During the excavation of large-scale rock slopes and deep hard rock engineering, the induced rapid unloading serves as the primary cause of rock mass deformation and failure. The essence of this phenomenon lies in the opening-shear failure process triggered by the normal stress unloading of fractured rock mass. In this study, we focus on local-scale rock fracture and conduct direct shear tests under different normal stress unloading rates on five types of non-persistent fractured hard rocks. The aim is to analyze the influence of normal stress unloading rates on the failure modes and shear mechanical characteristics of non-persistent fractured rocks. The results indicate that the normal unloading displacement decreases gradually with increasing normal stress unloading rate, while the influence of normal stress unloading rate on shear displacement is not significant. As the normal stress unloading rate increases, the rocks brittle failure process accelerates, and the degree of rocks damage decreases. Analysis of the stress state on rock fracture surfaces reveals that increasing the normal stress unloading rate enhances the compressive stress on rocks, leading to a transition in the failure mode from shear failure to tensile failure. A negative exponential strength formula was proposed, which effectively fits the relationship between failure normal stress and normal stress unloading rate. The findings enrich the theoretical foundation of unloading rock mechanics and provide theoretical support for disasters prevention and control in rock engineering excavations. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

The mechanical behavior of expansive soil in geotechnical engineering is significantly sensitive to loading rates, hydration, confining pressure, etc., where most engineering problems are attributed to the existence of montmorillonite in expansive soil. Here, the hydration, confining pressure, and loading rate effect on the mechanical behavior of montmorillonite were investigated through the triaxial tests and molecular dynamics (MD) simulation method, revealing their fundamental mechanism between the microscale and macroscale. The average basal spacing of hydrated montmorillonite system, the diffusion coefficient and density distribution of interlayer water molecules were calculated for the verification of MD model. The experimental results indicated that the stress-strain relationship of montmorillonite was the strain-hardening type. The failure stress did not increase monotonously with the increase in loading rate, and there were two obvious critical points. The failure stress of the soil sample increased with the increase of the confining pressure, and the decrease of the water content, where their fundamental mechanism between microscale and macroscale were adequately discussed. Furthermore, the stress-strain response, total energy evolution, deformation evolution of atomistic structure, and broken bonds evolution were analyzed to deeply understand the fundamental deformation mechanism at the microscale. The multi-scale studies could effectively examine the macroscopic mechanical behavior of expansive soil and elucidate its microscopic mechanisms.

Time-dependent effects of soils have been widely recognized as a key issue, which is particularly evident for coral sand due to its high frangibility. To determine the time-dependent behaviors of coral sand under triaxial stress states for enhanced engineering applications, a series of triaxial tests was carried out on two gradations of coral sand. Besides the conventional time-dependent tests, deviator-stress rate, creep with different stress histories, drop creep/relaxation, and postpeak creep/relaxation tests, which have been rarely investigated, have also been conducted. Additionally, the particle size and shape variations in the coral sand after the tests were analyzed to elucidate the time-dependent behavior mechanism. According to the test results, different from the tests with varying axial strain rates, over- and under-shooting phenomena were not clear when the deviator stress rate changed suddenly. Creep behavior was found to be noticeably influenced by the stress history and decreased with increasing preconsolidation pressure. The stress unloading could considerably reduce the subsequent creep or relaxation response, and the response diminished with increasing magnitude of deviator stress drop. The time-dependent behavior of coral sand was mainly determined by the level of particle breakage. The coral sand particles became smaller and more regular due to particle breakage, which would increase the compressibility of specimens and weaken interlocking between the sand particles, leading to more obvious time-dependent behaviors. The influence of particle breakage on the time-dependent behaviors of coral sand could be examined from two perspectives, i.e., the particle breakage during creep or relaxation processes and that during preloading processes. The effects of the unstable broken particles that formed during preloading were larger. In addition, a unique relationship was observed between the relative breakage and input energy for the same coral sand gradation, regardless of the test conditions, which was meaningful for the time-dependent constitutive modeling considering particle breakage.

Controllable shock wave fracturing is an innovative engineering technique used for shale reservoir fracturing and reformation. Understanding the anisotropic fracture mechanism of shale under impact loading is vital for optimizing shock wave fracturing equipment and enhancing shale oil production. In this study, using the well-known notched semi-circular bend (NSCB) sample and the novel double-edge notched flattened Brazilian disc (DNFBD) sample combined with a split Hopkinson pressure bar (SHPB), various dynamic anisotropic fracture properties of Lushan shale, including failure characteristics, fracture toughness, energy dissipation and crack propagation velocity, are comprehensively compared and discussed under mode I and mode II fracture scenarios. First, using a newly modified fracture criterion considering the strength anisotropy of shale, the DNFBD specimen is predicted to be a robust method for true mode II fracture of anisotropic shale rocks. Our experimental results show that the dynamic mode II fracture of shale induces a rougher and more complex fracture morphology and performs a higher fracture toughness or fracture energy compared to dynamic mode I fracture. The minimal fracture toughness or fracture energy occurs in the Short-transverse orientation, while the maximal ones occur in the Divider orientation. In addition, it is interesting to find that the mode II fracture toughness anisotropy index decreases more slowly than that in the mode I fracture scenario. These results provide significant insights for understanding the different dynamic fracture mechanisms of anisotropic shale rocks under impact loading and have some beneficial implications for the controllable shock wave fracturing technique. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Variations in excavation construction periods for fissured soil transportation engineering lead to differing unloading rates, which affect the soil's mechanical properties. This study utilizes a triaxial testing system to conduct monotonic and cyclic loading undrained shear tests on undisturbed fissured samples as well as remolded samples subjected to three distinct unloading rates. The K0 consolidated samples are regarded as soil mass that undergoes no unloading during testing. The findings indicated that the initial unloading rate influences the reloading shear mechanical properties of undisturbed and remolded specimens. The effects of unloading rates differ between undisturbed and remolded soil, a discrepancy attributed to inherent fissures. Specifically, undisturbed soil exhibits significant damage at low unloading rates due to fissures, while remolded soil experiences strength augmentation due to compaction with decreased unloading rates. Similarly, unloading will cause a loss of strength. Structural disparities result in the monotonic loading strength of undisturbed specimens being higher than that of remolded ones. In contrast, remolded specimens demonstrate greater dynamic strength under cyclic loading, likely because fissures deform, diminishing overall dynamic strength. Subsequent microscopic analysis, utilizing SEM images, along with a discussion of macroscopic inherent fissures, elucidated the impact of unloading rate on soil damage mechanisms, advancing the understanding of fissured soil behavior post- unloading. The study of mechanical properties of fissured soil following varying unloading rates is crucial for comprehending its damage mechanism and determining post-unloading soil strength parameters, providing valuable insights for practical applications in soil engineering.

