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To extract more in-depth information of acoustic emission (AE) signal-cloud in rock failure under triaxial compression, the spatial correlation of scattering AE events in a granite sample is effectively described by the cube-cluster model. First, the complete connection of the fracture network is regarded as a critical state. Then, according to the Hoshen-Kopelman (HK) algorithm, the real-time estimation of fracture connection is effectively made and a dichotomy between cube size and pore fraction is suggested to solve such a challenge of the one-to-one match between complete connection and cluster size. After, the 3D cube clusters are decomposed into orthogonal layer clusters, which are then transformed into the ellipsoid models. Correspondingly, the anisotropy evolution of fracture network could be visualized by three orthogonal ellipsoids and quantitatively described by aspect ratio. Besides, the other three quantities of centroid axis length, porosity, and fracture angle are analyzed to evaluate the evolution of cube cluster. The result shows the sample dilatancy is strongly correlated to four quantities of aspect ratio, centroid axis length, and porosity as well as fracture angle. Besides, the cube cluster model shows a potential possibility to predict the evolution of fracture angle. So, the cube cluster model provides an in-depth view of spatial correlation to describe the AE signal-cloud.

Gassy clays containing large, discrete gas bubbles are widely spread in deposits in shallow waters. The existence of gas bubbles may impair the original soil structure, resulting in the instability of offshore foundations and the occurrence of submarine landslides. Although gassy clay was found to exhibit different undrained shear behaviors with variations of the initial pore water pressures and initial gas volume fractions, more experimental studies and theoretical interpretations of the relationship between the effective confining pressure and undrained responses are required. A series of undrained triaxial compression tests is conducted to compare the responses of reconstituted gassy and saturated specimens at an effective confining pressure of 200, 400, or 600 kPa. An existing elastoplastic constitutive model based on the critical state is improved by updating its stress-dilatancy function and yield surface. The dilatancy equation proposed has the potential to quantify the effect of gas bubbles on the dilatancy of the soil matrix. The yield surfaces are reproduced reasonably well by optimizing the expressions of shape parameters under a variety of effective confining pressures. The model developed can describe the stress-dilatancy and stress-strain responses of both the gassy and saturated specimens.

In this paper, a series of suction-controlled triaxial tests were conducted to investigate the hydromechanical behaviour of an unsaturated kaolin mixture. The studied variables included the mean net stress, suction, and initial dry density. The suction was controlled over a wide range – in the span of 0–390 MPa using the axis translation method and the vapour equilibrium technique, aiming to holistically study the mechanical response of unsaturated soils. The discussion focused on the critical state behaviour and dilatancy characteristics of the tested soil. The experimental results indicate that the ductility of the soil decreases and shifts towards brittleness with increasing suctions. The critical state stress ratio (M a) of the soil exhibits a significant increase with suction, and this increase becomes more pronounced with higher initial dry density. Conversely, the impact of suction on the critical state line in the specific volume-mean net stress plane (v -ln p net plane) diminishes with higher initial dry density. Furthermore, suction notably influences the stress-dilatancy relationship of kaolin mixtures, while the effect of confining pressure appears to be marginal. The influence of suction on the stress-dilatancy relationship primarily stems from its impact on the critical stress ratio (M a), which can be normalized by dividing the stress ratio by the critical state stress ratio. Lastly, the stress-dilatancy equation of the modified Cam-clay model is extended to capture the effect of suction on the dilatancy characteristics of unsaturated soils. • The mechanism of suction-induced over-consolidation is analysed. • As suction increases over a wide range, the critical state stress ratio (M a) notably rises. • The impact of suction on dilatancy is a result of its influence on M a. • The suction-induced friction angle is unaffected by initial dry density. [ABSTRACT FROM AUTHOR]

The paper presents a revisitation and comparison of some interface model able to couple the damage evolution with the unilateral effect and the friction. In particular, the models derived from the initial interface Needleman proposal (Needleman, 1987) are reported and revised in a unified framework, introducing some novelties in their formulations. Then, the model proposed by Alfano and Sacco (Alfano and Sacco, 2006, Sacco and Toti, 2010) is illustrated and discussed. The responses of the investigated models are compared, illustrating the similarities and the differences. Moreover, the multiplane model proposed in Albarella et al. (2015), Serpieri et al. (2015) able to reproduce the dilatancy effect is presented, showing its performances and computing the dissipation during the loading history, useful information for many mechanical applications. The multiplane model is finally modified to account for a limited dilatancy in a new and physically clear way. Numerical applications are developed for the illustrated interface models for different loading histories. • Review and comparison of interface models coupling damage, contact and friction. • Similarities and differences between Alfano-Sacco and other interface models. • Microstructured interface model for reproducing the dilatancy effect. • New formulation for modeling limited dilatancy in a physically consistent manner. [ABSTRACT FROM AUTHOR]

