Low Normal Stress Research Articles

Study on the characteristics particles of coarse grain material in high bench dump

Particle shape is an important factor affecting the strength and deformation of coarse materials in dumps. However, conducting research on particle shape in current laboratory experiments, and there exists no classification standard for the particle shape of the dump. Considering the project on high-bench dump in Jiangxi Province, based on the random particle construction method, this study determined the characteristic particles of coarse grain material in the dumping site by combining field investigation results, large-scale direct shear tests, and numerical analysis. Further, the influence mechanism of coarse grain content and characteristic particles on the mechanical characteristics of the dump were analyzed. The results showed that the increase in coarse grain content enhanced the irregularity of particle shape, increased the internal friction angle between particles, and reduced the cohesion of the dump. Moreover, with increase in the normal stress, the shear strength changed from insignificant to gradually enhanced, although the increasing trend slowed down. The constructed random particle model can well solve the effects of occlusion, nesting, and friction between particles that is neglected by the standard spherical particle model, and the fitting degree is better. The particle characteristics of coarse-grain materials in the dump were primarily block, strip, and flake. The three types of shape characteristics under low normal stress did not affect the mechanical properties of the dump. Further, with the increase in normal stress, the stress-strain curve reflected the shape characteristics and content of the dominant characteristics in the dump. Specifically, the block sample exhibited the dominant characteristics and the most content under the same normal stress, followed by the strip sample, with the flake sample exerting the least influence. The research results provide a theoretical basis for long-term safe operation, disaster monitoring, and early warning of high bench dump.

Unstable Sliding of Plagioclase Gouge and Deformation Mechanisms Under Hydrothermal Conditions With Effective Normal Stresses of 100–300 MPa

AbstractPlagioclase feldspar is a major mineral in mafic crustal rocks. To better understand the deformation mechanism of plagioclase feldspar during frictional faulting, we conducted shearing experiments on simulated plagioclase gouge in a wide range of effective normal stress of 100–300 MPa, pore‐water pressure of 30–100 MPa, and temperatures ranging from 100°C to 600°C. The coefficient of friction is found to range from 0.65 to 0.74 across the entire temperature range, showing no significant thermal weakening process. Except for a case at 200°C with an effective normal stress of 300 MPa, the frictional sliding is velocity weakening over the whole temperature range, showing a steady‐state rate dependence (a−b) ranging from −0.5 × 10−3 to −8.6 × 10−3. This property facilitates nucleation of unstable slips in frictional faulting. Above 200°C, the direct rate effect parameter (a) and the evolution effect parameter (b) of friction increase with temperature up to a threshold of 400°C or 500°C, depending on the effective normal stress. This thermal enhancement suggests thermally activated creep at contact junctions governed by intergranular pressure solution, as evidenced by microstructural signatures indicating the prevalence of very fine precipitates formed at the surfaces of gouge particles as a result of pressure solution. In frictional sliding of plagioclase, a low effective normal stress of 100 MPa corresponds to a higher degree of velocity weakening and tends to facilitate seismic slip rather than slow slips, whereas the high effective normal stress of 300 MPa corresponds to a minor velocity weakening which may cause slow‐slip events in faults of limited size.

Experimental investigation of the roughness, freezing temperature and normal stress on the shear mechanical action of frozen saturated clay-rock interface

Recently, with the increase in global temperature, the permafrost degradation trend has intensified, and the soil-rock binary slope in permafrost regions has become more unstable. Therefore, this paper focuses on the shear strength of the clay-rock interface in the binary slope. The three-dimensional roughness, freezing temperature, and normal stress are key factors affecting the shear strength of the clay-rock interface. The influence of freezing temperature can be further quantified by using the unfrozen water content (UWC), which was measured by nuclear magnetic resonance (NMR) technology. By analyzing experimental results, it can be concluded that the three-dimensional roughness can effectively improve the shear strength of the clay-rock interface under freezing conditions, and the shear strength increases with the growth of the climbing angle (ic). The influence of temperature can be attributed to the effect of UWC on the internal friction angle and cohesion of saturated clay. Compared with the internal friction angle, the cohesion of saturated clay decreases faster with the increase of UWC. In addition, the shear strength of the clay-rock interface rises linearly as the normal stress increases, therefore the Mohr-Coulomb criterion also can be used to characterize the shear strength of the clay-rock interface. An interesting finding is that significant tensile cracks will appear in the clay part around the large bulge under low normal stress and high roughness. It further confirmed the contribution of large bulges to the prevention of shear slides of soft clay. The quantification understanding of the shear mechanical action of the clay-rock interface can provide a reference for scientific disaster reduction in cold regions.

