In blasting in open-cut mines the explosion energy is controlled by intraborehole delays.Simple delay schemes, with thebottom charge fired first, followed by the upper charge after 10 msec ('bottom" delay),have been replaced by more complex types, for example, the "herringbone" scheme, with alternate initiation of the borehole charges (either the upper of the lower charge is fired first). The delay between the firing of the borehole charges is calculated by simplified procedures and fluctuates between 10 and 25 msec. In calculations of the optimal delay, account is taken of the physicomechanical properties of the rocks and their fissuration, the type of explosive used, the stope parameters, the stemming, the blasting conditions, and the blasting scheme ("upper" or "bottom" delay).To determine the optimal delay in mines with nonsimultaneous initiation of the individual parts of the charge, we examined (for both blasting schemes) procedures for determining the delay either from the detonation velocity of the explosives and the wave velocity or from the detonation velocity of the explosives and the parameters of the stress field front (the velocity of the stress field front).The optimal delay for the first variant with the "upper" scheme (the upper charge is fired first) may be considered for blasting by means of detonating fuse (DF), when the delay is effected by KZDSh-58 pyrotechnical charge delayer placed in the gap between the parts of the charge, and also for electric blasting. In the first case the optimal delay is determined from the condition tl + t2 ~ t3 + tdel,hence tdel ---t, -~-t~ --t~ msec,where t I is the detonation time of the upper charge in msec, t 2 is the duration of movement of the blasting products from the upper charge to the primer of the lower charge in msec, and t 3 is the detonation time of the filament of the detonating wire over the entire length of the borehole in msec. Thus l g + le~c h ltch + lg + "/2.c h rflsec, tdel = I, ch_ .+ ..........where lch is the length of the upper charge in meters, 12c h is the length of the lower charge in meters, lg is the length of the inert gap between the charges in meters, v E is the detonation velocity of the explosives in m/sec, VDF is the detonation velocity of the detonating fuse in m/sec, and Vgas is the speed of the gases in the stemming in m/sec.Branch of the Institute of Geotechnical Mechanics, Academy of Sciences of the ukrainian SSR, Dnepropetrovsk.
