# Study on the Effect of Rock Mass Structure on CO2 Transient Fissure Excavation
## 5 Comprehensive Comparison and Analysis  
The transient cracking role of liquid carbon dioxide acts on the step rock. In the first step, the high-pressure gas is released from the pipe and ejected into the cracking hole wall rock to form the dynamic load role. The second step is the dynamic physical interaction after the expansion of high-pressure gas. The former leads to the damage and fracture of rock, forming lots of fissures; the latter extends the fissure, making rock fragments separate from the rock body. It shall be noted that, on account of the transient cracking role of carbon dioxide, some rock bodies in the upper area of the free face tumble under self-weight conditions, thus becoming crushed. After the rock in the local horizontal area of the cracking pipe is effectively crushed and separated from the original step rock body, the overlying rock may be damaged even with sufficient cracking.  
The comprehensive comparison results of four tests show that the rock structure plays a significant role in controlling the cracking effect. The specific analysis results are as follows:  
1. The comparison results of the cracking effect of three kinds of hole layout methods in the No. 1 test show that the hole layout of the consequent rock stratum is conducive to giving play to the role of high-pressure swelling gas along the soft (weak) structural surface in the rock, and is beneficial to the cracking and crushing of rock and separating from the original rock. The rock after cracking is great in fractal dimension, large in the effective square amount of cracking, and good in the cracking effect. The hole layout cracking on the inverse rock stratum is small in square amount, but the formed fissure is large in area and influence scope.
2. The comparison results of the No. 1 and No. 4 tests show that the cracking effect of carbon dioxide transient cracking role in the massive structure rock is superior to that in the whole structure rock. The carbon dioxide transient cracking is \"lazy\" because it would crush the weak position in the rock in priority. For the rock with good intactness, the rock structure controls the cracking effect in the dynamic pressure resistance of rock and tensile strength. The larger the mechanical strength of the rock is, the more difficult the rock is cracked under the role of transient cracking of the first stage. At this time, the cracking role mainly implicates the \"fissure creation.\" When the rock is poor in intactness, the rock structure controls the cracking effect in the development condition of the structural surface. The cracking role mainly expands the detritus along the original deep fissure weathering surface. The transient cracking focuses on the dynamic impact effect.
3. When the cracking pipe fails to crack the rock and form the fissure connecting to the surface, the high-pressure swelling gas cannot be emitted in the rock, then acts on the backfilling position of the cracking hole, leading to the \"washpipe\" and \"flying tube\". At this time, compared with the first generation of disposable cracking pipe, the second generation of recyclable cracking pipe's safety decreases greatly.
4. The soil interlayer in the rock would severely weaken the cracking effect, deepen the weathering surface, and certainly control the effective square amount of cracking.