Abstract: During block caving of hard rock orebodies,the pressure arch of the caving back intensifies the stress concentration at the caving boundary,and the caving process is easily hindered by the restraint on both sides of the arch feet. By calculating the ultimate self-stable balance of the pressure arch at the caving roof and analyzing the characteristics of the arch formation and the potential areas of caving cessation through numerical simulation,the engineering measures for pre-conditioning of hard rock by boundary weakening is proposed. The arching coefficient and caving damage factor are introduced to characterize the annual development trend of the plastic zone above the undercutting in different areas of the rock mass,and the relative position of the plastic zone is used to reflect the position and timing of slot cutting,thereby ascertaining the timeliness of boundary weakening. The results show that during the upward caving process of hard rock masses,the caving roof is prone to forming a self-stable arch,which causes the caving to slow down or cease. The vertical and horizontal pressures at the top of the caving back are the direct factors causing the continuous destruction of the pressure arch. As undercutting advances,the span of the caving pressure arch increases at a non-uniform rate,and the boundary of the pressure arch continuously moves forward with the advance of the undercutting,and the area in front of the undercutting line is always in a high stress concentration condition. When the plastic zone at the ore-rock boundary appears later than the undercutting forward line,slot cutting can be used to destroy the balance of the caving arch and promote propagative caving. The study results are helpful for determining the area and timing of boundary weakening of hard rock masses in block caving.

Key words: block caving,balancing arch,boundary weakening,caving damage factor