Recycled concrete aggregate is an environment-friendly and economical civil engineering material. The concrete blocks in a construction waste dump were crushed and screened to attain recycled concrete aggregate with different particle distributions. A series of monotonic direct shear tests were conducted through large direct shear apparatus to analyze the influence of different particle distributions on the shear characteristics of recycled concrete aggregate. Results show that during the shearing process, the various particle distribution samples experienced shear contraction then dilatation with the gradual increase of shear displacement. Shear strength of recycled concrete aggregate was optimal when the coefficient of uniformity was 13.67 and the coefficient of curvature was 2.44. An increase in normal stress led to an increase in the shear stress of each particle distributions. With the increasing of normal stress, the shear contraction of the shear surface increased, but dilatancy decreased. Under the same particle distribution, when the shear strength was maximized, shear dilatancy was also maximized. As the content of fine particles was increased, the apparent adhesion first decreased and then increased, while the internal friction angle first increased and then decreased. Multiple test results demonstrated that the effect of shear rate on shear characteristics is not obvious on the shear characteristics of the recycled concrete aggregate. An empirical formula for the dilatancy coefficient is established to describe the dilatancy characteristics of the recycled concrete aggregate quantitatively under different particle distributions and normal stresses. [ABSTRACT FROM AUTHOR]

In the era of high-speed trains, it is very important to ensure the safety and stability of rail tracks under adverse conditions including seasonal freezing and thawing. Freeze–thaw cycles (FTCs) affecting the engineering performance of coarse-grained soil (CGS) is one of the major reasons for track deterioration. The reported results of a number of static freeze–thaw triaxial tests on the shear behaviour of CGS are analysed herein. It was observed that confining pressure (σ3) and FTCs have a significant influence on the shear behaviour of CGS. In this paper, an empirical mathematical model has been proposed to capture the dilatancy of CGS subjected to FTCs during shearing. The empirical constants a, b, and c proposed in the model are a function of σ3 and FTCs. The results of the model have been compared with the laboratory experiments and are found to be in good agreement. [ABSTRACT FROM AUTHOR]

Experimental measurements during laboratory tests on salt specimens are important for the design of underground facilities in salt formations. These measurements, especially of the volumetric strain, can be influenced not only by the errors inherent to the experimental work, but also by the material heterogeneity. The natural variability due to the presence of impurities can lead to a non-negligible data scattering in the volumetric strain measurements and therefore raises questions concerning the specimens representativity. In this work, we study the effect of the presence of insoluble materials on the representativity of salt specimens during laboratory tests. An experimental investigation of this matter is delicate due to the shortcomings of laboratory work, thus we suggest a numerical method where salt specimens are modeled as pure halite matrices with insoluble inclusions. The insolubles distribution is described via a leveled Gaussian random field. Instead of using three different constitutive models, we use one law that is flexible enough to be adapted for each material involved (pure halite, inclusions and rock salt) with the adequate choice of parameter set. The modeling approach is presented, compared to the morphological modeling approach, and validated. It is employed to study the natural variability effect on strain measurements and to specify the size requirements to achieve representativity. The study proves that using non-representative specimens can misguide the rock characterization process and might even lead to optimistic design criteria. We propose a methodology that combines numerical and experimental work to overcome representativity issues. [ABSTRACT FROM AUTHOR]

This paper presents a generalized dilatancy angle equation of granular soil to cover not only the drained tests but also the undrained tests by introducing a generalized structure of soil: soil skeleton formed by soil particles and the fluid in soil voids, under the assumptions of the incompressibility of soil particles and the compressibility of the fluid in soil voids. For the drained tests, the generalized dilatancy angle equation of granular soil would be degenerated to its current dilatancy angle equation. However, for the undrained tests, the generalized dilatancy angle equation of granular soil was derived with a λ parameter that was related to the stress-strain state of soil and the nature of the fluid in soil voids. The λ parameter was determined by the initial dilatancy angles of granular soil at the onset of shearing on the same initial state of the soil in the drained and undrained tests. In addition, the generalized dilatancy angle equation of granular soil was verified for application in calculation of the dilatancy angles of sands in the drained and undrained tests. [ABSTRACT FROM AUTHOR]

