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Swarms are microearthquake clusters governed by aseismic deformation, fluid migration, and stress changes, but the underlying mechanisms for their recurrence remain elusive. In 2008, abundant swarms were observed on the westernmost Gofar transform fault. Microearthquake clustering reveals five distinct fluid chambers: three active before the September mainshock, generating quasi‐periodic swarms and inter‐chamber migration, and two new chambers that emerged afterward, exhibiting spatial independence and disrupted periodicity. A conceptual hydro‐mechanical fault model, incorporating creep‐driven compaction, shear‐induced dilatancy, elastic stress transfer, and rate‐and‐state friction, reproduces the main features of these swarms and perturbation by the mainshock. Results suggest that compaction‐dilatancy cycles within individual chambers, along with chamber interactions, control swarm periodicity. Mainshock‐induced stress resets the fault's hydro‐mechanical‐chemical state, and altered compaction rates could perturb swarm recurrence. This self‐organizing cycle links swarms to fluid overpressure and stress redistribution, offering an alternative physical framework for explaining various fault slip modes across active fault systems.

The mechanical behavior of rock under cyclic loading is quite complicated compared to monotonic loading or unloading conditions. The triaxial cyclic loading and unloading testing of rock specimens under 6 confining pressures (σ3) was carried out through the MTS 815 rock mechanics testing system, to explore the strength, deformation, and expansion characteristics of the rock specimens. The stress–strain curves of the rock specimens in the triaxial cyclic loading and unloading testing presented the hysteresis effect. Besides, as σ3 increased, the rock specimen strength increased, while the failure form brittle to ductile. The elasticity modulus (El) increased first and consequently decreased as the cycle index increased, while it increased as σ3 increased. However, the generalized Poisson’s ratio (μl) increased as the cycle index increased, whereas it decreased as the σ3 increased. Based on the Mohr–Coulomb strength criterion and plastic shear strain (γp) as the plastic parameter, the subsequent yield plane model of the loaded rock was characterized by generalized cohesion (c´) and generalized internal friction angle (φ´). Ultimately, the evolution rules of c´, φ´ and Ψ (dilatancy angle), with σ3 and γp were revealed. Moreover, the post-peak dilatancy angle models with regard to the influence of σ3 and γp on the volume dilatancy of the rock specimen were established, which indicated that Ψ increased first and consequently decreased along with the γp increase, whereas it decreased as the σ3 increased.

The crack initiation stress threshold (σci) is an essential parameter in the brittle failure process of rocks. In this paper, a volumetric strain response method (VSRM) is proposed to determine the σci based on two new concepts, i.e., the dilatancy resistance state index (δci) and the maximum value of the dilatancy resistance state index difference (Δδci), which represent the state of dilatancy resistance of the rock and the shear sliding resistance capacity of the crack-like pores during the compressive period, respectively. The deviatoric stress corresponding to the maximum Δδci is taken as the σci. We then examine the feasibility and validity of the VSRM using the experimental results. The results from the VSRM are also compared with those calculated by other strain-based methods, including the volumetric strain method (VSM), crack volumetric strain method (CVSM), lateral strain method (LSM) and lateral strain response method (LSRM). Compared with the other methods, the VSRM is effective and reduces subjectivity when determining the σci. Finally, with the help of the proposed VSRM, influences from chemical corrosion and confining stress on the σci and Δδci of the carbonate rock are analyzed. This study provides a subjective and practical method for determining σci. Moreover, it sheds light on the effects of confinement and chemical corrosion on σci.HighlightsA volumetric strain response method (VSRM) is proposed to determine the crack initiation stress threshold from the volumetric strain curve.Two novel parameters, i.e., the dilatancy resistance state index and the maximum dilatancy resistance state index difference, are proposed to help the VRSM determine σci.Rock’s compressive stage is divided into two stages: the interlocking stage and the shear sliding stage. The crack initiation stress threshold divides these two stages.Relationships between the crack initiation stress threshold and rock’s mechanical properties (i.e., Young’s modulus, Poisson’s ratio, mobilized cohesion and friction angle) are analyzed.

