## 3 Results and Discussion  
### Wave Velocity Degradation  
The rock's acoustic velocity is a crucial indicator of the engineering rock's physical and mechanical characteristics, as it is strongly related to the mineral composition, structure, pore space, fracture, stress condition, and others [34]. Sound waves are sent and received using pulse transceivers and probes, and oscilloscopes are used to see waveforms, time, and other information. The upper and lower sides of the test piece for the wave speed test must be uniformly coated with a coupling agent. The acoustic-wave-transmitting and -receiving probes are then positioned on the upper and lower sides of the test piece. The internal structure of the specimen can be evaluated without compromising the specimen's integrity by measuring the amount of time it takes for the sound wave to travel through the sample. Fissure-grouted rock specimens are susceptible to varying degrees of damage from dry and wet cycles. Moreover, monitoring the wave velocity values can be a quick and easy way to visually assess the interior structural damage of fissure-grouted rock specimens.  
Figure 3 displays the wave velocity values for several sets of dry and wet cycles on fracture-grouted rock specimens. The relationship between wave velocity and cycle number variation for fracture-grouted rock specimens after the action of dry and wet cycles was fitted and analyzed following the experimental data obtained from acoustic velocimetry. Moreover, the exponential relationship obtained between the two was very obvious. The fitted relationship equation is as follows:  
\[y=926.54e^{-\frac{n}{26.18}}+1248.93, \tag{6}\]  
As the quantity of dry and wet cycle operations increased, the wave velocity values of cracked-grouted rock specimens continuously dropped. The internal structure of the fracture-grouted specimens underwent numerous different alterations as a result of the dry and wet cycles occurring on the specimens. After the first 10 dry and wet cycles of action, the wave velocity rapidly decreases, primarily because of internal microstructural adjustment of the specimen, chemical erosion and dissolution, water swelling, erosion and expansion, and minute particle carryover [35], as shown in Figure 4. At this point, the fluid's dragging force acting on the particles causes the fracture-grouted rock specimen to dissolve, lubricate, and soften. Hence, the rock sample's porosity increases quickly, new fractures appear, and the rock-bearing state declines sharply, all of which cause the wave speed value to rapidly decrease. Moreover, after the last 10 wet and dry cycles, the amplitude of the wave velocity decrease in the crack-grouting sample is evidently weakened, and at this time, the microstructure adjustment inside the specimen gradually became smooth. Furthermore, the hydration reaction inside the fissure-grouted specimen gradually ended as well as the full reaction of cement components, and the influence of fluid on the particles became smooth. Then, the specimen entered the stable stage of water swelling, and the water swelling deformation gradually became stable with the growth in the number of cycles [36]. Therefore, at this time, the wave velocity value of fracture-grouted rock specimens shows a trend of slow decrease.  
According to the fitted relationship, the average wave velocity for specimens after 50, 100, 200, and 300 dry and wet cycles is 1386, 1269, 1249, and 1248 m/s, respectively. The impact of dry and wet cycles on fissure-grouted rock specimens decreases with an increase in the number of cycles until it eventually has no effect on the specimens' quality. Therefore, in this study, the number of dry and wet cycles of rock is mainly set at the stage of severe sample deterioration.  
Figure 3: Wave velocity values of rock specimens of fracture–grouted rock with different groups of wetting–drying cycles.