Experimental Study on Pre-splitting Blast of Underground Caverns in Fractured Rock Mass

The geological structure of the Ying Liang-bao hydroelectric underground power-house is complex due to the development of surrounding rock fissures, messy lithology, and a rock body with "hard, broken, miscellaneous" characteristics. Excavation and molding pose difficulties while pre-splitting blasting has poor effects. To address this issue, we conducted a systematic blasting test combined with pre-splitting for central groove construction on layer III of the power-house. In initial tests, both sides of the wall exhibited significant breakage after blasting and traces of pre-splitting holes were not clearly visible when linear charge density was nearly 100 g/m lower than standardized calculation values. Acoustic testing data revealed that average longitudinal wave velocity in the rock mass body was 4.03 km/s indicating overall poor integrity. Additionally, segmental wave velocities along axial depths from 0~1.5 m, 1.5~3.9 m and 3.9~7.4 m were found to be 2.59 km/s, 3.58 km/s and 4.70 km/s respectively suggesting segmented integrity differences in depth direction. Based on these findings an average single-hole linear charge density for pre-splitting blasts during excavation was determined to be between 0.123~0.284 kg/m with different densities selected according to varying depths while small charge rolls were evenly spaced for each section. The results obtained through testing and application have been positive ensuring basic formation of wall surfaces while significantly increasing half-porosity levels.