We applied multiple linear regressions to scrutinize the maximum variability produced in soil gas radon (Rn-222) by pressure, temperature and rainfall. Statistical methodologies were applied to discriminate the effect of pressure, temperature and rainfall for precise identification of seismic induced anomalies. High Rn-222 anomalies (HRA) as well as Low Rn-222 anomalies (LRA) were apparent. There were nine earthquakes (Mw ≥ 3) which were discernible to be manifested by preseismic soil gas Rn-222 anomalies. Correspondingly, observation centered on radon data set considering the precursory time 'T' (days), epicentral distance 'D' (km) and magnitude 'M' (Mw) has been derived as Log (DT) = 0.79 M + b. The discernible linear equation was found to be substantial with value of coefficient 'b' as 0.18 and approximately equal to those obtained by various investigators. In general coefficient 'b' is assigned as 0.15 for gaseous geo-seismic precursor. The value of coefficient 'a ~ 3.51', 'b ~ 0.18' and correction factor 'K ~ 0.49 (kBqm-3 d)−1/2 were estimated empirically for the first time in Tezpur (Eastern Himalaya), India region. The calculated empirical value of 'a' by us gives another form of precursory manifestation zone (D) equation as D ~ 100.58 M. The values of coefficient 'a', 'b' and correction factor 'K' estimated for Tezpur, Assam (Eastern Himalaya) region perhaps can be used for probable earthquake forecast in the region constrained by the peak of the radon anomaly. The number of earthquakes registered was meager and further long-term analysis can estimate more precise and robust values of these parameters for soil gas Rn-222. The investigation also reassures us in a physical sense that seismic induced soil gas Rn-222 perturbation do exist in nature. The investigation is a probable approach for identification of seismic induced anomalies in soil gas Rn-222 and their characteristics for possible earthquake forecasting. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is tightly related to the existence of tectonic coal. Because large blocks of tectonic coals are difficult to be collected, reconstituted coal specimens are often used to study the mechanical properties. In this study, reconstituted coal specimens were prepared under different external forces. Then, triaxial compression tests were conducted under different confining pressures. The deviatoric stress, volumetric strain, axial strain and dilatancy ratio were analyzed. From the laboratory tests, the cohesion of reconstituted coal is positively related to the applied external force. Bonds are developed between coal particles. It is found that the deviatoric stress mainly contributes to bond breakage and internal friction. Under low confining pressure testing condition, the specimen is in over-consolidation state. The failure process is similar to the failure of intact coal. The deviatoric stress reaches the peak value. Then a strain-softening effect is observed with the bond breakage. But for normal consolidation scenario, the bond-breakage stress is smaller than the maximum internal friction force. With the increase of deviatoric stress, the shear stress exceeds the threshold limit value of bond breakage first and then, it exceeds the maximum internal force. Before the new unstable fracture development, extensive bond breakage happens. When the shear stress exceeds the maximum friction force, internal slippage occurs with sharp increases of dilatancy ratio. From this study, the failure behaviors of reconstituted coal are closely related to the stress conditions. The understanding of tectonic coal failure process is enhanced. Highlights: Reconstituted coal samples were prepared under different external forces and were tested under triaxial confining conditions. The stress dilatancy relationship is established and dilatancy ratio is quantified. The deviatoric stress contributes to the internal friction and bonds breakage for the consolidated specimen. Based on the OCR (overconsilidation ratio), different failure mechanisms were found for the reconstituted coals. [ABSTRACT FROM AUTHOR]

Particle size and confining pressure are important factors that influence the shear characteristics of coral sand. In this study, multiple consolidated-drained triaxial shear tests were conducted under varying conditions, to explore the effect of particle size and confining pressure on the shear characteristics of coral sand, in terms of critical confining pressure and shear strength, respectively. Particle survival rates of coral sand under different conditions were compared. It was found that (i) the stress-softening coefficient and dilatancy coefficient had a semilogarithmic linear relationship with effective confining pressure. With an increase in effective confining pressure, the strain-softening and dilatancy of the coral sand weakened gradually. Compared with small-particle coral sand, the strain-softening and dilatancy of large-particle coral sand attenuated at a greater rate, and the mode conversion for deformation curves occurred prior to that for the strength curves. (ii) Under the same relative density, because of differences in the void ratio of coral sand with different particle sizes, the shear strength deviations increased gradually with an increase in effective confining pressure. For this reason, the internal friction angle of coral sand decreased with an increase in particle size, whereas its apparent cohesion presented an opposite trend. (iii) Under the combined action of single-particle crushing strength and particle coordination number, coarse coral sand (D = 1.0–2.0 mm) showed the largest amount of particle breakage. [ABSTRACT FROM AUTHOR]

Many shear strength criteria have been proposed for soils. The Mohr–Coulomb, Matsuoka–Nakai, and Lade–Duncan criteria are more frequently used for sands. For sands sheared in drained conditions, the general stress–dilatancy relationship is obtained from the frictional state concept. It is shown that, in failure states, the dilatancy for triaxial compression, the plane strain condition (biaxial compression), triaxial extension, and general states can be expressed by the ratio of the volumetric and axial strain increments for triaxial compression. By using the frictional state concept, the shear strength of sand for general states can be expressed by the critical state angle and the dilatancy for drained triaxial compression. It is shown that the calculated shear strength of the sand is similar to that obtained by using the Mohr–Coulomb, Matsuoka–Nakai, and Lade–Duncan criteria for the non-dilative, moderate-dilative, and high-dilative behaviors of sand, respectively. Therefore, the shear strength of sand has a purely dilative nature for deformations without breakage effects. [ABSTRACT FROM AUTHOR]

