Study on deterioration effect of mechanical properties of coal and rock under water-rock interaction
Authors: Chen Guangbo , Gao Ning , Li Tan 
Published: March 2023

Abstract
In geotechnical engineering and underground mining engineering, because coal and rock are often immersed in water, the mechanical properties of coal and rock are deteriorated to varying degrees under the action of water and rock. For this reason, water and rock interaction tests were carried out for coal and rock under six kinds of immersing times of 10days,20days,30days,40days,50days and 60days to study the deterioration effect, failure characteristics and deterioration mechanism of their compressive strength, impact characteristics and other parameters, and discuss the progressive instability and damage mechanism of multi-scale coal and rock structure under water rock interaction. The research show that:1) With the prolongation of immersion time, the water content of coal and rock samples increases in varying degrees, finally, it tends to be stable. The increase of water content of coal sample is more obvious than that of sandstone sample. 2) The compressive strength of coal and rock has deterioration effect obvious under the action of water-rock, and the deterioration effect of coal sample is more obvious than that of sandstone sample. The compressive strength of coal sample decreased from 22.55MPa to 11.15MPa, which reduced by 11.40MPa, and the compressive strength of sandstone samples decreased from 66.29MPa to 59.05MPa, which decreased by 7.24MPa, after being immersed in water for 60days. The deterioration effect of the compressive strength of coal and rock is non-uniform, with the prolongation of immersion time the deterioration effect gradually weakens. 3) With the prolongation of immersion time, the impact energy index of coal samples and sandstone samples decreases in varying degrees, and the impact tendency is gradually weakened. The impact energy index of sandstone samples dropped from 4.38 (moderate impact) to 2.65 (weak impact) after coal and rock samples were immersed in water for 60days. The coal samples dropped from 7.41 (strong impact) to 1.18 (non-impact). The impact tendency of coal samples has great impact by water-rock interaction. 4) In view of the crack distribution characteristics of coal-rock sample after failure, the failure characteristics of the samples have great influence by the time of immersion and the material of coal-rock, it is mainly in five aspects: the length of crack, the initiation location of crack, the angle between crack and specimen axis, the number of cracks, and the type of damage. 5) The deterioration effect of mechanical properties of coal under water-rock interaction the result that gradually accumulate by the internal damage of coal and rock. Under the action of water and rock, the micro damage of coal and rock samples are caused by the internal physical and chemical actions. When the damage accumulation reaches a certain degree, the samples will be damaged in the macro. 6) The multi-scale system of coal-rock structure under water-rock interaction is constructed. The water-rock interaction is analyzed from the micro-scale, meso-scale and macro-scale. The three scales are interrelated, influenced and promoted each other. In particular, this effect is more significant under high stress conditions, and the coal rock structure is deteriorated further, eventually leading to instability of system. The research results provide a reference for the prediction of mechanical properties and stability research of engineering rock mass under the environment in which both coal and rock are immersed.