In a VCR stope, blast holes of 165 mm diameter are drilled. For the blast hole to behave as a spherical charge, the maximum charge length, in m, is ________.

Steps to Solve

1. Determine the frequency of discontinuity per meter

From the diagram, the distance between points A and B is 2 meters, and there are several joints. Let's identify how many joints (discontinuities) are in the 2-meter length.

Assume there are ( n ) joints counted in that length.

2. Calculate the frequency of discontinuity

The frequency of discontinuity ( x ) per meter can be defined as:

$$ x = \frac{n}{2} $$

This tells us how many discontinuities exist per 1 meter.

3. Calculate Rock Quality Designation (RQD)

RQD is calculated using the formula:

$$ RQD = 100 \left(1 - \frac{x}{0.1}\right) $$

Where ( x ) is frequency of joints per meter. Substitute the value of ( x ) calculated from the previous step to find RQD.

4. Calculate the maximum charge length

For a blast hole to behave as a spherical charge, the maximum charge length is:

$$ L = \frac{D}{2} \times 3 $$

Where ( D ) is the diameter of the hole in meters (convert 165 mm to meters: ( D = 0.165 , \text{m} )).

Therefore:

$$ L = \frac{0.165}{2} \times 3 $$

5. Final calculations

Calculate ( L ) and round off to two decimal places as required.