Soft rock squeezing deformation mainly consists of pre-peak damage-dilatancy and post-peak fracture-bulking at the excavation unloading instant, and creep-dilatancy caused by time-dependent damage and fracturing. Based on the classic elasto-plastic and Perzyna over-stress viscoplastic theories, as well as triaxial unloading confining pressure test and triaxial unloading creep test results, an elasto-plastic and viscoplastic damage constitutive model is established for the short- and long-term dilatancy and fracturing behavior of soft rock squeezing deformation. Firstly, the criteria for each deformation and failure stage are expressed as a linear function of confining pressure. Secondly, the total damage evolution equation considering time-dependent damage is proposed, including the initial damage produced at the excavation instant, in which the damage variable increases exponentially with the lateral strain, and creep damage. Thirdly, a transient five-stages elasto-plastic constitutive equation for the short-term deformation after excavation that comprised of elasticity, pre-peak damage-dilatancy, post-peak brittle-drop, linear strain-softening, and residual perfectly-plastic regimes is developed based on incremental elasto-plastic theory and the nonassociated flow rule. Fourthly, regarding the time-dependent properties of soft rock, based on the Perzyna viscoplastic over-stress theory, a viscoplastic damage model is set up to capture creep damage and dilatancy behavior. Viscoplastic strain is produced when the stress exceeds the initial static yield surface fs; the distance between the static yield surface fs and the dynamic yield surface fd determines the viscoplastic strain rate. Finally, the established constitutive model is numerically implemented and field applied to the -848 m belt conveyer haulage roadway of Huainan Panyidong Coal Mine. Laboratory test results and in-situ monitoring results validate the rationality of the established constitutive model. The presented model takes both the transient and time-dependent damage and fracturing into consideration. [ABSTRACT FROM AUTHOR]

This paper focuses on the presence of nodules of insoluble materials within salt specimens, and their effect on the volumetric strain measurements and the dilatancy phenomenon. We analyzed experimental results of over 120 conventional triaxial compression tests, and found that in 20% of the cases, the volumetric strain measurements were atypical. We also noted that the natural variability of the specimens can lead to a non-negligible data scattering in the volumetric strain measurements when different specimens are subjected to the same test. This is expected given the small magnitude of those strains, but it occasionally implies that the corresponding specimens are not representative of the volumetric behavior of the studied rock. In order to understand these results, we numerically investigated salt specimens modeled as halite matrices with inclusions of impurities. Simulations of triaxial compression tests on these structures proved that such heterogeneities can induce dilatancy, and their presence can lead to the appearance of tensile zones which is physically translated into a micro-cracking activity. The modeling approach is validated as the patterns displayed in the numerical results are identical to that in the laboratory. It was then employed to explain the observed irregularities in experimental results. We studied the natural variability effect as well and proposed a methodology to overcome the issue of specimen representativity from both deviatoric and volumetric perspectives.

With rapid development of infrastructures like tunnels and open excavations in Shanghai, investigations on deeper soils have become critically important. Most of the existing laboratory works were focused on the clayey strata up to Layer 6 in Shanghai, i.e. at depth of up to 40 m. In this paper, Layers 7, 9, and 11, which were mostly formed of sandy soils at depth of up to 150 m, were experimentally investigated with respect to physico-mechanical behaviors. The stress–strain behaviors were analyzed by the consolidated drained/undrained (CD/CU) triaxial tests under monotonic loading. One-dimensional (1D) oedometer tests were performed to investigate the consolidation properties of the sandy soils. Specimens were prepared at three different relative densities for each layer. Also, the micro-images and particle size analyzers were used to analyze the shape and size of the sand grains. The influences of grain size, density, and angularity on the stress–strain behaviors and compressibility were also studied. Compared to the other layers, Layer 11 had the smallest mean grain size (D50), highest compressibility, and lowest shear strength. In contrast, Layer 9 had the largest mean grain size, lowest compressibility, and highest shear strength. Layer 7 was of intermediate mean grain size, exhibiting more compressibility and less shear strength than that of Layer 9. Also, the critical state parameters and maximum dilatancy rate of different layers were discussed.

We propose a dilatant, two-layer debris flow model validated by full-scale density/saturation measurements obtained from the Swiss Illgraben test site. Like many existing models, we suppose the debris flow consists of a matrix of solid particles (rocks and boulders) that is surrounded by muddy fluid. However, we split the muddy fluid into two fractions. One part, the inter-granular fluid, is bonded to the solid matrix and fills the void space between the solid particles. The combination of solid material and inter-granular fluid forms the first layer of the debris flow. The second part of the muddy fluid is not bonded to the solid matrix and can move independently from the first layer. This free fluid forms the second layer of the debris flow. During flow the rocky particulate material is sheared which induces dilatant motions that change the location of the center-of-mass of the solid. The degree of solid shearing, as well as the amount of muddy fluid and of solid particles, leads to different flow compositions including debris flow fronts consisting of predominantly solid material, or watery debris flow tails. De-watering and the formation of muddy fluid washes can occur when the solid material deposits in the runout zone. After validating the model on two theoretical case studies, we show that the proposed model is able to capture the streamwise evolution of debris flow density in time and space for real debris flow events. [ABSTRACT FROM AUTHOR]

