## 3 Results and Analyses
### Uniaxial Compressive Properties  
The stress-strain curve for the natural coal rock specimen is displayed in Figure 4 and reveals a calculated elastic modulus of 3.50 GPa, with peak compressive strength of 27.42 MPa and a Poisson's ratio of approximately 0.19. This curve is distinguished by four significant stages, namely the compression-dense stage, linear elasticity stage, pre-peak compression-dense stage, and strain softening stage. The first stage is the compaction stage (range marked as I), caused by the existence of natural microcracks and pores inside the sample. After compression, the sample undergoes brief compaction, and the curve exhibits nonlinear characteristics. The second stage is the linear elasticity stage (range marked as II), where the stress-strain curve experiences linear growth until it reaches point A, with an intensity of 24.71 MPa; elastic energy gradually accumulates, resulting in the curve accounting for the largest proportion during this stage. The third stage indicates pre-peak compaction, while both the first and second stages denote the pre-peak compression phase, with each represented as stage III. The third stage (III) represents the pre-peak compression stage. After the completion of stage II, the specimen fractures internally and cracks develop, causing a slight decrease in curve intensity. Due to the different particle sizes of broken particles within the specimen, the specimen experiences two rounds of compression. As a result, the intensity reaches the peak compressive strength (point B)  
Figure 3: Natural water absorption rate of absorbed coal rock. of 27.42 MPa. The fourth stage is the softening stage (range IV). The curve exhibits a \"zigzag\" decline, and the strain remains constant at point C as the stress decreases to 18.28 MPa, resulting in a decrease in the specimen's bearing capacity. As loading continues, the stress-strain curve shows many fluctuating peaks that are associated with crack closure, fracture surface friction, crack extension, penetration, and slip [28; 29]. This is due to internal particles becoming more densely compressed, causing a local rise in strength but an overall decline. Additionally, the surface of the specimen experiences local fracturing and the serious extension of cracks. An extensometer was used to measure the circumferential strain of the specimen, and the transverse strain during peak stress was \(-\)4.25 \(\times\) 10\({}^{-3}\). The transverse deformation of the coal rock specimen was categorized into three stages. During the initial stage, the specimen exhibited a brief transverse contraction, which corresponded to the axial strain (stage I, stage II). In the second stage of the curve, a portion of the linear growth became apparent, corresponding to the axial strain (stage II). The strength of the specimen was increased as its internal particles were compressed, and the curve exhibited a rising trend. In the third stage, macroscopic damage occurred, corresponding to the development of cracks. The specimens' internal particles were compressed, resulting in increased strength. Sustained macroscopic damage was observed during the third stage, with significant crack development (stage III). Following peak strength, the transverse strain gradually decreased. The transverse strain did not increase until the specimen completely fractured during the fourth stage, as indicated in the stress-strain curve, at 18.28 MPa. Figure 5 displays the secant modulus of natural coal rock based on a strain interval of 1 \(\times\) 10\({}^{-5}\). The results indicate that the secant modulus curve of the coal rock remains nearly horizontal when the axial strain is at 2.5 \(\times\) 10\({}^{-3}\)-4.5 \(\times\) 10\({}^{-3}\), with an average secant modulus of 3.5 GPa. The Poisson's ratio of 0.186 was obtained by measuring the axial strain and circumferential strain of the specimen at this point. Figure 4: The stress–strain curve of the coal specimen at stages I–IV.