Feasibility study on the application of carbon dioxide phase change fracturing technology in a foundation pit of an open cut tunnel

2.1. Controlled blasting

Controlled blasting is based on engineering requirements and specific conditions such as blasting environment, scale, and objects. Through careful design, various technical measures such as construction and protection are adopted to strictly control the release process of explosive energy and the crushing process of the medium, which must meet the expected requirements. The blasting and crushing effect must also control the blasting range, direction and the damage of blasting seismic waves, air shock waves, noise and broken and non-scattering within the prescribed limits.

According to the maximum allowable vibration speed of the blasting safety regulations, the maximum allowable explosive quantity at the time of a single explosion can be obtained by the following formulation [2, 12, 13]:

where: $R$ is the distance from the source of the explosion source to the object to be protected, m; $v$ is the allowable vibration velocity of the location point of the building, cm/s; $K$ is the medium correlation coefficient; soft rock takes 120-180; medium hard rock takes 80-120; hard rock takes 30-80; $\alpha$ is the attenuation index, which is generally 1.5 to 1.75.

Due to the fact that the foundation pit to be excavated and the adjacent structure of the adjacent exhibition center are only 4.6 m, the vibration and safety requirements of the buildings around the foundation pit cannot be met.

2.2. Hydraulic splitting

The hydraulic splitting machine captures the brittle nature of rock and concrete, and uses the wedge principle to design - hydraulic cracking of the rock, cracking and rock without vibration and sound [14, 15].

Process safety, environmental protection, flexibility of use, accuracy, but the cracking rod seal is easy to leak oil, frequent changes; only cracks can be generated after splitting, and the crusher is required to be used twice. Broken, construction efficiency is low. According to the actual production data statistics of a test section at the construction site, the amount of breakage during overtime work is less than 30 m³/day.

2.3. Carbon dioxide phase change fracturing technology

As a new and efficient cracking and excavation technology, carbon dioxide (CO₂) cracking technology has been gradually promoted in the field of rock mass mining and coal mine excavation. The basic components consist of a liquid storage tube, an ignition heat generating agent, an energy dissipating sheet and an unloading head [7], as shown in Fig. 1. The detonator is activated, and the high-voltage current causes a chemical reaction of the activator material in the blast tube to generate a large amount of heat, which causes the phase change of the liquid carbon dioxide to be converted into rapidly expanding high-pressure gaseous carbon dioxide. The volumetric expansion of liquid CO₂ is more than 600 times after thermal phase transformation, and the instantaneous bursting pressure ranges from 100 to 300 MPa, which can meet the requirements of cracking and displacement of coal rock and most rock mass [16-19]. The principle of this technology is that the high-pressure gas generated by the liquid CO₂ phase transition is instantaneously released, and the stress wave propagates to the surrounding. The stress wave and the high-energy gas cause new cracks in the rock/coal body on the one hand, and promote the expansion and development of the primary crack on the other hand. To achieve the effect of cracking. Due to the low tensile strength of the coal body, the coal block is divided into appropriate block degrees by rationally arranging the carbon dioxide-induced splitting pipe, and thrown into the mining space to achieve the purpose of increasing the lump coal rate.

Fig. 1CO2 fracturing structure

(1) Liquid storage pipe: storage of liquid CO₂;

(2) Filling head: filling liquid CO₂, closing the inner cavity of the liquid storage tube;

(3) Ignition heating component: after energization excitation, providing heat for liquid CO₂ gasification;

(4) Unloading piece (constant pressure damaged piece): sealing the inner cavity of the liquid storage pipe to control the amount of energy released by the CO₂ cracker;

(5) Leakage head: Provides a high pressure gas release passage.

The CO₂ phase change fracturing process is the instantaneous release of high-pressure gas generated by liquid CO₂ phase transformation, which causes the rock to break and loose, and will generate certain vibration. The cracking tubes used in the field are different, and their vibration effects are different. At present, there is no relevant theoretical calculation formula for the vibration effect of carbon dioxide cracking process and dynamic damage of rock mass, but many scholars have conducted field tests on the vibration effect of carbon dioxide cracking process and analyzed its attenuation law.

3.1. Fracture parameter design

According to the width of the excavation foundation pit and the distance from the building, three carbon dioxide phase change cracking parameters were designed, as shown in Table 1. At the same time, the TC-4850 blasting vibration self-recording instrument was used to monitor the vibration velocity and acceleration value of the rock mass during the cracking process. Combined with the characteristics of this project, the measuring line is arranged on the mid-perpendicular line of the broken hole connection line, and each measuring point is provided with three directions of vertical direction, horizontal radial and horizontal tangential direction.

3.2. Results analysis

Fig. 2 shows the final cracking effect obtained by using carbon dioxide phase change cracking technology in the field. According to the on-site cracking situation, the carbon dioxide phase change cracking technique mainly forms a wide crack and causes the rock to loosen. There are fewer new cracks in the fracture, mainly primary cracks. Rocks that are judged to be loose will expand and break in natural cracks, and the formed stones are larger.

Fig. 2On-site cracking effect

Fig. 3 shows the vibration attenuation law of carbon dioxide cracking process under three different cracking parameters. It can be found that the vibration decay of CO₂ phase change cracking process is fast and the influence range is small. Among the third cracking parameters, 5 meters from the source can already meet the safety requirements. The vibration response safety range obtained by using the carbon dioxide phase change cracking technology in the foundation pit excavation of Li Qiyue, Liu Xiaoxiong and Xie Xiaofeng [19-22] is the same, that is, when the distance between the monitoring points and the gun source is 5 meters, the vibration velocity is less than 2.5 cm/m.

Fig. 3Vibration attenuation law of carbon dioxide cracking process under three different cracking parameters