The energy consumption of particle breakage is added to the Cambridge energy balance equation so that the energy balance equation for coarse-grained soil is obtained. To reasonably measure the energy consumption of particle breakage, a function of friction coefficient relating to the axial strain is proposed to replace the friction coefficient as a constant in the energy balance equation based on the evolution rule of particle breakage. Then, according to the energy balance equation of coarse-grained soil, the energy consumption of particle breakage is calculated, and the particle breakage energy increases with axial strain increasing, which satisfies the thermodynamic law. Based on this energy balance equation, a simple stress dilatancy relationship is developed. In this stress dilatancy relationship, the relationship between d E b / p d ε s and M / M f can be described by a simple function with acceptable accuracy. This stress dilatancy relationship is validated with the satisfactory capability to predict the dilatancy behavior of coarse-grained soils, which can be the effective choice to build the constitutive model. [ABSTRACT FROM AUTHOR]

The Xigeda Formation has the characteristics of poor cementation, variable structure, and softening in water. It is prone to lead to failure of the initial support structures, arch collapse, and even roof fall. The key to the problem lies in the exploration of the interaction between surrounding rock and support. Therefore, based on the constraint loss theory of the tunnel face space effect, this paper constructively introduces multiple dependent variables, including the intermediate principal stress, the dilatancy of the surrounding rock, the timeliness of the shotcrete support stiffness, and the water content of the Xigeda formation, to decouple the whole process of the interaction between the surrounding rock and support, and discusses the influence of each variable. The research results show that: i) there is a critical support point in the tunnel under the influence of the unified strength theoretical parameter b and water content, and when the unified strength theoretical parameter b is constant, the critical distance of the support will decrease with the increase of the water content; ii) from the perspective of deformation control, the stress of support structure will increase with the increase of dilatancy angle of Xigeda surrounding rock; iii) considering the hardening characteristics of shotcrete, the initial support stiffness of Xigeda tunnel increases nonlinearly, and the whole process of stress and deformation can be divided into four stages; iv) the combined support structure of section steel and grille has little difference in the deformation control effect of the surrounding rock and the stress of the support structure, but the combined support structure of the grille steel frame is more sensitive to the hardening parameters. [ABSTRACT FROM AUTHOR]

The main objective of this study is to compare the mechanical behavior of two sands (Hostun or Dune sands) mixed with crushed rubber obtained from used tires. However, it is essential to ensure that his geotechnical application do not result in long-term negative impacts on the environment. The chemical properties of these two sands are given by energy dispersive analysis X-ray fluorescence spectrometry. The mineral composition of these two sands is performed by X-ray diffractometry. The morphological characteristics of the sand grains are given by the analysis of the images of the two sands given by the scanning electron microscope. This study is based on 120 direct shear tests performed on sand-rubber aggregate mixtures. The results show that the rubber content of the aggregates has a significant effect on the shear strength of sand-rubber mixtures in both cases of sand. In fact, the shear strength of the sand-rubber mixture increases with increasing crushed rubber up to 20% for different normal stresses. The analysis of the test results also shows the effect of the angular shape of the sand grains on the interparticle friction. The contribution of the structure effect in the mobilized friction is analyzed by comparing the shear test results of Hostun and dune sand mixtures. [ABSTRACT FROM AUTHOR]

This study presents results from a series of large-scale simple shear tests performed on sandy soil reinforced with randomly distributed fibers. The soil was mixed with polypropylene, glass and basalt fiber, respectively, with fiber content varying from 0% to 2% and sheared under a normal stress ranging from 100 kPa to 300 kPa. The effects of fiber type, content and normal stress on shear strength and the dilatancy behavior were examined. The friction state theory was employed to interpret the influence of fiber reinforcement on the dilatancy behavior. The results indicate that the additions of polypropylene and basalt fibers are beneficial to enhancing ultimate shear strength, whereas the glass fiber makes little contribution. It is shown that the fiber inclusion hinders the close packing and interlocking of soil particles and hence produces relatively looser soil fabric in the as-compacted state, leading to increased volume contraction accompanied by a negligible improvement or even reduction in shearing stiffness over a small to moderate strain range. The most obvious increase in shear strength generally occurs at a shear strain exceeding 10%, where the fibers and particles are in tight contact with each other, allowing full interface interaction and the mobilization of fiber tensile strength. [ABSTRACT FROM AUTHOR]

The apparent resistivity imaging at multiparametric geophysical observatory (MPGO), Ouguri Hills, Tezpur was carried out since 31st August, 2016 for fixed interval time of 3 days to investigate precursory signature prior to earthquake events. Anomalies in apparent resistivity prior to earthquake events are observed. The anomalies are not influenced by the rainfall activity in the region during the investigation period. We adopt Weibull distribution technique with observed apparent resistivity data to look for and revalidate the observations. The Weibull parameters, i.e. K and m are estimated to be 0.26 and 1.04, respectively. No precise relation between magnitude and earthquake precursory time is found. Simultaneously, an attempt has been made to establish a relation between the earthquake magnitude (M) and precursory time (T) and a relation between the earthquake precursory time (T) and distance of the earthquake (Δ) from the observation point. Weibull probability distribution indicate the probability of an earthquake occurrence exceeds the measure up to 80% (9 days) after the precursory signal is observed. A comparison of probability analysis ascertains a mean precursory time and its standard deviation.

