Flyrock: theory and calculation
The main purpose of blasting is the adequate fragmentation of the rock mass, with secondary purpose (at times) of moving as much as possible of the rock mass to minimise additional ground movement using trucks, draglines or other heavy equipment from the blast area. Unfortunately, a portion of the explosive energy is lost due to the generation of blast rock that may result in face bursting, cratering and rifling.
Fly rock is generally perceived as the rock propelled beyond the blast area. IME (1997) has defined fly rock as the rock(s) propelled from the blast area by the force of an explosion. Generally, fly rock is caused by a mismatch of the explosive energy with the geo-mechanical strength of the rock mass surrounding the explosive charge. Factors responsible for this mismatch include:
- Abrupt decrease in rock resistance due to joint systems, bedding layers, fracture planes, geological faults, mud seams, voids, localized weakness of rock mass, etc.
- High explosive concentration leading to localized high energy density,
- Inadequate delay between the holes in the same row or between the rows, Inappropriate blast design,
- Deviation of blast holes from its intended directions,
- Improper loading and firing practice, including secondary blasting of boulders and toe holes.
The potential throw distances of fly rock can be estimated using tables or empirical formulas highlighted below:
Face Bursting = DFB = (k² × g-1)(m0.5 × SH-1)2.6
Cratering = Dc = (k² × g-1)(m0.5 × SH-1)2.6
Rifling = DR = (k² × g-1)(m0.5 × SH-1)2.6
Where: 𝜃 = drill hole angle (worse case 80°), DFB, DC, DR = maximum throw (m), m = charge weight/m (kg/m), B = burden (m), SH = stemming height (m), g = gravitational constant (9.81 m/s2) k = a constant (can be refined with measurements).
Ghasemi et al (2012) also developed an empirical formula, based on the analysed data from various blasts, with this formula considering various input parameters:
Flyrock = DFR = B-0.796 × S0.783 × SH1.994 × H1.649 × D1.766 × PF1.465 × Q-1.465
Where (if not defined above): S = Spacing (m), H = Depth of borehole (m), d = Borehole diameter (m), PF = Powder Factor (kg/m3), Q = mean charge per blast-hole (m).
Reference
Tendele Mining (Pty) Ltd, blasting impact assessment for the development of an Anthracite Mine as part of the Somkhele Mine Expansion Project North-west of Mtubatuba, KwaZulu-Natal.
