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• A novel thermo-mechanical coupling scheme is developed based on the combined finite-discrete element method (FDEM) to capture the mechanical response of the rock to the isotropic/anisotropic thermal conduction process. • The mechanisms of the creation and development of the excavation damaged zone (EDZ) around the wellbore subject to excavation unloading, drilling mud support, and thermal change in the surrounding rock are analyzed. • The influences of the thermal-mechanical properties of the rock mass, such as the isotropic/anisotropic rock strength and thermal conductivity, thermal expansivity, as well as drilling mud-rock thermal interaction process, on the EDZ development, are discussed. Insights into fracture development and excavation damaged zone (EDZ) formation around wellbore excavation sites are essential for understanding wellbore stability. In this study, a novel thermo-mechanical scheme is implemented in the 2D combined finite-discrete element method (FDEM) to investigate the fracturing process around a wellbore in a high-temperature subsurface environment. The developed scheme captures the isotropic/anisotropic thermal conduction characteristics within the rock formation. The coupled scheme and stress distributions around the wellbore subject to in-situ stress, drilling mud pressure, and temperature changes are validated by closed-form solutions. Fracture development and progressive EDZ formation by various mechanisms are analyzed. The results show that for an unsupported wellbore, fractures initiate in the region of the most severe stress concentration due to excavation unloading and constitute an EDZ that resembles the logarithmic-spiral rupture zone captured by experimental observations and Mohr-Coulomb models, resulting in potential water inrush due to the significantly increased fracture transmissivity of the EDZ and wellbore collapse. The shape and extent of the EDZ for wellbore excavation in the layered rock mass are dominated by the stress redistribution and the presence of low-strength bedding planes favorably oriented for bedding slippage. The excavation unloading-induced stresses can be effectively counteracted by properly applying drilling mud pressure, and the disturbance of the stress field occurs only within a small region, greatly reducing the extension of the EDZ. Thermal contraction of the surrounding rock due to its convective interaction with the cold drilling mud induces extra fractures around the wellbore. The extension of the EDZ is greater with increased thermal expansivity of the rock formation. Overall, the results indicate that the adopted thermo-mechanical FDEM simulation can provide unique geomechanical insight into wellbore stability behavior analysis, in which explicit consideration of the fracturing and fragmentation processes is of great significance. [ABSTRACT FROM AUTHOR]

• The long-term degradation behavior of rock strength was systematically summarized. • The long-term degeneration laws of cohesion and internal friction angle were proposed. • FDEM was used to study the long-term crack propagation of tunnel surrounding rock. • The conclusion of this paper was verified by engineering case. The time-dependence of rock strength is an issue that cannot be ignored in the rock engineering field. This paper summarizes previous study results on the degradation of the uniaxial compressive strength of rock (including component cohesion and internal friction angle), tensile strength and Young's modulus over time. Based on previous studies, the degeneracy formulas of intact rock cohesion and internal friction angle are derived. Taking circular tunnel excavation under hydrostatic pressure as an example, the surrounding rock response is studied under different degradation degrees of rock strength and stiffness, including the radius of the excavation damage zone, damage degree of surrounding rock, lining support pressure and tunnel boundary displacement. The results show that with the increasing degree of degradation of the surrounding rock compressive strength, the above four parameters increase in the form of an exponential function and that weakening of the tensile strength had little effect on the surrounding rock response. In addition, as the degradation degree of rock stiffness increases, the radius of the excavation damage zone remains unchanged, whereas the other three parameters values are reduced. An actual engineering case simulation using the combined finite-discrete element method shows that results consistent with field observations can be obtained if the rock strength degradation law proposed in this paper is adopted. [ABSTRACT FROM AUTHOR]

Core material softening method has been widely used in two-dimensional tunnel excavation simulation by the combined finite-discrete element method (FDEM). In numerical studies, softening stress paths (generally described by critical kinetic energy (CKE), softening time step (STS) and softening curve (SC)) usually have a significant influence on simulation results and computational efficiency. There is not a direct way to determine these three parameters. In the present study, a series of numerical studies by the FDEM are performed to discuss the effects of these three parameters on the simulation results and calculation efficiency in a circular tunnel excavation project with a 4 m diameter. The results show that the CKE is closely related to the model size and rock density. The CKE can be obtained from the model kinetic energy curve, and a selection method is proposed. The STS value of 50 000 steps is optimal. To obtain a uniform and stable propagation process of the surrounding rock crack, the SC should be in an exponential function. The engineering case shows that the simulation results are consistent well with the field observations as the softening stress paths are calibrated by the proposed method. [ABSTRACT FROM AUTHOR]