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The characterization of time‐dependent brittle rock deformation is fundamental to understanding the long‐term evolution and dynamics of the Earth's crust. The chemical influence of pore water promotes time‐dependent deformation through stress corrosion cracking that allows rocks to deform at stresses far below their short‐term failure strength. Here, we report results from a study of time‐dependent brittle creep in water‐saturated samples of Darley Dale sandstone (initial porosity, 13%) under triaxial stress conditions. Results from conventional creep experiments show that axial strain rate is heavily dependent on the applied differential stress. A reduction of only 10% in differential stress results in a decrease in strain rate of more than two orders of magnitude. However, natural sample variability means that multiple experiments must be performed to yield consistent results. Hence we also demonstrate that the use of stress‐stepping creep experiments can successfully overcome this issue. We have used the stress‐stepping technique to investigate the influence of confining pressure at effective confining pressures of 10, 30, and 50 MPa (while maintaining a constant 20 MPa pore fluid pressure). Our results demonstrate that the stress corrosion process appears to be significantly inhibited at higher effective pressures, with the creep strain rate reduced by multiple orders of magnitude. The influence of doubling the pore fluid pressure, however, while maintaining a constant effective confining pressure, is shown to influence the rate of stress corrosion within the range expected from sample variability. We discuss these results in the context of microstructural analysis, acoustic emission hypocenter locations, and fits to proposed macroscopic creep laws.

Understanding the mechanical behavior of calcareous sand under repeated loading-unloading is important to engineering design for island-reef underground caverns and pile foundation projects. In this study, we conducted conventional consolidated-drained triaxial shear tests on calcareous sand collected from the South China Sea under different effective confining pressures, and then, we performed multiple sets of triaxial repeated loading-unloading tests on calcareous sands with equivalent physical properties under different deviatoric stress levels and effective confining pressures. Based on the results, we conducted an in-depth investigation of the effects of the loading mode and effective confining pressure on the strain softening behavior and shear strength angle of calcareous sand. We studied the variation in the unloading rebound modulus as the effective confining pressure and deviatoric stress level changed, verified the applicability of the Dunkan-Chang model’s estimation equation for the unloading rebound modulus to calcareous sand, and established the relationship among the volume contraction during unloading, the effective confining pressure, and the deviatoric stress level. The results of this study provide an important basis for selecting appropriate parameters for island-reef construction engineering.

Gas production by depressurization can significantly increase the effective stress in hydrate-bearing sediments. Therefore, strength and deformation characteristics of sediments under high effective confining pressure should be fully understood before large-scale extraction. In this study, a series of triaxial tests on artificial methane hydrate-bearing specimens were conducted under effective confining pressures of 0.2–20 MPa, and the effects of effective confining pressure and hydrate saturation on the strength parameters and the stress–dilatancy characteristics were discussed. The results demonstrate that the strength and stiffness of hydrate-bearing sediments increase with increasing effective confining pressure. Shearing under high effective confining pressure leads to significant breakage of host particles, which is independent of the hydrate saturation. The increase in the effective confining pressure decreases the internal friction angle while increasing the cohesion of hydrate-bearing sediments. By linking the effective confining pressure and hydrate saturation with strength parameters of Mohr–Coulomb criterion and Drucker–Prager criterion, the strength of sediments in a high range of effective stress can be accurately predicted. With increasing effective confining pressure, the shear–dilation transfers to shear–contraction, the critical stress ratio gradually decreases to attain a constant value, and the effects of hydrate saturation and effective stress on the dilatancy characteristics gradually become less notable.

The stress-dependent permeability (SDP) of post-failure rock is pivotal for mine goaf reuse and environmental protection. In this study, we employed a triaxial servo system to investigate the SDP of post-failure sandstone subjected to a mining-induced stress path. Initially, intact sandstone specimens underwent axial loading and confining unloading under various initial hydrostatic pressure, seepage pressures, and loading–unloading rates. Subsequently, the SDP and crack porosity were quantified by incrementally increasing the effective confining pressure to characterize how these properties evolve under different stress conditions. Notably, the induced crack patterns resulting from the loading–unloading stress path (LUSP) encompassed tensile dominated cracks, shear dominated cracks, and tensile-shear composite cracks, which exhibited dependence on the specific test conditions. Interestingly, the measured permeability and crack porosity of the anisotropic complex crack network exhibited less sensitivity to changes in effective confining pressure compared to that of single cracks and pore medium. This is likely due to the directional correlation between confining pressure and crack surfaces. Our experiments suggest that a power law provides a more accurate description of sandstone behavior post-failure under low uniform stress conditions than the exponential model. Gas permeability was found to be higher than water permeability, but within one order of magnitude. The sensitivity exponent of crack porosity ranged from 1.13 to 6.33, reflecting variations in crack morphology and complexity under different test conditions.

Calcareous sand is the main geomaterial available for island-reef reclamation construction projects in the South China Sea. To clarify the effect of particle breakage on the dilatancy of calcareous sand, multiple consolidated-drained triaxial shear tests were conducted on calcareous gravelly sand (CGS) under different conditions. On this basis, dilatancy assessment indices were constructed from the perspectives of stress ratio and dilatancy ratio, and their relationships with the initial physical parameter and stress level of CGS were established. Next, the variations in particle breakage of CGS with compactness and stress level were explored, and a physical model was proposed to predict particle breakage according to plastic work. Finally, the relationship between dilatancy and particle breakage was established for CGS. The results lay a research foundation for developing an elastoplastic constitutive model considering the effect of particle breakage for CGS. Graphical abstract In this study, the dilatancy of calcareous sand was investigated under varying relative densities and effective confining pressures. Then, the variations in the particle breakage of calcareous sand with compactness and stress level were revealed. Finally, the relationship between dilatancy and particle breakage of calcareous sand was established.

