Abstract: The emergence of blasting tight bottom seriously affects the effect of bench blasting and subsequent construction procedure. The dynamic analysis method of multi-degree of freedom system is applied to the structural stress analysis of bench blasting, and the influence mechanism of lithologic change on the formation of blasting tight bottom is analyzed based on the structural characteristics of the rock mass. The results indicate that 50% weakening of the lithology of the middle rocks could increase damage difficulty of bottom rock, because the shear force of rock mass at the bottom is reduced by 7.8% and the bending moment is reduced by 6.6%. And the weakening effect increases with the decrease of mechanical properties of rock mass. In addition, the strengthening of the lithology of the bottom rocks could increase the internal force of bottom rock slightly, but rock strength is greatly improved, which makes the bottom rock more difficult to break. Furthermore, the influence mechanism of different factors on the formation of blasting tight bottom is analyzed based on the rock mass structure failure criterion. It is found that the uneven distribution of lithology and the changes of structural stiffness and bottom stress conditions are the main reasons for the formation of bench foundation. Adjusting the initiation point below the bench can weaken the influence of weak-lithology segment. Increasing the stress conditions and reducing the structural stiffness of the bottom rock mass can also avoid the formation of blasting tight bottom. The relevant research results can provide reference for similar projects.

Key words: bench blasting, rock mass, toe rocks, structural mechanics, control method