A Comparative Study of Different Blasting Excavation Methods of the Gentle Slope in High Dam Foundation

Gentle slope is the transition from the steep slope section to the horizontal riverbed section in the high dam foundation. When the gentle slope is excavated by the traditional blasting method, the blastholes are arranged along the slope (BHAS). Due to the special structural characteristics of the gentle slope, this arrangement of blastholes results in poor drilling efficiency and accuracy, seriously influencing the construction schedule. Thus, engineers try to adopt the way of reverse slope drilling (BHRS) and energy-relief blasting (ERB). However, the excavation effects, blast-induced damage, and blasting vibration under those methods are still unclear. In this study, based on the excavation of the gentle slope in the Baihetan hydropower station, onsite blasting experiments and numerical simulation are conducted to compare the differences induced by the BHAS, BHRS, and ERB. Moreover, the inherent causes of these differences are investigated using the numerical models with single blasthole of different inclination angles. Finally, the characteristics of different blasting methods are discussed to provide references for the selection of the blasting method of the gentle slope. The results demonstrate that the inherent cause of the differences induced by the BHAS and BHRS is the variation of the arrangement of production blastholes. The way of reverse slope drilling in the BHRS improves the drilling efficiency and increases the inclination angle between production blastholes and slope. As the inclination angles increase, the radial projected area and average pressure gradually decrease, thereby reducing the explosion energy transmitted into the remaining rock mass. Therefore, the PPVs and damage depths induced by the production blastholes in the BHRS are smaller than those in the BHAS. In the ERB, the presplitting blastholes are removed and the shock-reflection device is arranged on the bottom of the blasthole to protect the remaining rock mass. The PPVs induced by the production blastholes in the ERB are larger than those in the BHAS and BHRS, but the damage depths are smaller than those in the BHRS and BHAS. The excavation effects, blast-induced damage, and blasting vibration in the BHAS, BHRS, and ERB are well controlled, meeting the requirements of the gentle slope excavation. However, on the slope with varying inclination angles, these methods demonstrate different drilling efficiency and accuracy. Therefore, it is advisable to consider the influences of topographic features when selecting the blasting methods of the high dam foundation.