To investigate the meso-fabric characteristics of calcareous sand (CS) under undrained shear conditions, multiple consolidated-undrained triaxial shear tests were conducted on CS under different effective confining pressures ( σ 3 ′ ), and the particle gradations and shapes of CS before and after triaxial shear tests were measured using screening tests and particle shape scanning tests, respectively. According to the results, (I) the consolidated-undrained triaxial shear tests on CS can be regarded as a process of transforming from weakening frictional strength to reinforcing cohesive strength. (II) The relative breakage ratio ( Br ) and modified relative breakage index ( Br ∗ ) of CS both increased in power function form with increasing σ 3 ′ . The Br of CS was less than its Br ∗ under the same σ 3 ′ . The connection between the Br and plastic work of CS under undrained shear conditions was established. Splitting and abrasion were the main particle breakage patterns of CS during consolidated-undrained triaxial shear. (III) After consolidated-undrained triaxial shear tests, the particle shape of CS became increasingly regular. Due to the diversity of particle breakage patterns, the mean particle shape parameter of CS did not show any monotonic change with increasing σ 3 ′ .

The permeability of sandstone during hydrostatic compaction and triaxial deformation was measured using a rock triaxial servo-controlled system. The gas permeability was also measured using an integrated permeability and porosity test system to study the difference between gas and water permeability. The experimental results suggested that gas permeability is larger than water permeability by almost one order of magnitude. This phenomenon is due to the slippage effect. The modified permeability is much closer to the water permeability. An empirical exponential relationship can describe the stress-dependent permeability of sandstone, while a power law is suitable to describe the relationship between porosity and effective confining pressure. During triaxial deformation, permeability initially decreases and then begins to increase at an accelerating rate. The peak value of permeability is hysteretic to peak stress. The initial permeability, lowest permeability, peak permeability, and stable value of permeability all decrease with the increase of effective confining pressure. The volumetric strain has a great effect on permeability. The turning point where permeability starts to increase coincides well with the onset of dilatancy.

In this study, a series of resonant-column experiments were conducted on marine clays from Bohai Bay and Hangzhou Bay, China. The characteristics of the dynamic shear modulus (G) and damping ratio (D) of these marine clays were examined. It was found that G and D not only vary with shear strain (γ), but they also have a strong connection with soil depth (H) (reflected by the mean effective confining pressure (σm) in the laboratory test conditions). With increasing H (σm) and fixed γ, the value of G gradually increases; conversely, the value of D gradually decreases, and this is accompanied by the weakening of the decay or growth rate. An intelligent model based on a back-propagation neural network (BPNN) was developed for the calculation of these parameters. Compared with existing function models, the proposed intelligent model avoids the forward propagation of data errors and the need for human intervention regarding the fitting parameters. The model can accurately predict the G and D characteristics of marine clays at different H (σm) and the corresponding γ. The prediction accuracy is universal and does not strictly depend on the number of neurons in the hidden layer of the neural network.

Coral sand is the main filling material for the island–reef foundation. Under tidal actions, the saturation (Sr) of coral sand layers varies with the specific depths in the reclaimed foundation. Studying the Sr effect of coral sand’s mechanical behaviors is crucial for the stability of the reclaimed foundation of island–reefs. In this study, a “quantitative injection method” was designed to prepare coral sand with saturation ranging from 90% to 100%, and unconsolidated–undrained (UU) triaxial shear tests were conducted on coral sand under different effective confining pressures (σ3′). The results indicated that the stress–strain curves of coral sand under various conditions were of the strain-softening type. When σ3′ = 200, 400, 600, and 800 kPa, the shear strength of coral sand decreased exponentially by 13.1, 9.1, 16.8, and 15.2%, respectively, with the increase in Sr from 90% to 100%. As Sr rose, the internal friction angle (φ) dropped by 3.77°. The cohesion (c) was not significantly affected by Sr compared to φ. In consideration of the physical susceptibility of coral sand to breakage, relative breakage ratio (Br) and modified relative breakage index (Br*) were introduced to evaluate the particle breakage behaviors of coral sand samples with different Sr levels in the triaxial shear process. It was found that Br and Br* increase linearly with increasing Sr; the effect of Sr on the particle breakage of coral sand weakens significantly when σ3′ is sufficiently large. The median particle size (d50) of coral sand decreases with increasing Sr, and presents a negative linear correlation with both Br and Br*. Based on comparing the strength and particle breakage characteristics of coral sand samples with varying Sr levels, this study suggests that 92.5% should be considered as the Sr value of coral sand available for testing.

This study conducted experimental tests on the undisturbed Nanjing Yangtze River floodplain soft soil using the bender element instrument to determine the maximum dynamic shear modulus of the Yangtze River floodplain overconsolidated soft soil. The Gmax of floodplain soft soil with different overconsolidated ratio OCR, initial effective confining pressure σ3c′, and void ratio e are discussed. The results indicated that Gmax reduced as e rose for given σ3c′ and OCR. In addition, an increase in OCR contributed to a gradual decrease in the decay rate of Gmax, while the Gmax decay rate is insensitive to the change of σ3c′. The void ratio-normalized maximum shear modulus Gmax/F(e) improved with the increase in the stress-normalized initial effective confining pressure σ3c′/Pa, whereas the growth rate gradually drops, and a power relationship is then obtained between Gmax/F(e) and σ3c′/Pa. Based on the regression analysis, a Gmax prediction method is established for reasonably characterizing Yangtze River floodplain soft soils with various over-consolidation states, initial stress conditions, and compactness levels, with a prediction error of less than 10%.