Beyond a threshold applied compressive stress, porous rocks typically undergo either dilatant or compactant inelastic deformation and the response of their physical properties to deformation mode is key to mass transport, heat transport and pressure evolution in crustal systems. Transitions in failure modes-involving switches between dilatancy and compaction-have also been observed, but to date have received little attention. Here, we perform a series of targeted mechanical deformation experiments on porous andesites, designed to elucidate complex post-failure deformation behaviour. By investigating a sample suite and effective pressure range that straddles the transition between positive and negative volumetric responses to compression, we show two post-failure critical stress states: a transition from compaction to dilation ( C ∗ ' ), and a transition from dilation to compaction, which we term C ' ∗ . We demonstrate that multiple switches in deformation mode can be driven by stress application under conditions relevant to the shallow crust. While the effect on fluid flow properties of compaction-to-dilation switching may be masked by a net reduction in sample porosity, samples that underwent dilatant-to-compactant failure mode switching exhibited an increase in permeability of approximately two orders of magnitude, despite only slight net volumetric change. Such a substantial permeability enhancement underscores the importance of post-failure deformation in influencing solute and heat transfer in the crust, and the generation of supra-hydrostatic fluid pressures in volcanic environments., Competing Interests: We declare we have no competing interests., (© 2024 The Author(s).)

Earthquakes result from fast slip that occurs along a fault surface. Interestingly, numerous dense geodetic observations over the last two decades indicate that such dynamic slip may start by a gradual unlocking of the fault surface and related progressive slip acceleration. This first slow stage is of great interest, because it could define an early indicator of a devastating earthquake. However, not all slow slip turns into fast slip, and sometimes it may simply stop. In this study, we use a numerical model based on the discrete element method to simulate crustal strike‐slip faults of 50 km length that generate a wide variety of slip‐modes, from stable‐slip, to slow earthquakes, to fast earthquakes, all of which show similar characteristics to natural cases. The main goal of this work is to understand the conditions that allow slow events to turn into earthquakes, in contrast to those that cause slow earthquakes to stop. Our results suggest that fault surface geometry and related dilation/contraction patterns along strike play a key role. Slow earthquakes that initiate in large dilated regions bounded by neutral or low contracted domains, might turn into earthquakes. Slow events occurring in regions dominated by closely spaced, alternating, small magnitude dilational and contractional zones tend not to accelerate and may simply stop as isolated slow earthquakes. Plain Language Summary: Numerous research studies indicate that earthquakes may start progressively by a slow unlocking of the fault surface followed by fast slip acceleration. This first slow‐slip stage could then define an early indicator of a devastating earthquake. However, not all slow‐slip become fast, and some may simply stop. In this study, we use a numerical model to simulate faults that generate both slow earthquakes and fast earthquakes, sharing similar characteristics to natural cases. The main goal is to understand the conditions that allow slow events to turn into earthquakes, in contrast to those that cause slip to abort. Our results suggest that fault surface geometry and induced strain patterns play a key role. Slow events that initiate in large dilated regions might turn into earthquakes whereas the ones occurring in regions showing more complex strain pattern including contracted zones may simply stop as isolated inoffensive slow earthquakes. This work could have significant implications for the development of new monitoring systems devoted to anticipate the occurrence of destructive earthquakes. Key Points: A discrete element model is used to investigate earthquake nucleation mechanismsFault geometry and precursory dilation/contraction patterns along strike might be predictive by controlling the earthquake energy balanceSlow events in dilated zones bounded by neutral areas turn into earthquakes whereas those in alternating dilated/contracted zones stop [ABSTRACT FROM AUTHOR]

This paper focuses on the influence of volume change boundary condition on the instability and post-peak softening of sand. The laboratory program comprises an extensive series of Direct Simple Shear (DSS) tests with different degrees of linear coupling between volumetric and shear strains. It is observed that progressive loosening of sand associated with the volumetric expansion in a coupled strain path intensifies instability susceptibility, whereas volumetric contractive strains reduce vulnerability to instability and post-peak softening. Analysis of the experimental data using Hill's second-order work criterion designates a family of state-dependent relational trends for instability lines, normalized peak shear strengths, and maximum pore-water pressure ratios depending on the degree of coupling between the volumetric and shear strain rates. It is found that a simple state-dependent constitutive model can predict relational trends in sand specimens suffering from instability and post-peak softening under different degrees of the linear coupling between the volumetric and shear strains. [ABSTRACT FROM AUTHOR]

