated shear tests were conducted on compacted loess from the 6th loess layer in Yan’an, China, using GDS unsaturated soil triaxial apparatus. The saturation, volume and moisture change of compacted loess were analyzed, and the influence of matric suction (ua-uw) and net confining pressure (σ3-ua) on the deformation and strength characteristics of compacted loess were discussed. It was found that the deviatoric stress versus deviatoric strain curves exhibited a strain hardening state, and the volumetric strain transitions from shear contraction to shear dilatancy. In the testing pressure range (ua-uw ≤ 100 kPa, σ3-ua ≤ 400 kPa), both matric suction and net confining pressure significantly influence the deformation and strength characteristics of compacted loess. Under certain matric suction conditions, the shear strength of the compacted loess increased with increasing net confining pressure, the peak value of the shearing shrinkage increased, and the shear dilatancy decreased. Under a constant net confining pressure, the shear strength increased with the increasing matric suction. However, the peak values of shearing shrinkage and shear dilatancy decreased with increasing matric suction. In the unsaturated shear test, φ increased with the matric suction. The cohesion of unsaturated soil is affected by many factors and the data points from this research are limited owing to time constraints, hence, further analysis of cohesion strength is needed. Data availabilityThe data used to support the findings of this study are available from the corresponding author upon request.

Large-scale triaxial tests of reinforced coral sand with different grain size distributions

The strength, deformation and particle breakage behaviors of reinforced coral sand were analyzed by performing a series of large-scale consolidated drained triaxial tests on unreinforced and two-layer geogrid reinforced samples. Two grain size distributions, namely, the natural coral sand in the South China Sea (ISG) and the uniformly graded coral sand (UG), were used to investigate the effect of gradation on unreinforced and geogrid-reinforced specimens under dry and saturated conditions. The results indicated that the inclusion of geogrid offered a great improvement to the coral sand with respect to increasing its shear strength and reducing the shear dilation. The reinforcement effect of geogrid on dry sample was better than that on saturated one. Moreover, the UG sample exhibits high compressibility and distinct strain hardening behavior due to the significant particle breakage resulting from the lack of fine content compared with the ISG sample, which induced the better reinforcement effect of geogrid on ISG sample. The increase of particle breakage further led to the decrease of peak-state friction angle.