Page 267 - Civil Engineering Formulas

SOIL AND EARTHWORK FORMULAS 201

determining the burden. The length of the burden relative to the depth of the cut
has a significant effect on fragmentation.

Langefors’ Formula Langefors suggested that the burden determination was
based on more factors, including diameter of hole, weight strength of explosives,
degree of packing, a rock constant, and the degree of fracture.
Langefors’ formula for burden determination is

Ps
V (d b 33) (8.64)
Bcf(E V)
where V burden, m
d diameter of drill bit, mm
b
P the degree of packing 1.0 to 1.6 kg/dm 3
s weight strength of explosive (1.3 for gelatin)
c rock constant, generally 0.45
f 1 degree of fraction, for straight hole 1
E spacing
E/V ratio of spacing to burden

Konya Formula Currently the best formula for burden determination is one
developed by C. J. Konya. This formula uses the diameter of the explosives in
relation to the specific gravity of the explosive and of the rock.
The Konya formula is

3 SG e
B 3.15 D e (8.65)
BSG r
where B burden, ft
D diameter of the explosive, in
e
SG specific gravity of the explosive
e
SG specific gravity of the rock
r
The optimum length-to-burden ratio (L/B) is 3.

Spacing Determination
The spacing is calculated in relation to the burden length; that is, it is necessary
to complete the burden calculations before determining the spacing. Spacing is
the distance between blast holes fired, on the same delay or greater delay, in the
same row.
For a single-row instantaneous blast, the spacing is usually 1.8 times the bur-
den; that is, for a burden of 5 ft (1.5 m) the spacing would be 9 ft (2.7 m). For
multiple simultaneous (same-delay) blasting where the ratio of length of bore-
hole to burden (L/B) is less than 4, the spacing can be determined by the formula
S BL (8.66)