Freeze-thaw damage evolution of fractured rock mass using nuclear magnetic resonance technology

Taking sandstone as a research object, this study investigated the damage evolution law of rock mass with different macroscopic and microscopic defects under freeze-thaw cycles. For the first time, macroscopic defects were simulated by prefabricating 0 degrees, 45 degrees, and 90 degrees fractures in real rock materials. Initial microscopic damage was caused by applying prestressing forces equal to 0%, 30%, 50%, and 70% of the ultimate compressive strength of the specimen. The microstructure changes of samples during freeze-thaw cycles were tracked by nuclear magnetic resonance (NMR) technology, and the characteristic parameters such as porosity, aperture distribution, and T-2 spectral distribution were obtained. Through freeze-thaw cycle tests and uniaxial compression tests on specimens undergoing 0, 20, 40, and 60 freeze-thaw cycles, the influence of different damage levels on the changes in the surface, mass, volume, compressive strength, and elastic modulus of rock mass was investigated. The experimental results show that the porosity of the intact specimen is greater than that of the fractured specimen during freeze-thaw cycles. The mechanical properties of the intact specimen and the 90 degrees fractured specimen are controlled by the pore distribution, and the strength drop increases first and then decreases during freeze-thaw cycles, while the mechanical properties of the 0 degrees fractured specimen and the 45 degrees fractured specimen are affected by the prefabricated fracture, and the strength drop increases with the increase of the fracture damage. The initial damage can accelerate the damage degradation of the rock mass, and when the damage level exceeds a certain threshold, which is between 30% and 50%, the deterioration effect will be more significant.