Engineers from Huizhou Hydropower Construction Engineering (HHCE), a Chinese company specializing in hydraulic engineering and tunnel construction, have proposed using carbon dioxide to build mountain tunnels, underground railway systems and complex hydraulic facilities. They estimate that this technology could potentially replace conventional blasting in areas where heightened requirements on safety and minimal impact on surrounding structures are in place.
The problem that the Chinese specialists aimed to solve is well-known to authorities in many cities. Today, tunnels are increasingly being dug in mountains and under built-up areas, where blasting operations are invariably accompanied by vibrations, noise and the risk of damage to buildings and infrastructure. While conventional blasting is effective, it causes shock waves that travel long distances, causing dangerous cracks. The Chinese engineers are proposing an alternative based on the physics of carbon dioxide phase transitions rather than chemical explosions.
This method goes as follows. High-pressure liquid CO₂ gets injected into pre-drilled boreholes. After initiation, a membrane inside the device gets destroyed, after which the carbon dioxide transforms into a gaseous state almost instantly. Its volume increases tenfold, causing a pressure of hundreds of megapascals inside the borehole. Unlike explosives, this method does not generate a sharp shock wave; instead, its action is smoother and more guided: it splits the rock along naturally weak zones. This results in effective rock mass destruction without loud noise, fragment dispersion or long-range vibrations.
Until recently, this technology was mostly used by trial and error, based on experience and test runs. Engineers had no precise answers to the question of how much energy CO₂ releases during the phase transition and how safe it is for buildings above ground. In their study, the researchers offer detailed calculations of the energy effect of this process for the first time in history. For a standard 1.5-liter tank with a burst pressure of 280 MPa, they obtained a value of about 1,190 kJ, which corresponds to roughly 281 grams of TNT in the construction equivalent. This is fundamentally important, since it makes it possible to compare CO₂ fractionation with conventional blasting operations directly, as well as to apply existing standards and methods for assessing vibration impacts.
The resulting calculations were tested on a real site during the excavation of a water tunnel in a mountainous area. The tunnel was constructed under residential buildings, including those with vulnerable wooden structures. In line with regulations, the ground vibration velocity near these buildings must not exceed 1 cm/s. Calculations showed that the energy impact of CO₂ devices was merely about 20% of the maximum permissible level. Instrumental measurements confirmed these estimates: the maximum recorded vibration by the nearest buildings was 0.18 cm/s, which was several times lower than the limit values and did not pose a danger to the structures.
Currently, the HHCE specialists propose making it mandatory to calculate CO₂ phase transition energy and its conversion to TNT equivalent when designing tunnels and other underground structures. They also stress the need for a comprehensive approach to safety, which has to combine engineering calculations, continuous vibration monitoring and regular drone surveys during construction near residential areas.
