Disclaimer: This article is published in the book entitled: «Flyrock in surface mining» by K. Raina
Burden is one of the most important blast design variables that influences flyrock distance. Burden, however, has a direct relationship with drill diameter. The optimum breakage burden (Bob) can be estimated by the Livingston (1956) crater theory (Equation 1) in terms of total charge mass (Q) that were further defined in terms of burden by Dick et al. (1990) and Da Gama (1984) as in Equations 2 and 3, respectively.
Bob = kQ1/3 (1)
Log(Bob2 + ra2)1/2 = 1.846 + 0.312logQ (2)
rc = (3Bm2 – B2)1/2 (3)
where rc is the crater radius (m) in rock, Bob is the optimum breakage burden distance or charge depth in m, k is a constant of proportionality expressing rock and explosive properties, Q is the mass of explosive in kg, and ra is the apparent crater radius and the equivalent TNT charge mass (Q).
Accordingly, an optimum burden will thus produce the desired results in terms of fragmentation and throw. However, two conditions of the burden, i.e. lesser than optimum and more than optimum, arise. In the case of low burden, the throw will transform to excessive throw and even flyrock travelling towards the free face. However, in the case of excess burden than optimum, the confinement of the explosive will be greater, and thus may induce flyrock from the top of the face that can be launched in any direction, including back of the free face.