Influence of Apparatus Scale on Geogrid Monotonic and Cyclic/Post-Cyclic Pullout Behavior in Cohesive Soils

Geosynthetics have increasingly been applied to geotechnical engineering works due to their numerous advantages, including cost-effectiveness and their significant role in sustainable development. When geosynthetics are used as reinforcement in earth structures, such as embankments, retaining walls and bridge abutments, soil–geosynthetic interface shear behavior is a critical parameter involved in the design. This paper presents a series of monotonic and cyclic/post-cyclic pullout tests carried out to examine the apparatus scale effect on the pullout response of a geogrid embedded in two different soils. To assess the small-scale equipment feasibility, comparisons were made between pullout test parameters derived from small- and large-scale equipment. The test results indicate that, under a low confining stress of 25 kPa, using a smaller-sized apparatus results in lower values of geogrid pullout resistance and maximum mobilized shear stress, but higher values of confined tensile stiffness at low strains. On the other hand, as the confining stress increases (i.e., 50 kPa and 100 kPa), the difference between the results becomes less significant and similar trends are observed regardless of the equipment type. Adopting small-scale equipment enables obtaining soil–reinforcement interaction parameters using test procedures that are less time-consuming than those associated with large-scale pullout tests. However, proper scale effect correction factors may be considered for more consistent estimates of the interface strength parameters under low normal stress values.

Optimization of two-wheeler Bike engine among three different designs using design of experiments

This research presents the results of a fin heat transfer study of three different models of the motorcycle engine. The objective of this study is to minimize the external body temperature because the optimum design will be found when the heat transfer rate is maximized. The study obtains the procurement of a balance between the outer boundary temperature due to convective heat transfer and air-cooled fin design. Heat transfer coefficient varies according to wind velocity (Km/hr) and Nusselt Number. The analysis was performed on three different engine models, named A, B and C. The optimum design was design B through simulation which has lower temperature gain, lower deformation and lower normal stress. DOE (Design of Experiments) was performed on the optimum design of the engine among all with three parameters thickness of fin, size of fin, and shape of fin, and again analysis was performed according to DOE cases. The material used for manufacturing the models was aluminum alloy 6061 which has a thermal conductivity of 200 W/mK. The study was performed on the designed models by taking the outer boundary temperature of 750oC. The heat transfer coefficient was about 77.28 W/m2K at 40 Km/h velocity.

Frictional Properties and Healing Behavior of Tectonic Mélanges: Implications for the Evolution of Subduction Fault Zones

AbstractThe frictional velocity dependence and healing behavior of subduction fault zones play key roles in the nucleation of stick‐slip instabilities at convergent margins. Diagenetic to low‐grade metamorphic processes such as pressure solution are proposed to be responsible for the change in frictional properties of fault materials along plate interfaces; pressure solution also likely contributes to the acceleration of healing according to previous studies. Here, we report friction studies for temperatures of 20–100°C and normal stresses from 20 to 125 MPa on samples collected from ancient subduction fault zones, the Lower Mugi and Makimine mélanges of the Cretaceous Shimanto belt. The two mélanges correspond to the updip and downdip limits of the seismogenic zone and include deformation features that indicate lower and higher degrees of pressure solution. Our data show that the Lower Mugi mélange exhibits velocity‐weakening to velocity‐neutral frictional behavior under low normal stress and that the Makimine mélange sample shows velocity‐strengthening behavior under high normal stress. We suggest that mineralogical changes due to diagenesis and metamorphism influence fault slip behavior. We measure frictional healing in slide‐hold‐slide experiments for the Lower Mugi mélange sample and document the role of pressure solution in fault healing. Our results show that frictional healing increases at higher temperatures. The microstructures related to pressure solution found in the post‐experimental gouges support the idea that the enhanced healing is related to pressure solution.

