## 4 Discussion  
This experimental study of the effect of delay time on rock fragmentation shows there is a certain delay requirement related to obtaining optimal blast excavation quality in multi-hole bench blasting. The location of stress waves between boreholes plays an important role in wave history and possibly influences rock damage and fragmentation. Therefore, a brief analysis of the stress waves produced by the boreholes was performed in the 2D horizontal plane. Figure 19 shows the P-wave propagation produced by the borehole at three delays. The solid line represents the P-wave, and the dashed line represents the reflected P-wave. Figure 19: Brief analysis of P-wave propagation: (**a**) P-wave propagation for simultaneous detonation; (**b**) P-wave propagation of borehole A at 50 \(\upmu\)s and 100 \(\upmu\)s. The letters A–D represent the numbers of the boreholes. Figure 18: Median sizes and blast excavation weights of all specimens with different delays. After the explosive column is detonated in the borehole, the stress wave propagates within the bench specimen. Figure 19a shows the P-wave propagation of simultaneous detonations, with the most significant superposition of P-waves occurring in the area between boreholes. The location of the P-wave superposition changes when there is a delay between the boreholes. P-wave propagation at 50 \(\upmu\)s after the detonation of borehole A is shown in Figure 19b. If the inter-hole delay is 50 \(\upmu\)s, the reflected P-wave of borehole A formed at the free surface will interact with the P-wave generated by borehole B in the burden. When the inter-hole delay is 100 \(\upmu\)s, the P-wave propagation of borehole A when borehole B is detonated is as shown in Figure 19b. At this time, the reflected wave of borehole A formed at the free surface propagates to the back of borehole B, indicating that there is no P-wave superposition in the burden. The same P-wave analysis method can be applied to bench specimens BS4-BS6. When the inter-hole delay increases, the interaction of the P-waves in the burden becomes weaker until it disappears. BS1, with the most significant stress wave superposition in the blasting experiments, did not produce the smallest median size. When the inter-hole delay was increased to 50 \(\upmu\)s, BS2 produced an \(x_{50}\) value similar to that of BS1. In the coarse zone, the particle size of BS2 was decreased compared to BS1, which means rock fragmentation was improved in this zone. Unfortunately, the further increase in delay time did not continue to improve fragmentation, and BS3 obtained the worst fragmentation results. Although BS3 recorded the maximum blast excavation weight, the particle size in the coarse zone of BS3 was also significantly increased, with the \(x_{\text{max}}\) value increasing by nearly 21.3% compared to that of BS1. In the blasting experiments of BS4-BS6, the interaction of the stress waves gradually became weaker until it disappeared, and the particle size in 1-80% mass passing decreased with the increased inter-hole delay. BS6, having the longest delay time of all experiments, obtained the smallest \(x_{50}\) value, i.e., the best fragmentation result. Compared to BS1 with simultaneous detonation, the \(x_{50}\) value of BS6 was improved by nearly 46%. In this study, simultaneous detonation and short delays did not obtain excellent fragmentation results. When the inter-hole delay was longer than 100 \(\upmu\)s, fragmentation was improved with increased delay time, even though stress wave interaction gradually disappeared. This indicates that other factors besides stress wave interaction should be considered in multi-hole delay blasting to control fragmentation. In studying stress wave propagation in rock using different delay times, the rock medium is usually treated as an intact continuum, and the propagation of cracks in the rock mass is often ignored. During blasting, the velocity of the blast-induced crack is lower than that of the stress wave. By conducting model blast tests, [PERSON] et al.