## 6 Conclusions  
The objective of this research is to investigate the mechanical properties and cracking behavior of frozen fissured sandstone, aiming to provide valuable insights for conducting safe rock engineering in frozen underground rock. High-speed photography and acoustic emissions are employed to observe frozen fissured sandstone with three open en echelon fissures under uniaxial compression. The strength, deformability, and failure processes are thoroughly examined, revealing the influence of freezing on the mechanical properties. The main conclusions are as follows:  
1. The mechanical properties are influenced by the inclination angle of fissures, saturation, and temperature. In the frozen-saturated and dried states, the UCS curve decreases first and then increases with an increase in inclination angle, reaching its minimum value at 30\({}^{\circ}\). However, in the saturated state, the UCS shows a continuous increase. The ranking of UCS can be stated as follows: frozen-saturated state > dried state > saturated state.
2. The AE events in the dried and frozen-saturated states exhibit three peaks during the compression process. The first peak corresponds to the initiation of cracking and propagation between the three fissures, while the second peak corresponds to initial cracking and propagation at the outer tip within fissures 1 and 3; the third corresponds to specimen failure. However, in the saturated state, there are only two peaks observed during the compression process. The first peak represents initial cracking and propagation between all three fissures, whereas the second peak indicates specimen failure.  
Figure 14: Failure process in the frozen saturated state.  
Figure 13: Failure process in the saturated state.  
3. The initial crack angles decrease with a rise in fissure inclination, while temperature and saturation have no influence on the initial crack angle. However, the initial crack stress and time increase with fissure inclination in all three states. Furthermore, the ranking of the initial crack stress and time at a given inclination is as follows: frozen-saturated state > dried state > saturated state.
4. The strength of saturated sandstone increases when subjected to freezing, and this phenomenon can be attributed to four underlying mechanisms: (1) ice provides braced force under compression, (2) ice fills microcracks, and (3) an unfrozen water film provides a cementing effect under tension or shearing load, while (4) the frost damage effect causes softening. The first three mechanisms counteract the softening effect, resulting in an increase in the UCS of saturated fissured sandstone during freezing.
5. The mechanical properties and crack behavior of frozen fissured rock containing a single pre-existing flaw, as well as the influence of ice within the flaw on stress distributions at the fissure tip, remain unclear. These aspects require further investigation in future studies.  
Conceptualization, W.L. and L.H.; methodology, W.L.; validation, L.T. and D.W.; formal analysis, L.H.; investigation, D.W.; resources, H.J.; data curation, L.H.; writing--original draft preparation, W.L.; writing--review and editing, L.H.; supervision, L.T.; project administration, H.J.; funding acquisition, H.J. All authors have read and agreed to the published version of the manuscript.  
This study was funded by the National Natural Science Foundation of China (grant No. 41702334).  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
Author [PERSON] was employed by the company CCCC First Highway Consultants Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.