Shear strength enhancement at the sand-steel interface: A pioneering approach with Polyurethane Foam Adhesive (PFA)

This study explores a novel application of Polyurethane Foam Adhesive (PFA) injection to augment shear resistance at the interface, investigating various parameters such as PFA content, injection pressure, curing time, sand density, and surface roughness. Results demonstrate that injecting 10 % PFA increased the friction angle (ϕ) from 6.4 to 27.4° and 32.6–42° for sand-smooth and rough surface interfaces, respectively, under relative density (Dr) of 30 %. Correspondingly, cohesion (c) values rose from 0 to 124 and 62 kPa. Samples with lower Dr (30 %) exhibited a more significant increase in shear strength post-PFA injection, attributed to PFA penetration and enhanced shear performance of loose sands, showing a maximum 23 % difference compared to Dr = 90 % for the sand-smooth surface interface. Curing time notably influenced injected samples, particularly at lower normal stress levels, leading to increased mobilized shear strength with longer curing periods, with 28-day shear strength averaging 2.34 and 2.74 times higher than one-hour shear strength for smooth and rough surfaces, respectively. A substantial shear strength increase was observed one hour after injection, with more pronounced behavior after seven days. Additionally, increased injection pressure expanded the stabilized area at the interface, enhancing mobilized shear strength. At the sand-smooth surface interface, increasing injection pressure from 25 to 225 kPa resulted in average mobilized shear strength increases of 84% and 117% for Dr 30 % and 90 %, respectively. Examination under a Scanning Electron Microscope (SEM) revealed that the inclusion of PFA additive serves to interconnect adjacent grains, consequently enhancing the mechanical properties of the mixture.

Temporal evolution of shear-induced dilatancy of rock fractures: controls from surface roughness and normal stress

SUMMARY Understanding the shear-induced dilatancy of rock fractures is important for assessing the permeability evolution and seismic hazard in shale and geothermal reservoirs. The displacement dependence of fracture dilation has been well studied, while the influence of slip velocity is poorly constrained. In this study, we combined displacement- and velocity-dependent aperture models to reproduce the transient shear-induced dilatancy of fractures in sandstone in 16 normal stress unloading tests. Our results show that the combined aperture model can describe the transient fracture aperture evolution during accelerating slip induced by normal stress unloading better than the model dependent only on slip displacement. Slip velocity could enhance the aperture increase on smoother fractures at lower normal stresses and higher slip velocities. Both the dilation factor and characteristic slip distance decrease with increasing normal stress and surface roughness, signifying reduced contribution of slip velocity to transient shear-induced dilatancy at higher normal stresses and surface roughness. The dilation angle increases with the increase of surface roughness, and this increase diminishes at higher normal stresses primarily attributable to more severe asperity wear. These findings highlight the importance of slip velocity in controlling the transient evolution of aperture and permeability of a rock fracture. Our study also provides constraints on the constitutive parameters in the combined aperture model for describing transient shear-induced fracture dilatancy. We suggest that it is crucial to incorporate the velocity-dependent aperture model to simulate the nonlinear evolution of fracture aperture in future analytical and numerical models involving coupled hydromechanical processes in geoenergy systems.

Determining the optimal sampling interval for 3D morphology measurements of different-sized natural rock joints

Efficiently studying the scale effect of surface roughness and peak shear strength of natural rock joints, especially unfilled and well-matched ones, requires an optimal sampling interval to realize cost-effective three-dimensional morphology measurements for different-sized natural rock joints with advanced non-contact optical devices. However, rationally determining such an optimal sampling interval remains controversial. To this end, inspired by digital signal processing, the optimal sampling interval is creatively determined by Shannon's sampling theorem using the Fourier series' optimum order. An empirical formula relating the joint size to the optimum order is correspondingly developed. The proposed empirical formula was found to work well under conditions of high normal stress, where the relative errors for the surface roughness and peak shear strength of series-sized natural rock joints were mostly less than 10% and 5%, respectively, but should be used cautiously under conditions of low normal stress. Furthermore, different from previous perceptions, a higher joint roughness level was discovered to not necessarily imply the need for a smaller optimal sampling interval, and conversely. The proposed empirical formula can facilitate the determination of the optimal sampling interval for measuring different-sized natural rock joints' surface morphologies in engineering practice.

