Page 268 - Civil Engineering Formulas

202 CHAPTER EIGHT

where B burden, ft
L length of borehole, ft

If the length-to-burden ratio is greater than 4, then the spacing is twice the
burden. Therefore, if the L/B is 5 the spacing is determined by S = 2B. For
example, if the burden were equal to 7 ft (2.1 m), the spacing would be

S 2(7) ft
14 ft
again showing the relationship of the length of borehole to the burden and spac-
ing dimensions.

Stemming
There must be some substance put into the top of every borehole to prevent
the explosive gases from escaping prematurely. This substance is called
“stemming.”
Generally the amount of stemming required will range from 0.7B to 1B.
Therefore, in the case of burdens of 8 ft (2.4 m) the stemming will be some-
where between 5.6 ft (1.7 m) and 8 ft (2.4 m). That is, the ratio of stemming to
burden can vary through this range depending on existing conditions. The con-
dition most likely to affect the amount of stemming is the structural integrity of
the material in the area of the borehole collar. If the material is very competent,
as is a homogeneous granite, the stemming will approach 0.7B. However, if the
material is fractured rock with many fissures and dirt seams, the required ratio
of stemming to burden will be approximately 1. In material other than rock,
such as overburden, the ratio of stemming to burden will be even greater. Over-
burden is generally treated in a 2:1 ratio over rock; that is, 2 ft of overburden is
approximately equal to 1 ft of rock for stemming purposes. Therefore, if there
is 4 ft (1.2 m) of overburden on a shot that requires a burden of 7 ft (2.1 m), the
stemming is calculated as follows.
Using the ratio of 0.7B, the stemming would ordinarily be 5 ft (1.5 m).
However, with 4 ft (1.2 m) of overburden, for stemming purposes equal to 2 ft
(0.6 m) of rock, the actual amount of stemming is 7 ft (2.1 m). Another way of
expressing it is to compute the required stemming and then add one-half the
overburden depth. Thus
OB
Stemming 0.7B (8.67)
2

where B burden
OB overburden
To compute stemming for a side hill cut, use a right triangle and 2 to 1 ratio,
as shown in Fig. 8.2.