Soil dilatancy behavior is crucial in the numerical modeling of slopes for safety factor (SF) computation. The effect of dilatancy on the SF of stabilized expansive soils has been rarely reported. The present study thus focused on the effect of dilatancy on the numerical modeling (NM) of coconut shell ash (CSA)-treated expansive soil slopes using the strength reduction finite element analysis in PLAXIS for SF computation. The NM input parameters were mainly obtained from laboratory experiments performed for the stabilization of the expansive soil using CSA. The degree of non-associativity, which was incorporated during the NM for SF computation, was considered as a crucial variable to check the dilatancy effect on the SF. Predictive modeling of the SF was subsequently done using multiple linear regression (MLR), artificial neural network (ANN), adaptive neuro-fuzzy inference system (ANFIS) and support vector regression (SVR). The results obtained show that the dilatancy has a pronounced effect for overconsolidated clays. Furthermore, consideration of the degree of non-associativity as a variable quantifying the dilatancy behavior resulted in better predictions of SF. Through parametric studies, optimal parameters for the ANN, ANFIS and SVR that affected the SF prediction were established. Overall, the ANN and ANFIS had superior predictive capability when compared with the MLR and SVR. [ABSTRACT FROM AUTHOR]

This paper presents the application of digital image correlation to uniaxial and diametral compression tests to identify the Drucker-Prager/cap shear failure and the elastic parameters of a lead zirconate titanate powder (PZT). The calibrated parameters were validated through finite element simulation of the cold isostatic pressing of a hollow hemisphere used in piezoelectric transducers. The results showed that the dilatancy stresses were 50–65% lower than the fracture stresses, demonstrating that adopting the fracture stress may overestimate material cohesion. The simulation using the calibrated model achieved an excellent agreement with the measured geometry, with a difference of only 1.1% in the dimension and 2.0% in the average density. The results highlight how digital image correlation can obtain more accurate parameters with fewer tests. The proposed methodology can be used to calibrate other powder materials, especially those in which dilatancy plays a key role. [Display omitted] • Image correlation was used to partially calibrate the Drucker-Prager/cap model. • More model parameters were obtained with fewer tests using image correlation. • Image correlation identified the dilatancy stress which occurs before fracture. • Finite element simulation of a hollow hemisphere of PZT validated the methodology. • Calibration considering dilatancy data improved the powder compaction simulation. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Advanced Engineering Science / Gongcheng Kexue Yu Jishu is the property of Advanced Engineering Science Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Wet granular materials develop both shear stresses and normal stresses during simple shear. Over a certain range of shear rate levels, both the shear stress and the normal stress that develop are linearly proportional to the shear rate. In addition, such materials exhibit permanent increase in volume during simple shear. In this article, a model is developed to capture these features of the mechanical behavior of such wet granular materials. The model is developed within a Gibbs-potential-based thermodynamic framework utilizing the notion of the body possessing multiple natural configurations. Specific functional forms are assumed for the Gibbs potential and the rate of dissipation and the constitutive relations are obtained by the maximizing the rate of dissipation (rate of entropy production). Dilatant behavior is captured by coupling the dissipation due to volume changes to the dissipation associated with the distortion of the material. [ABSTRACT FROM AUTHOR]

Fine-grained calcareous silt interlayers with a non-uniform thickness are distributed discontinuously in the hydraulically reclaimed foundation of island-reefs in the South China Sea. Because of their high compressibility and low strength, these interlayers commonly cause differential foundation settlement and result in the cracking of buildings, which poses a potential safety hazard. In this study, consolidated-drained triaxial shear tests were conducted to investigate the effect of fines content (FC) on the shear strength of calcareous sand. The brittleness index and dilatancy coefficient were introduced to quantify the strain-softening and dilatancy of calcareous sand. The mechanism by which the FC affects the calcareous sand strength was revealed in terms of the particle shape and soil structure. Under a constant effective confining pressure, an increase in FC led to a gradual decrease in peak strength, a weakening of strain-softening characteristics, less dilation, and a linear decrease in apparent cohesion for calcareous sand. The internal friction angle of calcareous sand peaked at a FC of 10%. Compared with quartz sand, calcareous sand has the more irregular particle shape. An increase in FC weakened the interlocking between calcareous sand particles, which decreased the shear strength. The research results are of great significance for slope stability analysis and for the foundation treatment of island reefs. [ABSTRACT FROM AUTHOR]

To clarify the effects of compactness and stress level on the strength and deformation characteristics of coral sand in the reclaimed foundation of island reefs in the South China Sea, consolidated-drained triaxial shear tests were conducted on coral sand with four different relative densities under a constant confining pressure range. Brittleness index and dilatancy coefficient were introduced to quantitatively express the strain softening and dilatancy characteristics of coral sand. The results showed that when σ 3 ′ was between 150 and 300 kPa, the strain softening and dilatancy characteristics of coral sand decreased with increasing effective confining pressure and decreasing compactness. Brittleness index and dilatancy coefficient exhibited a good linear relationship with effective confining pressure and relative density, respectively. The peak friction angle and dilatancy angle of coral medium sand both decreased with increasing effective confining pressure and decreasing relative density. Based on this behavior, the attenuation amplitudes of peak friction angles of medium dense and dense coral medium sand with increasing confining pressure logarithm were determined. The critical-state friction angle of coral medium sand was determined based on Bolton's stress–dilatancy relationship. The relational expressions of peak friction angle/relative dilatancy index with mean effective stress and relative density were established, respectively. [ABSTRACT FROM AUTHOR]