Analysis and prediction of contact characteristics of rock fracture surfaces under normal loading

The real contact area of rock fractures is smaller than the nominal area, and actual stresses on contacts are significantly higher. Evaluating contact characteristics and predicting potential contact areas are crucial for understanding mechanical and hydraulic properties of geological fractures under compression or shear. We measured the real contact area of mated tensile fractures using pressure-sensitive films and studied their contact characteristics and evolution under different normal stresses through point cloud analysis. It is revealed that the initial aperture of mated tensile fractures determines the contact state. Under conditions of low normal stress, regions exhibiting higher mean curvature are more susceptible to contact formation, particularly when aperture sizes are comparable. This propensity is further influenced by the directional orientation of the normal vector components within these regions. Increasing normal stress leads to faster development of new contacts in regions with smaller initial apertures. Additionally, we predicted contact characteristics and real contact area of tensile fractures using back propagation artificial neural network. Comparison between predictions and laboratory measurements demonstrated accurate prediction of contact distribution and area values by the neural network. Our findings enable the prediction of real stress states, potential failure areas, and hydraulic conductivity of rock fractures in slope and underground rock engineering.

Experimental study of interface performance between calcareous sand and steel pile with different surface roughness and temperatures

The shear characteristics of calcareous sand, a typical deposit in the South China Sea, could significantly influence the safety and stability of marine structures. Previous studies showed that many factors, such as temperature and surface roughness, could affect the mobilized shear strength between the calcareous sand and steel surface. Therefore, studies on the shear behaviors of the steel-sand interface under various thermal conditions are necessary. In this study, a new multifunctional sediment shear system has been developed and employed to investigate the shear performance. For this purpose, steel plates with different surface roughness were tested under various normal stress values, shear displacement rates and temperatures. Shear strength and deformation were analyzed using test data to reveal the relationships between several factors and the shear strength of the steel-sand interface. Results from test demonstrate that the interfacial shear strength on rough steel interfaces will be 1.45–3.15 times higher than that on the smooth steel interface. Besides, the shear strength of the interface rises significantly with the roughness of the steel plate. The peak friction angle increases rapidly at the start and then tends to stabilize with the increase of normalized roughness. The interfacial shear strength could be enhanced in low-temperature environments but would reduce in high-temperature environments, e.g. the energy pile, particularly under low normal stress. The shear displacement rate also has a positive effect on the interface shear strength for most cases in this study. Finally, a new nonlinear function is proposed to describe the peak friction angle ratio of foundation interface to consider the effect of temperature and shear displacement rate reasonably.

Tension-shear extension criteria used in PFC2D to reveal a brittle failure of rock bridges in rock slopes with stepped joints

Stepped failure is a typical mode in alpine valleys and usually manifests as brittle fractures determined by joints. Tension-shear extension criteria were proposed to reflect the intrinsic mechanism of this failure mode. These criteria combined the theory of linear elastic fracture mechanics and the Mohr-Coulomb (M−C) strength criterion, including propagation trends and fracture conditions, and reflected the fracture behavior of rock bridges determined by joints. The proposed extension criteria were numerically applied in the 2D Particle Flow Code (PFC2D). The mesoscopic strength parameters were calibrated according to a mathematical relationship, which could satisfy the relationship between stress and strength with the proposed criteria. For massive rock slopes dominated by stepped joints, it is determined through theoretical and experimental analysis that rock bridges undergo tensile failure under low normal stress levels, which gives a reasonable explanation of why rock bridges observed in situ generally undergo tensile fracture. The internal mechanism of rock bridge failure is revealed through mechanical analysis of fracture behavior, which is considered in the numerical method. Taking the Xiaowan Hydropower Station as an example, it is confirmed that the simulation results truly reproduce the mesoscopic mechanism of the stepped failure of the slope.

Thermal efficiency of stick slip events along experimental analog faults

Most of the elastic energy released by earthquakes is believed to convert into heat; however, the reported experimental thermal efficiencies, defined as the ratio of frictional heat to elastic energy, are largely scattering, i.e., 50% or 90%. In this study, we conducted double direct shear experiments using halite powders as fault gouge under normal stresses of 10, 20, 30, 40, and 50 MPa to investigate the thermal efficiency of stick slip events. The results revealed two types of temperature changes during stick slip, resulting from stress drop and frictional heating. The temperature change caused by stress drop reduced thermal efficiency by 10%–20%. The scaling relationships between the friction heat, displacement and normal stress were also determined, which bridge the gap between the previous experimental conditions with only low and high normal stresses. The thermal efficiency was 50%–85%, and the specific value was inversely proportional to the fraction of stress drop. The thermal efficiencies was not a constant value and was closely related to the coseismic stress variation. The findings of this study suggest that the widely assumed 90% earthquake heat fraction is a consequence of specific stress change conditions.

