## 3 Experimental Results and Analysis  
### Mechanical Properties of Argillaceous Silitstone Under Different Water Temperatures  
Figure 4 presents the trends in the peak strength and elastic modulus of argillaceous siltstone as a function of water temperature, highlighting distinctive concave curves that clearly illustrate the relationships between temperature and these mechanical properties. As the water temperature increases, both the peak strength and elastic modulus initially decrease, followed by a subsequent increase. Notably, under extreme temperature conditions, specifically at 5 and 35 \({}^{\circ}\)C, the peak strength and elastic modulus are significantly higher than at intermediate temperatures of 15 and 25 \({}^{\circ}\)C. Additionally, with an extended soaking duration, the temperature's effect on the peak strength and elastic modulus becomes less pronounced. During the initial soaking period (1-3 days), the impact of water temperature on the peak strength and elastic modulus of argillaceous siltstone is very significant. However, as the soaking duration increases, the influence of water temperature on the peak strength and elastic modulus of argillaceous siltstone diminishes. To investigate the mechanical properties of argillaceous siltstone under varying water temperatures, stress-strain curves were generated after immersion in solutions at different temperatures for 1, 3, 7, and 14 days, as shown in Figure 5. These stress-strain curves not only reveal the deformation characteristics of the rock but also provide foundational data for studying its mechanical properties and establishing its constitutive relationship, particularly characteristics beyond peak compressive strength. In the early stages of immersion (1-3 days), the stress-strain curve displays four distinct phases: the microcrack compaction phase, elastic deformation phase, plastic yielding phase, and strain-softening phase. During this period, the increase in internal microcracks due to hydration is especially pronounced, highlighting a plastic yielding phase that is absent under natural conditions. Here, the influence of water temperature on the stress-strain curve remains negligible. Figure 3: Diagram of the experimental scheme. Figure 4: Physical parameters of clay powder under different water temperatures. (**a**) Peak strength. (**b**) Elastic modulus. Figure 5: Stress–strain curve of argillaceous siltstone. Soaked for (**a**) 1 day, (**b**) 3 days, (**c**) 7 days, and (**d**) 14 days. As shown in Figure 5, After 3 days of immersion, specimens soaked at 15 and 25 \({}^{\circ}\)C began to exhibit complete five-stage stress-strain curve with increasingly distinct plastic characteristics. This effect intensified with extended immersion time, as hydration and ion exchange between the mineral components and water, along with the adsorption and dissolution of certain minerals, lead to continuous \"secondary porosity\" formation within the siltstone. Furthermore, in the yielding phase, the increased immersion duration and reduced strength facilitate the micro-rearrangement and slippage of particles under stress, resulting in more rapid propagation of internal cracks compared to the early stages of immersion. This produces a more pronounced extension of the compaction and plastic yielding phases. The process of microcrack initiation, propagation, and coalescence into macroscopic fractures is the reason for the progressive damage and eventual instability failure of rock specimens. The types of microcracks and macroscopic fractures help reveal the mechanical mechanisms of rock specimen instability and failure under different water temperatures. Based on the failure mechanism, macroscopic fracture types in rocks can be categorized into tensile cracks and shear cracks. Tensile cracks initiate and propagate from the crack tips, extending linearly toward both ends of the rock specimen. If this process continues along an inclined curve, it forms shear cracks.