In this work, a novel constitutive model is developed within the framework of fractional plasticity to delineate the coupling between inelastic deformation and damage of quasi-brittle materials. Faced with the common challenge of determining plastic flow direction, we resort herein to the Riemann–Liouville definition of fractional derivatives, instead of introducing an additional plastic potential. The pre-peak hardening behavior is described using an exponential function, while the post-peak softening response is viewed as the consequence of material damage. For describing damage evolution, a damage criterion is constructed in terms of plastic volume dilation related to micro-crack growth. This is conducive to supply a new insight for describing the complex influence of the non-orthogonality of plastic flow on damage evolution. For numerical applications, a semi-implicit return mapping algorithm is proposed. The predictive performance of the model is evaluated by comparing numerical simulations with experimental data under various loading paths. [ABSTRACT FROM AUTHOR]

As a common phenomenon in granular flow, grain segregation plays a great role in affecting the behavior of granular soil by causing a great change of grain-void distribution in granular soil. This paper presents an experimental study on the influence of grain segregation on the behavior of sand, by a number of triaxial tests to interpret the characteristic behavior, friction and dilatancy behavior, excess pore water pressure behavior and critical state behavior of sand incorporating grain segregation. An index - grain segregation index Igs was proposed to quantify grain segregation. Grain segregation affected greatly the characteristic behavior of sand, causing the movement of void ratio-dilatancy relation of sand towards the increase of void ratio and dilatancy of sand. In the drained tests, the mobilized friction angle of sand showed a decrease followed by an increase but the mobilized dilatancy angle of sand increased, with increasing grain segregation index. An increase in grain segregation index impaired the basic friction of sand. In the undrained tests, the mobilized friction angle of sand showed an increase followed by a decrease with increasing grain segregation index. However, grain segregation caused an increase of the mobilized dilatancy of sand followed by a different development. An increase in grain segregation resulted in a higher summit of the dilatancy of sand but with a faster decrease along axial strain. In the q-p′ plane, grain segregation caused a reciprocating rotation of the dilatancy line and failure line of sand. Grain segregation resulted in enhancement of the peak-state dilatancy of sand, affecting greatly peak-state friction angle and peak-state basic friction angle of sand as well as the normalized excess pore water pressure. The excess friction angle of sand showed an increase followed by a decrease in the drained tests but increased linearly in the undrained tests, with increasing grain segregation index. The excess friction angle-over-maximum dilatancy angle of sand decreased in up convexity while increasing grain segregation index. Grain segregation resulted in rotation and translation of the critical state line of sand in the e-p′α=0.7 plane. However, in the q-p′ plane, the critical state line of sand showed an anticlockwise rotation followed by a clockwise rotation with increasing grain segregation index. [ABSTRACT FROM AUTHOR]

The effects of fibre (0–3 %) and cement (0–3 %) additives, on Toyoura sand were examined under consolidated drained compression and extension loading conditions. All samples were prepared to a target dry density value (e.g., ρ d = 1.489 g/cm3) of Toyoura sand using under-compaction moist tamping technique. In compression, the unreinforced specimens exhibited a behavior of medium dense sand and reached a peak deviator stress (qp) at approximately 4 % axial strain ( ϵ a ) for the varying mean effective stresses, pʹ (i.e. 50–400 kPa). The peak drained strength increases in fibre reinforced cemented specimens were found to be up to 132 % (lower effective stresses) and 243 % (higher effective stresses), while, the drained strength increases at critical state for the fibre reinforced cemented specimens were found to be up to 105 % (lower effective stresses) and 245 % (higher effective stresses). Overall, the fibre and cement additives increased the stiffness, peak and strength at critical state of pure Toyoura sand but were found to be least effective in extension loading. Moreover, the stress ratio, peak and critical state stress ratios increase with the addition of fibres and cement. The secant modulus shows limited increases for the fibre reinforced specimens. However, a significant improvement in the secant modulus is observed for the fibre reinforced cemented specimens. For both unreinforced and reinforced specimens there is a decrease in volumetric strain with greater effective stresses or in other words, the rate of dilation decreases with increases in effective stresses. The fibre and cement additives also increased the strength parameters (frictional angle, cohesion), dilatancy angle, slope of the critical state line, and decreased the state parameter of pure Toyoura sand. [ABSTRACT FROM AUTHOR]

An accurate theoretical model to predict the extent and mechanical behavior of the excavation damage zone (EDZ) in the surrounding rock of deep-buried tunnel is of great importance for the practical engineering applications. Using the elastic-plastic theory and combined with the analysis on the stress characteristics of the tunnel surrounding rock, this paper present a predict model for the EDZ formation and evolution. A three-zone composite mechanical model was established for the tunnel surround rock and the corresponding stress state and displacement field of three zones were derived. The effects of the strain softening and dilatancy during rock deformation was taken into account. The correctness of the proposed model was validated by the existing theoretical models. A sensitivity analysis for different influencing factors in this model was also performed. The results can benefit for the future numerical and experimental studies.