Strength weakening characteristics and microscopic mechanism of coal samples with different water contents based on penetration strength

The use of goafs in abandoned mines as water storage space plays an important role in the ecological environment and economic development of the mining area. The strength of coal and rock mass under the action of water immersion directly affects the stability of underground storage. Aiming at the characteristics of strong anisotropy of coal in the closed Datai mine, this paper studies the correlation between the penetration strength and the compressive strength and tensile strength of coal samples. On this basis, the variation law of the same coal sample strength with different water contents was studied, and the internal mechanism of water-immersed pore evolution and strength weakening was analyzed by means of CT scanning. The research results show that: The uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) of coal samples with different water contents have a linear relationship and a negative exponential relationship with the coal sample penetra-tion index (NPI), respectively. The NPI can well match the UCS and BTS of coal samples. With the increase of water content, the penetration strength of coal samples decreases exponentially. Therefore, the penetration index can be used to evaluate the strength of the coal sample, thus realizing the low damage cycle test of the same coal sample with different water content. It reduces the influence of coal sample anisotropy on the ex-perimental results, and also provides an effective means for on-site rapid testing of coal sample strength. From drying to saturation, the penetration strength of coal samples decreased by 86.55%. Before and after soaking, the porosity of connected and isolated pores increased by 1.58% and 0.97% respectively, and the overall porosity increased by 2.55%, while the volume proportion of mineral components decreased by 1.96%. During the water soaking process of coal samples, the clay minerals in the connected pores swelled and dissolved in water, resulting in changes in the porosity of coal samples. The increase in porosity (isolated pores and connected pores) makes damage fractures develop along pores, making it easier to form through-fractures, which in turn leads to a decrease in strength. In addition, the water-filled connecting joints greatly reduce the its strength, which makes the damaged fractures more likely to expand along the weak surface, thereby reducing the strength of the coal sample.