Evolution law of ultrasonic characteristics and its relationship with coal-measure sandstone mechanical properties during saturation and desaturation

Abstract

Underground buildings are easily destabilised by water and there is a strong link between their mechanical properties and ultrasonic characteristics. To grasp the above features and the relationship among them, the mechanism of sandstone degradation under water erosion was briefly reviewed. Using the coal-measure sandstone as the object, experimental studies on the dynamic evolution of water content, ultrasonic wave velocity, waveform, uniaxial compressive strength (UCS), elastic modulus (E), and damage mode during saturation-desaturation were carried out. The findings demonstrate that coal-measure sandstones usually exhibit more mechanical parameter deterioration following saturation than non-coal-measure sandstones. During saturation, the water absorption velocity slows down; the P-wave velocity (VP) first fluctuates down, then fluctuates up and finally decreases slightly (VP increases after saturation), and the wave velocity anisotropy coefficient (δi) decreases; the waveform shows a trend of unequal time and unequal amplitude → equal time and equal amplitude → unequal time and equal amplitude, and the first wave reaches a progressively shorter time. During desaturation, the dehydration rate slows down, VP first drops rapidly and then fluctuates slightly and rises slightly near drying (VP decreases after desaturation), δi increases, and the waveform changes in the opposite trend to saturation. Saturation treatment significantly weakened the mechanical properties, while desaturation restored some of the strength lost due to immersion. The sandstone mechanical properties and ultrasonic features are more closely related during saturation than desaturation.