The evolution of isolated cavities and hydraulic connection at the glacier bed – Part 2: A dynamic viscoelastic model

Abstract. Many large-scale subglacial drainage models implicitly or explicitly assume that the distributed part of the drainage system consists of subglacial cavities. Few of these models, however, consider the possibility of hydraulic disconnection, where cavities exist but are not numerous or large enough to be pervasively connected with one another so that water can flow. Here I use a process-scale model for subglacial cavities to explore their evolution, focusing on the dynamics of connections that are made between cavities. The model uses a viscoelastic representation of ice and computes the pressure gradients that are necessary to move water around basal cavities as they grow or shrink. The latter model component sets the work here apart from previous studies of subglacial cavities and permits the model to represent the behaviour of isolated cavities as well as of uncavitated parts of the bed at low normal stress. I show that connections between cavities are made dynamically when the cavitation ratio (the fraction of the bed occupied by cavities) reaches a critical value due to decreases in effective pressure. I also show that existing simple models for cavitation ratio and for water sheet thickness (defined as mean water depth) fail to even qualitatively capture the behaviour predicted by the present model.

Effect of fiber properties and orientation on the shear rheology and Poynting effect in meat and meat analogues

The development of plant-based meat analogues is an important aspect of the transition towards using more plant proteins. Production of such products with texture (and rheology) adequate for consumer acceptance like meats is a great challenge. However, there is a current lack in quantitative methods to compare whole cut meats and plant-based analogues that also consider its fibrous structures. We tackled this problem by combining non-linear shear rheology and shear-induced normal stress measurements on whole cut meats (beef and chicken) and plant-based fibrous model systems for meat analogues (soy and pea protein isolates combined with wheat gluten). We found different response in the linear viscoelastic regime for different samples, and the non-linear viscoelastic regime provided a deeper understanding into the role of structural components. Meat samples showed rearrangements at small strains and lower energy dissipation, higher intracycle strain stiffening, and a larger normal stress response at large strains than plant-based fibrous products, due to more elastic fibers. The plant-based fibrous products behave more solid-like until a larger strain, but the fibrous structures break down at large deformations resulting in higher energy dissipation, and show a lower normal stress response than meat. We therefore explore the importance of fiber properties and orientation on the nonlinear rheology of fibrous model systems for meat analogues, and provide a robust methodology to quantitatively compare different mechanical responses of meat and plant-based fibrous meat analogues. These results thereby allow comparison and tuning mechanical responses of plant-based meat analogue products towards real meat.

An evaluation on Barton-Bandis shear strength criterion for discontinuities in weak materials under low normal stresses

An evaluation on Barton-Bandis shear strength criterion for discontinuities in weak materials under low normal stresses

A critical state three-dimensional multi-shear model for soil-structure interfaces under monotonic and cyclic loading

In this paper, a three-dimensional(3D) multi-shear model of soil-structure interface is established by using the plasticity theory of boundary surface. State parameters are introduced to predict the interface response under different soil densities and normal stress. A constitutive equation is established following the assumption that the interface response is decomposed into macro-response and a series of directional-dependent micro-shear responses. The calculation method of plastic microshear strain is modified, a new calculation method of microscopic plastic shear modulus is introduced, and the calculation parameters are reduced. The proposed model can reasonably describe the variation of stress–strain relationship under 3D shearing conditions and predict the three-dimensional non-linear mechanical behaviour of the contact surface. The proposed model has a unified calculation formula for loose and dense interfaces under different loading conditions. The proposed model can be accurately described the dilatancy characteristics under low normal stress and shear shrinkage under high normal stress. The proposed model can simulate the cyclic response characteristics of the interface, e.g., the normal displacement increases with increasing cycle number, however, the growth rate decreases with increasing cycle number. The validity of the proposed three-dimensional multi-mechanism shear model of soil-structure interface under different boundary and loading conditions is verified by comparing the calculation results with the experimental results.

