Fracture (fault) reactivation can lead to dynamic geological hazards including earthquakes, rock collapses, landslides, and rock bursts. True triaxial compression tests were conducted to analyze the fracture reactivation process under two different orientations of Q2, i.e. Q2 parallel to the fracture plane (Scheme 2) and Q2 cutting through the fracture plane (Scheme 3), under varying Q3 from 10 MPa to 40 MPa. The peak or fracture reactivation strength, deformation, failure mode, and post-peak mechanical behavior of intact (Scheme 1) and pre-fractured (Schemes 2 and 3) specimens were also compared. Results show that for intact specimens, the stress remains nearly constant in the residual sliding stage with no stick-slip, and the newly formed fracture surface only propagates along the Q2 direction when Q3 ranges from 10 MPa to 30 MPa, while it extends along both Q2 and Q3 directions when Q3 increases to 40 MPa; for the pre- fractured specimens, the fractures are usually reactivated under all the Q3 levels in Scheme 2, but fracture reactivation only occurs when Q3 is greater than 25 MPa in Scheme 3, below which new faulting traversing the original macro fracture occurs. In all the test schemes, both epsilon 2 and epsilon 3 experience an accumulative process of elongation, after which an abrupt change occurs at the point of the final failure; the degree of this change is dependent on the orientation of the new faulting or the slip direction of the original fracture, and it is generally more than 10 times larger in the slip direction of the original fracture than in the non-slip direction. Besides, the differential stress (peak stress) required for reactivation and the post-peak stress drop increase with increasing Q3. Post-peak stress drop and residual strength in Scheme 3 are generally greater than those in Scheme 2 at the same Q3 value. Our study clearly shows that intermediate principal stress orientation not only affects the fracture reactivation strength but also influences the slip deformation and failure modes. These new findings facilitate the mitigation of dynamic geological hazards associated with fracture and fault slip. (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 behaviour of soils subjected to any stress path in which deviatoric stresses are present is heavily characterised by non-linearity, irreversibility and is strongly dependent on the initial state of stress. The latter, for the majority of geotechnical applications, is normally determined by the at-rest earth pressure coefficient K0, even though this state is valid, strictly speaking, for axisymmetric conditions and for zero-lateral deformations only. Many expressions are available in the literature for the determination of this coefficient for cohesive and granular materials both for normal consolidated and over-consolidated conditions. These relations are available for low to medium stress levels. Results of an extensive experimental investigation on two sands of different mineralogy up to very high stress (120 MPa) are reported in the paper. For reach very high vertical stresses, a special oedometer has been realised. In the loading phase (normal consolidated sands), the coefficient K0n depends on the stress level. It passes from values of about 0.8 to values of about 0.45 in the range of effective vertical stress sigma ' v = 0.5-4 MPa. Subsequently, K0n is about constant and varies between 0.45 to 0.55 up to very high vertical effective stresses (120 MPa). For the sands employed in the tests, Jaki's relation did not lead to reliable results at relatively low pressures, while at high pressures, the same relationship seems to lead to reliable predictions if it refers to the constant volume angle of shear strength. For the over-consolidated sands, K0C strongly depends on the OCR, and for very high values of OCR, K0C could be greater than Rankine's passive coefficient of earth pressure, Kp. This result is due to the very locked structure of the sands caused by the grain crushing, with intergranular contact of sutured and sigmoidal, concavo-convex and inter-penetrating type, that confer to the sand a sort of apparent cohesion and make it similar to weak sandstone.

The occurrence of geological hazards and the instability of geotechnical engineering structures are closely related to the time-dependent behavior of rock. However, the idealization boundary condition for constant stress in creep or constant strain in relaxation is not usually attained in natural geological systems. Therefore, generalized relaxation tests that explore the simultaneous changes of stress and strain with time under different stress levels with constant pore-water pressure are conducted in this study. The results show that in area I, area II, and area III, the stress and strain both change synchronously with time and show similar evolutionary laws as the strain-time curve for creep or the stress-time curve for relaxation. When the applied stress level surpasses the s ci or s cd threshold, the variations in stress and strain and their respective rates of change exhibit a signi ficant increase. The radial deformation and its rate of change exhibit greater sensitivity in response to stress levels. The apparent strain deforms homogeneously at the primary stage, and subsequently, gradually localizes due to the microcrack development at the secondary stage. Ultimately, interconnection of the microcracks causes the formation of a shear-localization zone at the tertiary stage. The strain-time responses inside and outside the localization zone are characterized by local strain accumulation and inelastic unloading during the secondary and tertiary stages, respectively. The width of the shear-localization zone is found to range from 4.43 mm to 7.08 mm and increased with a longer time-to-failure. Scanning electron microscopy (SEM) reveals a dominant coalescence of intergranular cracks on the fracture surface, and the degree of physiochemical deterioration caused by water-rock interaction is more severe under a longer lifetime. The brittle sandstone 's time-dependent deformation is essentially controlled by microcrack development during generalized relaxation, and its expectancy-life is determined by its initial microstructural state and the rheological path. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting 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/).