User's Manual

98
RIOTRONIC X+
User Manual
If there is a layer of hard rock between weaker materials, the primer can be placed within
the tougher rock. This could assist to break the hard rock, and may also prevent charges from
easily blowing out through the weaker material.
Several primers may also be used in each blast-hole, for many different reasons. Multiple
primers provide “insurance” in long blast-holes, and are desirable if the rock contains cracks
or faults that could cut off charges during the blast. Extra primers are also recommended if
explosives are likely to be damaged after charging (e.g. by water).
9.4. Influence of blast timing on fragmentation and heave
Whilst the blast mechanism is an extremely complex event with many unmeasured factors,
it is possible to significantly alter the outcome of a blast by changing the initiation timing.
This is not to say that timing will fix a poorly drilled pattern or compensate for poor
explosive performance. However, in a correctly drilled and loaded pattern, the application of
consistent timing with appropriate, well-chosen delays will significantly enhance
performance.
Assuming there is standard bench geometry, the overall timing can be considered as being
comprised of two timing elements - the timing between holes along the row and the timing
between rows. The time between holes within the row is known as the intra-row or spacing
timing and the time between rows is known as the inter-row or burden timing.
It is reasonable to consider that the intra-row delay has the dominant influence on
fragmentation and the inter-row delay has the dominant influence on heave or throw. This
is simplifying matters - however, it helps to reach a good first estimate which can be fine-
tuned in the field. When actual firing times are considered for optimization, it is necessary
to refer to timing in terms of milliseconds per meter of burden or spacing for a particular
hole. This enables generalizations to be made about blast performance in certain rock types
irrespective of the blast geometry.
9.4.1. Spacing timing influence on fragmentation
For extremely brittle hard rocks the intra row delay should be reduced as the response time
of the rock mass is reduced. In contrast, a porous, plastic, highly jointed rock mass will
require more time between detonations of adjacent blast holes.
If the intra-row delay is made very short (consider the extreme case of firing a row of holes
instantly), preferential cracking will occur between holes in a row resulting in increased
forward movement and reduced fragmentation.