Page 266 - Civil Engineering Formulas

200 CHAPTER EIGHT

Blasting Operations
Blasting operations are used in many civil engineering projects—for basements
in new buildings, bridge footings, canal excavation, dam construction, and so on.
Here are a number of key formulas used in blasting operations of many types.*

Borehole Diameter
Generally there are three criteria in determining the borehole diameter to be used:
the availability of equipment, the depth of the cut, and the distance to the nearest
structure. To reduce the amount of drilling, the blaster will usually use the largest
hole that the depth of the cut and the proximity of structures will permit. The
maximum borehole diameter that can be effectively used depends on the depth of
the hole. Conversely, the minimum depth to which a hole can be drilled is depen-
dent on the diameter and can generally be represented by the formula:
(8.62)
L 2D h
where L is the minimum length of the borehold in feet and D is the diameter of
h
the borehole in inches. (For metric, multiply the hole diameter by 25.4 to obtain
the minimum hole depth in millimeters.)

Burden and Spacing Determination

The burden is the distance from the blast hole to the nearest perpendicular free
face. The true burden can vary depending upon the delay system used for the
blast; therefore the delay design should be determined before the drill pattern is
laid out.

Anderson Formula One of the first of the modern blasting formulas was
developed by O. Andersen. He developed his formula on the premise that burden
is a function of the hole diameter and the length of the hole. The Andersen
formula is
B dL (8.63)

where B burden, ft
d diameter of borehole, in
L length of borehole, ft
We have learned since this formula was developed that there are more fac-
tors involved in burden determination than just hole diameter and length. How-
ever, Andersen was quite correct in his assumption that length is a factor in

*The formulas presented here are the work of Gary B. Hemphill, P.E., who at the time of their pre-
sentation was Construction Manager, Hecla Mining Company, and presented in Blasting Operations,
McGraw-Hill.