•Different definitions and interpretations of dilatancy measures are discussed.•Significance of dilatancy angle in concrete damaged plasticity (CDP) model is shown.•Dependence of numerical results on dilatancy angle is examined for two benchmarks.•Recommendations for use of CDP model in structural simulations are formulated. The so-called concrete damaged plasticity (CDP) model is frequently employed by ABAQUS users to simulate the behaviour of concrete. One important aspect of the model, namely the representation of material dilatancy, is evaluated in the paper. The role of the dilatancy angle in pressure-dependent plasticity models is reviewed. The plastic potential adopted in the CDP model is discussed. It is shown that the definitions of the angle in the CDP model and in the Burzyński–Drucker–Prager (BDP) plasticity model for a continuum can lead to different angle magnitudes. Two tests on concrete configurations are simulated to illustrate how strongly the angle influences the results: the Kupfer benchmark of a panel under uniaxial or biaxial compression and the punching shear response in a slab-column connection. The importance of viscosity in cracking simulation is thereby mentioned, the results are compared with experimental ones and mesh sensitivity is verified. Recommendations for analysis of concrete mechanics problems are formulated.

Using deep salt caverns for underground energy storage is a globally recognized method of energy storage. The safety of gas storage and the utilization rate of salt mine formation resources are related to the pillar design of salt cavern gas storage. Existing theoretical research on the pillar design of salt cavern gas storage prioritizes universality. This may affect the accuracy of regional research. In this study, triaxial mechanical tests are conducted on salt rock under varying confining pressures. Moreover, a linear dilatancy limit equation is fitted according to the stress state at the dilatancy point. Based on the dilatancy failure mechanism of salt rock pillars, considering the influence of the cavern parameters of the salt rock and internal pressure during gas storage operation, the criteria for pillar failure were improved, and a theoretical method for pillar stability evaluation that is suitable for the Jintan area was established. Through the comparison of numerical simulation and theoretical results under different working conditions, the influence laws on pillar stability in terms of pillar width, cavern air pressure and salt hole depth were obtained, and the accuracy of the theoretical method for pillar stability evaluation was verified. Using the pillar stability analysis method to calculate the actual working conditions of a gas storage cavern in Jintan, the minimum operating pressure obtained was in line with the field working conditions. This method can provide a reference for future pillar design of salt caverns for gas storage in the Jintan area.

Rock salt under a generalised state of stress is either under the compaction or dilatancy domain. The compaction domain is more favourable for engineering use than the dilatancy domain. This study explores the possible application of the AE technique to identify the favourable or unfavourable state of rock salt. The present study analyses AE data of high-halite percentage Khewra rock salt (> 95% halite mineral) under a quasi-static unconfined compressive strength test. The study employs a unique approach to identifying AE events using a detection function. It has the advantage over a conventional threshold-based method of isolating individual events from the short temporal burst of larger numbers of AE events. The source mechanisms of isolated AE events are identified using the AF–RA method. The results from the analysis conclude that 79% of AE events are due to tensile microcracks, and 21% of AE events are due to shear microcracks. Tensile microcracks release 85% of AE energy. The fracture source of AE events in the case of rock salts is unique, and it differs from the reported literature of other geomaterials where shear microcracks constitute a significant amount of AE events. In the present study, the results of the AF–RA analysis are divided into three stages. The stress path of the three stages is compared with the established dilatancy boundary of rock salts. It is observed for the Khewra rock salt that the stress path of Stage I closer to the compaction domain (favourable, stable) reveals that 42% of AE events originated from shear microcracks, and 58% of AE events originated from tensile microcracks. In the case of Stages II and III, the stress path is in the dilatancy domain (unfavourable, unstable); more than 80% of AE events sources originated from tensile microcracks. The results indicate that in-situ AE monitoring and its AE source mechanism identification can be used to identify rock salts’ favourable and unfavourable conditions.

Giant rockslides are widespread and sensitive to hydrological forcing, especially in climate change scenarios. They creep slowly for centuries and then can fail catastrophically posing major threats to society. However, the mechanisms regulating the slow-to-fast transition toward their catastrophic collapse remain elusive. We couple laboratory experiments on natural rockslide shear zone material and in situ observations to provide a scale-independent demonstration that short-term pore fluid pressure variations originate a full spectrum of creep styles, modulated by slip-induced undrained conditions. Shear zones respond to pore pressure increments by impulsive acceleration and dilatancy, causing spontaneous deceleration followed by sustained steady-rate creep. Increasing pore pressure results in high creep rates and eventual collapse. Laboratory experiments quantitatively capture the in situ behavior of giant rockslides and lay physically-based foundations to understand the collapse of giant rockslides. Giant rockslides creep slowly for centuries and then can fail catastrophically, posing major threats to society. Here, the authors use observational and experimental evidence to quantitatively capture the full spectrum of giant rockslide behaviour until collapse, that is modulated by hydro-mechanical response to short-term fluid pressure perturbations.

