Experimental Study on the Influence of Blast Hole Bottom Cushion Medium on Blasting Damage Characteristics and Strain Evolution of Rock Mass

The blasting method involving the placement of a cushion at the bottom of blast holes has found widespread application in the construction of foundations for hydraulic and hydroelectric projects. This study employs comprehensive techniques including CT scanning, three dimensional reconstruction, fractal damage theory, and digital image correlation methods. It conducts both three-dimensional and two dimensional blasting model experiments using foam, fine sand, and fine sand combined with steel balls as the bottom cushion medium. Comparative analysis is performed on the damage characteristics and strain evolution of the rock mass subjected to bottom cushion blasting. The results indicate that specimens with foam as the cushion medium at the bottom of the blast hole exhibit the highest degree of damage and strain peak values, while the overall damage degree of the rock mass in the exploded segment is the lowest. Conversely, specimens utilizing fine sand and steel balls as the cushion medium at the blast hole bottom of the blast hole demonstrate the lowest damage degree and strain peak values, with the overall damage degree of the rock mass in the exploded segment being the highest. Employing fine sand combined with steel balls as the cushion medium at the bottom of the blast hole can mitigate the impact of blasting stress waves and blasting gas, enabling the efficient utilization of explosive energy. This approach not only safeguards the integrity of the rock mass at the blast hole bottom but also strengthens the fracture degree of the rock mass in the exploded segment, thereby achieving a dual-purpose outcome.