Cracking behaviors of rocks significantly affect the safety and stability of the explorations of underground space and deep resources. To understand deeply the microscopic cracking process and mechanical property of rocks, X-ray micro-computed tomography (X-μCT) is applied to capture the rock microstructures. The digital color difference UNet (DCD-UNet)-based deep learning algorithm with 3D reconstruction is proposed to reconstruct the multiphase heterogeneity microstructure models of rocks. The microscopic cracking and mechanical properties are studied based on the proposed microstructure-based peridynamic model. Results show that the DCD-UNet algorithm is more effective to recognize and to represent the microscopic multiphase heterogeneity of rocks. As damage characteristic index of multiphase rocks increases, transgranular cracks in the same grain phase, transgranular and intergranular cracks of pore-grain phase, intergranular and secondary transgranular cracks and transgranular crack between different grains propagate. The ultimate microscopic failure modes of rocks are mainly controlled by the transgranular cracks-based T1-shear, T3-shear, T1-tension, T2-tension and T3-tension failures, and the intergranular cracks-based T1-tension, T1-shear and T3-shear failures under uniaxial compression. 

Microscopic cracking behaviors of rocks under uniaxial compression with microscopic multiphase heterogeneity by deep learning

CO2 geological storage in tight sandstone reservoirs is an effective method of CO2 capture, utilization, and storage (CCUS). CO2 migration and flow characteristics in sandstone during geological storage affect the physical and mechanical properties of rocks significantly. In this study, laboratory injection experiments combined with X-ray computed tomography (CT) scanning and scanning electron microscopy (SEM) tests, and computational fluid dynamics (CFD) numerical simulation have been conducted to investigate the effect of geological and engineering parameters on CO2 migration and flow characteristics. The results show that: (1) The injection rate and injection pressure affect the Sc-CO2 migration path. Moreover, the fluid velocity and fluid pressure affect the pressure distribution in pores significantly. Limiting the injection rate and pressure during the CO2 geological storage is beneficial for the stability of the storage. (2) The pressure distribution of the fluid changes with orders of magnitude depending on the injection pressure. However, the overall transient relative pressure is almost consistent and the fluid pressure in the bending section of the bending pores exhibits a linearly decreasing trend. (3) SEM tests provide a connection between laboratory experiments and CFD simulation on microscopic scale. The injection pressure and the acoustic emission (AE) energy have a positive correlation to a certain extent. The Sc-CO2 migration path could be forecasted preliminarily through the fluctuation of the injection pressure-time curves and AE response. The results enrich the theory of CO2 migration and flow characteristics during CO2 geological storage. 

Investigations on CO2 migration and flow characteristics in sandstone during geological storage based on laboratory injection experiment and CFD simulation

To further improve the durability of cement-stabilized macadam and guarantee the use quality and sustainability of a semi-rigid base, the current study was carried out. With the help of a dry shrinkage test, temperature shrinkage test, freeze–thaw bending test, and fatigue test, the effect of incorporating PVA fiber on the deformation characteristics of cement-stabilized macadam was analyzed, and the changes in low-temperature residual toughness of the mixture before and after modification were compared. The low-temperature toughness of PVA fiber cement-stabilized macadam was evaluated with the help of the standard toughness evaluation method. The fatigue life prediction equation of PVA fiber cement-stabilized macadam was established based on the Weibull distribution. The results showed that PVA fiber can effectively improve the deformation characteristics, low-temperature toughness, and fatigue performance of cement-stabilized macadam. The low-temperature residual flexural tensile strength and low-temperature bearing capacity were increased by 10.3% and 55.3%, respectively. The residual toughness indices were increased by 58.6%, 88.1%, and 98.3% and the residual strength index was increased by more than 100%. The fatigue life was improved by 178~368% under different stress intensity ratios. The fatigue life values obeyed the two-parameter Weibull distribution, and the correlation between the fatigue life prediction equation and the measured data was significant. The fatigue life prediction error was between 0.03 and 4.9% under different stress intensity ratios.

https://www.mdpi.com/2071-1050/14/24/16953/pdf?version=1671702460

Durability Performance of PVA Fiber Cement-Stabilized Macadam

Brittle failure, or spalling, occurs around openings excavated in hard rock masses with high in situ stresses. It takes place due to the nucleation and growth of cracks around the excavation boundary, induced by the redistribution of stresses following the excavation. Modelling this failure process is a tough challenge. The hybrid finite-discrete element method (FDEM) can overcome the boundary between continuum and discontinuum, capturing emergent discontinuities associated with brittle fracturing processes. In this study, FDEM is applied using a commercial code to show its applicability to model brittle behaviour around deep underground excavations in the case of the Torino-Lyon Base tunnel in three different stress conditions. Except for the hydrostatic condition, cracking is triggered immediately after the excavation. Spalling occurring around the tunnel is quite extended; therefore, an accurately designed support must be installed to prevent blocks from falling from the tunnel boundary. The obtained results are aligned with previous results existing in the literature. However, in this case, a deeper spalling is caused by the shape change due to the gradual stress redistribution. Such a phenomenon underlines the importance of using a code able to identify crack propagation, opening, and the formation of loose blocks that progressively modify the tunnel contour.

Application of a Finite-Discrete Element Method Code for Modelling Rock Spalling in Tunnels: The Case of the Lyon-Turin Base Tunnel

Research progress on the development mechanism and exploratory protection of the scaling off on earthen sites in NW China

A novel random angular bend (RAB) algorithm and DEM modeling of thermal cracking responses of sandstone

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A novel random angular bend (RAB) algorithm and DEM modeling of thermal cracking responses of sandstone