User's Manual

RIOTRONIC X+

User Manual

Although the mechanical efficiency of staggered patterns exceeds that of square or

rectangular patterns, the difference in blasting results decreases with a decrease in the

strength of the rock mass. In weak or closely fissured rock the difference in blasting results

may not be noticeable.

Main Points - blast-hole pattern

 Staggered patterns are more economical than square or rectangular patterns in

strong rock.

 There is little difference in blast result in weak, highly fissured rock.

 Burden to spacing ratio varies from 1: 1 up to 1: 1.5 but it may go higher depending on

rock structure, orientation and required blast result.

9.3.5. Subdrilling

Subdrilling or subgrade is the length of blast-hole drilled beneath bench floor level below the

required floor of the excavation.

Subdrilling is carried out to ensure explosives are placed below the bench floor level to assist

with floor grade control. The correct amount of subdrilling is critical to the entire operation.

Too little subdrilling and digging becomes difficult, floor grades will be poor and tear on

equipment will be increased. Too much subdrilling and drilling is adversely affected due to

excessive preconditioning of the next bench down.

Note that if there is a weak or valuable horizon at grade (such as when blasting to coal) the

blast holes will be drilled to finish above the grade line. This is the opposite of subgrade and

it is known as ‘Standoff’.

The optimum amount of subdrilling depends on:

 Strength and density of the rock mass

 Diameter and inclination of blast-holes

 Type of explosive and, more particularly, the energy generated per meter of blast-

hole

 Mean inclination of the face and the effective toe burden distance

 Location of primers in the charge.

Rule of thumb #4:

Subgrade = 8-12 blast-hole diameters