User's Manual

99
RIOTRONIC X+
User Manual
In places where the intra-row delay is made too long, blast-holes function individually with
reduced fragmentation and movement. This is due to the relaxation of blast induced cracks
in the burden rock of the adjacent blast-hole, which then both stop the development of radial
cracks from the blast-hole firing and act as escape paths for explosive energy.
The optimum spacing timing will be such that all radial cracks from the first blast-hole have
developed as far as possible, and explosive gases have started to move the burden rock of
that blast-hole away from the face. The second blast-hole then “sees” only the correct burden
of rock, still stressed due to the energy of the first blast-hole, with all blast induced cracks
still tight, allowing new cracks to pass through into the solid rock on the other side.
9.4.2. Burden timing influence on heave
To achieve maximum throw in multiple row blasts, timing between the rows must be long
enough that previously fired material is well away and does not hinder the forward
movement of the mass about to be fired. Consider the extreme case of firing all rows
together; in this case, only the front row will see a free face forward, for all the remaining
rows the bench will effectively be the free face.
This puts a lower limit on the inter-row delay below which choking of the blast will occur
with restricted forward movement and a possible increase in over break and vertical heave.
This minimum time is variable depending on the response time of the rock mass.
Increasing the burden timing to excessively long inter-row delays does not have an added
effect on performance. In fact, excessive inter-row delays will limit throw due to displaced
rock mass coming to rest against the face hindering further movement.
If the case of the ‘optimum’ timing for the rock being blasted is being considered, then the
front row blast-hole will have fired, and will just have started to move away from the face,
so that the next row only sees one burden of unbroken rock.
The timing should not be so long that relaxation can occur in the burden rock. When the next
row of blast-holes is fired, the moving burden will not be confined by the row in front and
the rows of blasted rock will maximize forward movement. The combination of spacing and
burden timing elements is often described as “relief”. Unless there is a strong reason not to
do it, initiation timing will have a constant relief throughout the blast. This promotes
optimal timing and consistent results across the blast. Blast sequencing programs will show
the blast timing as “relief” in milliseconds per burden meter and can be used to apply
consistent timing.