Study on directional fracturing mechanism and field application of two-dimensional tensile blasting

The drilling and blasting method has been widely used due to its efficiency, ubiquity, and cost-effectiveness in the field of rock excavation, and directional fracture-controlled blasting is one of the important applications. Therefore, in response to the characteristic that the tensile strength of rock mass is much lower than its compressive strength, a blasting technique called two-dimensional tensile blasting (2D blasting) is proposed, which only produces an approximate two-dimensional single crack surface in the predetermined direction of the hole wall after blasting. The directional fracturing principle of 2D blasting was analyzed, the visualization experiments on the expansion of two-dimensional tension blasting cracks were conducted using a self-developed experimental system, and a directional roof cutting test using 2D blasting technology waw conducted at an actual coal mine. The research results indicate that: (1) The 2D blasting generates high-temperature and high-pressure gas in the hole through the instantaneous reaction of expander, which produces instantaneous expansion and cracking effect on the medium, meanwhile controls the direction of crack initiation and propagation through 2D blasting device. (2) The 2D blasting generates concentrated tensile stress on the hole wall in presplitting direction through the control of the 2D blasting device, resulting in mode I cracks through tensile fracturing, transforming the random and partitioned rupture of conventional blasting into a directional and single rupture. (3) Compared with conventional blasting, 2D blasting has a longer reaction time after initiation, about 200 ms, while conventional blasting only has less than 0.2 ms, a difference of 1000 times. This indicates that 2D blasting technology has a much smaller loading rate than conventional explosive blasting, resulting in a much smaller strain rate in the specimen than conventional explosives. Therefore, the 2D blasting technology eliminates the blasting crushed zone and makes it easier to achieve the desired directional fracture effect. (4) The 2D blasting can achieve ideal directional fracture effect under the condition of composite roof. The observation results of 10 cut holes within a 100 m test section on site show that the average crack rate inside the holes can reach over 90%. Therefore, 2D blasting technology is expected to replace explosives and become a new type of directional pressure relief technology for the mine roadway. The research results contribute to revealing the mechanism of 2D blasting and promoting the application of 2D blasting technology in the field of directional rock breaking.