Study on Cyclic Shear Properties of Siliceous Sand–Steel Interface under Different Normal Loading Conditions

It is of great significance to deeply understand the stress damage mechanism of the pile–soil interface under cyclic loading for the safety control of engineering entities. Large-scale self-developed shear equipment was used to conduct cyclic shear tests of the interface between steel and siliceous sand, and the macroscopic shear characteristics and particle crushing characteristics were analyzed. Finally, the interface micro characteristics were analyzed through numerical simulation. The results indicate that the interface peak shear stress under constant stress conditions mainly exhibits strengthening characteristics, while under constant stiffness conditions it exhibits weakening characteristics. The position of the relationship curve between shear stress and normal stress gradually moves towards the direction of low normal stress as the experiment progresses, and the distance between the curves gradually decreases. The degree of particle breakage increases with the number of cycles but is mainly concentrated in the first few cycles. The principal stress is proportional to the normal stress, and its rotation degree gradually weakens with the normal stress. The contact number of particles at any angle increases with the normal stress.

Friction of hornblende and tremolite at hydrothermal conditions with low effective normal stress and high pore pressure

In order to understand the friction properties of hornblende and tremolite as common minerals in deep fault zones, we used natural hornblende and tremolite as the simulated gouge material to perform shearing experiments at temperatures from 303 to 607 °C under an effective normal stress of 20 MPa, a pore pressure of 100 MPa, and displacement rates of 0.0488–1.22 μm/s. The steady-state friction coefficient of hornblende ranges from ~0.76 to ~0.88. The corresponding steady-state rate dependence is velocity-weakening (negative a-b) at temperatures from 303 to 403 °C, which transitions to velocity-strengthening (positive a-b) at temperatures above 500 °C. Steady-state friction coefficients of tremolite exhibited values between 0.70 and 0.76. The corresponding steady-state rate dependence is velocity-strengthening (positive a-b) for all the tested temperatures. The direct rate effect (a value) for hornblende roughly increases with temperature, while the corresponding evolution effect (b value) essentially declines with temperature. The direct rate effect (a value) of tremolite increases with increasing temperature for temperatures above 400 °C, with a quite minor evolution effect (b value) that indicates viscous-like slip behavior at all the tested temperatures. There are a large number of remnants of large particles distributed pervasively over sheared gouges, and significant particle size reduction is only observed in localized shear zones. Pervasive cataclastic flow seems to dominate the shear deformation of hornblende for temperatures above 400 °C and that of tremolite for all the tested temperatures. The velocity-weakening behavior of hornblende at low effective normal stress may apply to amphibole-bearing fault rocks in subduction zones where unstable slip events can occur at a narrow temperature range of 300–400 °C. In contrast, the very strong velocity-strengthening observed for tremolite provides a favorable condition for blocking the nucleation of unstable slip events.

Softening/Hardening Damage Model and Numerical Implementation of Seabed Silt-Steel Interface in Yellow River Underwater Delta

The interaction between soil and structure is a research hotspot in ocean engineering, and the shear performance of interfaces is an essential factor affecting the bearing capacity of offshore structures. Taking the Yellow River Underwater Delta as the research area, the Softening/Hardening damage model of the silt–steel interface and the determination method of model parameters are proposed based on the statistical damage theory. Through the interface monotonic shear test under the conditions of different normal stress, roughness and water content, the shear mechanical properties and volumetric deformation laws on the silt–steel interface are analyzed, and the damage model parameters are obtained. Finally, a FRIC subroutine for the damage model was developed based on ABAQUS. The research results indicate the following: (1) The interface between silt and steel exhibits two characteristics, softening/hardening and shear shrinkage/expansion, under different conditions. Roughness significantly impacts interfacial cohesion, while water content mainly affects the internal friction angle. (2) The softening model based on the classic rock damage model can better simulate the stress–strain relationship of the silt–steel interface under high normal stress and low water content. In contrast, the hardening model based on the classic hyperbola model can better simulate the stress–strain relationship under low normal stress and high water content. The calculated results of the softening/hardening model agree with the experimental results, and the model has 7 parameters. (3) The developed FRIC subroutine can effectively simulate the nonlinear mechanical behavior of the interface between silt and steel. The research results provide a reference for exploring the stability analysis of offshore structures considering interface weakening effects.