## 3 Results
### Stress and Damage Characteristics of Ore Rocks Under Different Delay Time Conditions
The velocity time-history curves under 25 ms, 50 ms, and 75 ms delay conditions were obtained, as shown in Figure 8.  
Figure 7: _Cont._  
Under the 25 ms delay condition, the initiation velocities of the second and fourth rows of boreholes increased most significantly, exhibiting a secondary velocity increase  
Figure 8: Velocity–time curves of rock movement under different delay time conditions following blast-hole detonation. Figure 7: Trend curves of rock mass movement under different delay conditions (various elevation levels). due to the apparent effects of subsequent blasting. For the first, third, fifth, and sixth rows, the velocity increase due to subsequent row explosions was not significant, displaying a slow and steady increase. Post-initiation, a velocity decrease was observed across all boreholes, with a more pronounced decrease in the second and fourth rows, while the other rows showed minor decreases. This phenomenon might have been attributed to the short delay time, where the detonation of subsequent boreholes continuously applied explosive forces to the preceding rock masses, causing collisions and sustained velocity increases. Additionally, the second row displayed minimal velocity changes post-explosion, indicating difficulty in outward rock mass ejection. Under the 50 ms delay condition, each subsequent borehole initiation promoted a velocity increase in the previously initiated rock masses. The velocities of rock masses in the first, second, and third rows continued to rise due to subsequent borehole explosions. After the initiation of the fourth, fifth, and sixth rows, the velocities showed significant decreases, but the velocities in the fourth and fifth rows exhibited abrupt increases after the subsequent borehole detonations. Each borehole's velocity showed a more substantial drop post-initiation than at 25 ms, indicating a reduction in rock mass density and collision frequency in a confined space. Overall, rock mass velocities continued to accumulate, suggesting ongoing collisions leading to continuous velocity accumulation, showing a clear and consistent pattern. At this delay, the second row also exhibited minimal velocity changes post-explosion, but the movement speed was higher than at 25 ms, indicating an enhanced ability for rock mass ejection compared with 25 ms. Under the 75 ms delay condition, the velocity time-history curves showed a different pattern compared with the 25 ms and 50 ms delays. The subsequent borehole initiations did not significantly enhance the velocities of the previously exploded rock masses. Post-initiation velocities of the first and second rows decreased continuously, indicating sparse rock mass density in the area, with most rock masses already ejected to the free face, preventing sustained velocity accumulation from subsequent explosions. The third and fourth rows exhibited slight velocity recoveries after initial decreases, suggesting some impact from subsequent explosions, but overall the rock mass density was low, and collision impacts were weak, with most rock masses effectively ejected. The fifth and sixth rows showed immediate velocity decreases post-initiation, without significant velocity fluctuations. Therefore, the velocity trend under the 75 ms delay indicated that the number of fragmented rock masses in the confined blasting space was sparse, with most already ejected to the free face. The rock masses in the second row, which initially moved slowly, showed continuous velocity increases post-explosion, indicating an enhanced ability for outward movement and ejection. [PERSON]'s casting model experiments revealed that approximately 20 ms after explosive detonation, the acceleration of the fragmented material was completed [36]. This study found that in underground mine blasting, the rock completed its initial acceleration within approximately 15 ms following the detonation of the explosives. [PERSON] et al.