# Study on the Effect of Rock Mass Structure on CO2 Transient Fissure Excavation
###### Abstract  
As a new rock breaking method, CO2 transient cracking has been widely used in rock excavation projects in recent years. However, in the actual construction process, there are often situations where the fracturing effect varies due to different rock mass structures. Through theoretical analysis and on-site cracking tests, this article studies the effect of CO2 transient cracking under the control of different rock mass structures. The results show that: (1) the dynamic compressive strength of rock directly determines the number and range of dynamic impact fractures; the original fractures of rock mass and those caused by dynamic impact in the first stage jointly determine the effect of high-pressure gas expansion in the second stage. (2) The arrangement of holes along the strata is conducive to the action of high-pressure expanding gas along the soft structural plane in the rock mass, which is conducive to the fracturing of the rock mass; the amount of crack formation is small, but the influence range is large. (3) The cracking effect of carbon dioxide transient cracking applied to massive rock mass is better than that of monolithic rock mass, while the cracking effect of layered rock mass with soil interlayer is poor. The research results are of great significance for improving the effectiveness of carbon dioxide transient-induced cracking excavation and guiding actual construction. transient cracking; rock mass structure; cracking effect; fractal dimension +
Footnote †: journal: Medicalconstraint of a hole wall, expansion pressure is generated along the radial direction to crack the rock mass. Due to low efficiency, long construction time, high construction cost, etc., the method is difficult to apply to large-scale construction. The mechanical crushing method refers to the use of mechanical equipment (such as a rock drill), impact rock, and dynamic rock crushing. This method has some problems, such as high fuel consumption and high equipment wear. The transient cracking technology of liquid carbon dioxide, a kind of new non-blasting rock excavation method, is simple and convenient in operation, safe, efficient, etc. [4]. Its small scope of vibration and low strength would avoid the strong vibration, which can damage buildings and overcome the drawback of the traditional rock breaking method. Originating from the 1950s and proposed by American researcher [PERSON] for the first time [5], such technology is mainly used to exploit the high-gas coal mine. China introduced the cracking technology at the beginning of the 1990s [6]. Many researchers have deeply researched the control mechanism of the cracking effect. The research results of [PERSON] et al. show that, in the isothermal condition, the entire process of the high-pressure gas blast could be deemed the cooling and expansion process of high-pressure gas. He proposes that, with the increase of medium strength, the pressure needed for crushing of the medium presents nonlinear growth [7]. [PERSON] et al. propose such technology is most applicable to the porous and fragile materials for cracking [6]. The research results of the foreign S.P.Singh show that the cracking rock aims to form the penetrating cone-shaped fissure [8]. [PERSON] et al. start from the theoretical level to divide the entire cracking process into the rapid gasification stage, slow rising stage, dramatic decrease stage of energy leakage, and remaining stage [9]. [PERSON] et al. carry out a multigroup contrast test, which starts from the difference in tube body structure. The research results indicate that the thickness of a constant pressure sheet would significantly affect the transient cracking role effect of carbon dioxide [10]. [PERSON] et al. think that the method of improving the cracking effect by increasing the filling weight of carbon dioxide is relatively applicable to the rock, which is loose and poor in leakproofness as a whole [11].