Pore fluid pressure plays an important role in the frictional strength and stability of tectonic faults. We report on laboratory measurements of porosity changes associated with transient increases in shear velocity during frictional sliding within simulated fine‐grained quartz fault gouge (d50 = 127 μm). Experiments were conducted in a novel true triaxial pressure vessel using the double‐direct shear geometry. Shearing velocity step tests were used to measure a dilatancy coefficient (ɛ = Δϕ/Δln(v), where ϕ is porosity and v is shear velocity) under a range of conditions: background shearing rate of 1 μm/s with steps to 3, 10, 30, and 100 μm/s at effective normal stresses from 0.8 to 20 MPa. We find that the dilatancy coefficient ranges from 4.7 × 10−5 to 3.0 × 10−4 and that it does not vary with effective normal stress. We use our measurements to model transient pore fluid depressurization in response to dilation resulting from step changes in shearing velocity. Dilatant hardening requires undrained response with the transition from drained to undrained loading indexed by the ratio of the rate of porosity change to the rate of drained fluid loss. Undrained loading is favored for high slip rates on low‐permeability thick faults with low critical slip distances. Although experimental conditions indicate negligible depressurization due to relatively high system permeability, model results indicate that under feasible, but end‐member conditions, shear‐induced dilation of fault zones could reduce pore pressures or, correspondingly, increase effective normal stresses, by several tens of megapascals. Our results show that transient increases in shearing rate cause fault zone dilation. Such dilation would tend to arrest nucleation of unstable slip. Pore fluid depressurization would exacerbate this effect and could be a significant factor in generation of slow earthquakes, nonvolcanic tremors, and related phenomena.

To simulate debris-flow behaviour from initiation to deposition, we derive a depth-averaged, two-phase model that combines concepts of critical-state soil mechanics, grain-flow mechanics and fluid mechanics. The model's balance equations describe coupled evolution of the solid volume fraction, m, basal pore-fluid pressure, flow thickness and two components of flow velocity. Basal friction is evaluated using a generalized Coulomb rule, and fluid motion is evaluated in a frame of reference that translates with the velocity of the granular phase, vs. Source terms in each of the depth-averaged balance equations account for the influence of the granular dilation rate, defined as the depth integral of vs. Calculation of the dilation rate involves the effects of an elastic compressibility and an inelastic dilatancy angle proportional to mmeq, where meq is the value of m in equilibrium with the ambient stress state and flow rate. Normalization of the model equations shows that predicted debris-flow behaviour depends principally on the initial value of mmeq and on the ratio of two fundamental timescales. One of these timescales governs downslope debris-flow motion, and the other governs pore-pressure relaxation that modifies Coulomb friction and regulates evolution of m. A companion paper presents a suite of model predictions and tests.

Coral sand is characterized by unique particle morphology and pore structure, which result in pronounced dilatancy and a high internal friction angle during shear. The dilatancy angle is a critical parameter for finite element analyses of sand foundation bearing capacity; the inappropriate selection of this parameter can lead to significant computational errors. In this research, a series of consolidated drained triaxial tests were conducted on coral sand samples from the South China Sea to investigate the dilatancy behavior and its effect on shear strength parameters. A dilatancy equation for coral sand was proposed, incorporating the dilatancy index, relative density, and mean effective stress. The results indicate the following: (1) Within the confining pressure range of 25–400 kPa, the stress–strain curves exhibit varying degrees of strain softening. When the effective confining pressure reaches 400 kPa, the dilatant behavior is nearly suppressed, resulting in a transition from dilatancy to contraction; (2) The peak internal friction angle decreases significantly with increasing effective confining pressure. However, the sensitivity to confining pressure varies for samples with different relative densities (Dr = 30–90%), with denser samples showing a more rapid reduction in peak friction angle; (3) At a confining pressure of 25 kPa, the maximum dilatancy angle of coral sand samples reaches 44.2°, significantly exceeding the typical range observed in terrestrial quartz sands. This difference may be attributed to the irregular and angular characteristics of the coral sand particles; (4) Based on Bolton’s dilatancy theory, a dilatancy equation applicable to coral sand was developed, demonstrating a strong linear relationship among the dilatancy index (IR), relative density (Dr), and peak mean effective stress (p′f). These findings provide valuable guidance for the selection of strength parameters for engineering applications involving coral sand.