The soil behavior is rate-dependent as observed in the laboratory and field tests, and the undrained shear strength of clay is shown to increase with the strain rate in different shear modes. In practical situations, the foundations can be loaded at various time and rate scales, which will result in a wide range of magnitudes and inhomogeneous distribution of strain rates in the surrounding soil. This may cause difficulties in calculating the undrained bearing capacity of clay using the undrained shear strength from standard laboratory and field tests at a reference strain rate. In addition, the rate-dependent soil behavior will also affect the interpretation of in situ tests conducted at different loading rates (e.g., CPT, T-Bar, and pressuremeter tests) using procedures based on rate-independent soil models. This paper investigates the effect of loading rate on the undrained bearing capacity of clay using finite element analyses and a rate-dependent constitutive model, the MIT-SR, based on two classical problems in soil mechanics (i.e., the deeply-embedded rigid pile/pipe section, and the rigid strip footing). Computed results suggest that the undrained bearing capacity of clay is strongly affected by the loading rate of foundations, which is consistent with the model and field tests. It also highlights the difficulty to select appropriate undrained shear strength used for practical design, and the uncertainty to interpret field tests using bearing capacity factors derived from analytical solutions.

To capture the influence of loading rate on the deformation process of structured soft clay, applying the approach used in the unified hardening model (UH model) to describe time effects, an equivalent time term is introduced into the yield function (t) of the structured UH model, and then a structured UH model for soft clay considering loading rate is extended. In the equivalent time term, the internal variable R of the original structured UH model is transformed into Rt to account for time-driven strain. The presented structured UH model considering time effects comprises a total of 8 parameters, all of which can be determined through routine soil tests. Comparisons between model predictions and experimental data demonstrate that the presented model is qualified to reflect the influence of loading rate on structured soft clays reasonably.

Assessing the dynamic properties of rocks remains a foundational pursuit in the field of rock engineering, providing crucial insights into their mechanical behaviors across a spectrum of loading conditions, including static, cyclic, and dynamic scenarios. This paper expounds upon the utilization of the nonresonance (NR) torsional shear test and its implications for understanding rock responses, particularly in the context of low and medium loading rates. The NR method serves as a pivotal tool for investigating model rock materials subjected to loading conditions characterized by low frequencies and amplitudes. Renowned for its efficacy, this method allows the simultaneous determination of two critical dynamic parameters: shear modulus (G) and damping ratio (D), all at a specific loading frequency. It has been ascertained that the loading rate increased as the loading frequency and applied amplitude of loading increased. With increasing loading rate, the shear modulus consequently increased while the damping ratio decreased. It is observed that the dynamic responses of both ramp and sinusoidal loading waveforms increase concurrently with the amplitudes of the applied torque and loading frequencies. The sinusoidal waveform exhibits greater dynamicity than the ramp waveform at a certain loading rate. Furthermore, this study delves into the intricate analysis of the nonlinear viscoelastic dynamic response exhibited by rocks, utilizing the modified hyperbolic (MH) model and the Ramberg-Osgood (RO) model as analytical tools. The findings derived from curve fitting exercises unequivocally underscore the superior applicability of the Ramberg-Osgood model, particularly in characterizing modulus reduction behavior. Conversely, the modified hyperbolic model emerges as the preferred choice for comprehensive damping ratio analyses. This study enhances the comprehension of rock dynamics and responses under diverse loading conditions, contributing valuable understanding to rock engineering. Insights into loading and strain rate effects aid informed decisions and preventive measures for rock deformation and collapse risks. This research suggests vital findings regarding the response of intact model materials to various dynamic loading conditions, providing significant insights for comprehending the mechanical response of rock structures exposed to cyclic loading conditions, which have the potential to create weaknesses in rocks resulting in untimely failures. The research can be utilized to assess the response of rocks in the context of seismic incidents, specifically those characterized by shear waves at particular frequencies. This study evaluates the response of rocks during earthquakes by establishing a correlation between the amplitude of torsional shear loading and the peak ground displacement linked to seismic events. In addition to its seismic implications, this research aids in advancing accurate predictive models and instruments that assess the stability and integrity of rock formations under different loading rates-a consequence of the symbiosis between frequency and amplitude. Professionals may ensure efficient risk mitigation, make well-informed decisions, and execute preventative measures concerning rock collapse and deformation by virtue of their comprehensive awareness of the delicate relationship between loading rate and dynamic rock properties. Incorporating the findings into engineering design standards and codes can bring about substantial improvements, augmenting the overall reliability and protection of rock structures.