Double spiral cut is a type of parallel hole cut which is frequently used in tunnels with sequential blasting technique. Proper charge distribution in the cut holes is of primary importance to achieve good pull and fragmentation. Though Langefors and Kihlstrom recommend equal concentration of charge in all the holes in the inner Spiral, the experiments conducted on small models made of cement mortor using aluminium detonators as micro-charge revealed that if the concentration of charge in the first two holes nearest to the empty hole is increased, there is considerable improvement in pull and fragmentation. This is mainly because the value of the aperture angle for these holes is small as compared to other holes thus requiring higher concentration of charge.
Despite increasing popularity of tunnel boring machines and road headers for rock excavation in tunnels, drill and blast system is still commonly used particularly in strong and abrasive rock formations. Amongst the various drilling patterns adopted for blasting, parallel hole cuts are generally adopted when longer pull is required. Double spiral cut is a type of parallel hole cut which, besides giving longer pull, also gives good fragmentation but correct estimation of explosive charge in the cut holes is necessary for the success of the blast. Langefors and Kihlstrom (1978) have developed empirical relation to calculate the explosive charge when blasting towards narrow openings. However, they advise equal quantity of Charge in all the cut holes in the inner spiral of the double spiral cut. The author has critically examined these relations and also conducted experiments on models made of cement mortar using aluminium detonators as micro-charge. The results of the investigations give a new direction for charge estimation in the cut holes.
According to Langefors and Kihlstrom(1978) explosive charge required for full breakage when blasting towards a narrow circular opening is given by the following relation (Figure 1). 0.55 (A -3/2/2) / (Sin v) ••• (1) The charge concentration when blasting towards narrow rectangular opening is given by (Figure 2) e 0.35 V/(sin v)3/2 ••• (2)
(Equation in Full Paper)
The equations differ only in the value of the constant, which in blasting towards a circular opening is 60% greater than for the rectangular one, owing to greater fixation in case of former.
It is clear from the above relations that the necessary charge concentration for proper loosening of the rock is mainly governed by the aperture angle and the distance of the charged hole from the free surface.
Cement mortar was used to cast models in cylindrical mild steel shell. This material was found to be ideal synthetic substance for blasting experiments because its property can be easily controlled by controlling the percentage of cement and sand and it can be formed into blocks of any desired shape.