Drilling and blasting should be thought of as a team effort, without good
drilling practices blast outcomes will always be less than ideal. The best
blast design, operational practices and explosive product will at best lessen
the impact and definitely not overcome poor drilling practices. In too many
cases the shotfirer or explosives supplier is asked to explain a sub-optimal
result from a blast when the principal root cause can be traced to poor drilling.
Unfortunately in these cases there is usually little record of what the effectiveness
of the drilling was except an invoice for x amount of drilling and a poor blast
on the ground.
The days of blast patterns being marked out as the drill trams onto the shot
because the quarry is out of rock should be numbered however this is still
too common an occurrence. As all astute Quarry Managers know getting the blast
results right will allow optimisation of his excavation and processing operations.
These statements are not new, however the knowledge of what works in drill
and blast at a particular site is often forgotten or misapplied and the entire
process is compromised. This especially happens when changes in personnel,
equipment, excavation area or even expectations of results occur.
How Drilling Effects Blast Outcome
Assuming that a given design will produce an efficient outcome, the results
of the blast will depend on the position in the rock mass and the physical
state of the blastholes. This can be expressed as the Drilling Effectiveness.
Factors that influence Drill Efficiency include:
- The Design, what is required to be carried out i.e. the baseline
- Geology, the material and its physical characteristics
- Drill Operations, how the drilling occurs
The first two groups of factors are usually controlled by operational processes
that an astute Quarry Managers will have implemented to mitigate their negative
influences i.e. standard designs and procedures based on rules of thumb and
modified by site knowledge and experience. The best example of this is the
orientation of Quarry faces in such a way as to minimise the negative impacts
of the geology of the rock i.e. firing faces across dominant jointing not parallel
to them. The last group, Drilling operations, however, is controlled predominately
by the driller and his equipment, be this site owed or sub-contracted.
Drilling Operations
Drilling operational factors that influence Dill Efficiency include:
- Setup Geometry of the Drill Rig
- Drill Operation Parameters
- Drill String, use of Guide Tubes and Bit Size
- Blasthole Angle
- Blasthole Collar Position
- Deviation of Drill String
The blast design needs to take into account the physical limitations of the
drill rig. Most modern drill rigs require a well prepared bench free from large
rocks and slopes to operate safely and efficiently. When a quarry bench that
had not been prepared to an adequate standard is allowed to be drilled it unfortunately
will result in compromises being made. This will be to the detriment of the
blast result. Figure 1 shows a plan of bore-tracking results from an area of
a blast where cleanup severely effected drill setup. The driller had difficulty
ensuring the blasthole orientation; collar position and angle were drilled
as designed. After the blast, material from this area was less fragmented and
toe was evident. This result could be directly related back to poor bench preparation.
Figure 1. Example of resultant blasthole deviation due to poor cleanup. (Black
Squares represent blasthole collars and the red lines represent the bore-track
of each blasthole)
It is common to see errors in drill angle and blasthole orientation due to
setup of drill rigs even when bench preparation is of adequate standard. It
is essential that the driller clearly understands the required design orientation
and angle of the blastholes.
In Table 1 some simple effects of variation in blasthole angle are given.
| Depth |
5° toe offset from Vertical |
10° toe offset from Vertical |
| 10m |
0.87m |
1.76m |
| 15m |
1.30m |
2.64m |
| 20m |
1.75m |
3.53m |
Table 1: Varying Blasthole offset with Depth
As can be seen by offsets given in Table 1, if a driller were to drill a 5
degree hole when the plan calls for a 10 degree hole on a 15m bench, it will
add 1.3m extra burden on the blasthole. This extra burden will effect the movement
and breakage of the muckpile. On a 20 metre bench every 1 degree change moves
the toe of the blasthole by 0.36m. If the blastholes are on the front row it
is very easy to move from an optimum burdened blasthole to a heavily overburdened
blasthole thus severely affecting the blast result. An extreme example of the
effect of this was recently seen when a new drill equipped with faulty electronic
inclinometer drilled a pattern at a quarry in North Australia. Blastholes designed
at 10 degrees were in fact were drilled at 17 degrees due to the faulty equipment.
As could be guessed there was insufficient burden on the blastholes and fly-rock
resulted. In this case no personnel was injured or equipment damaged, however
under different circumstances the result could have been dramatic. The customer
did not see the full value in Technical Services before the incident and no
bore-tracking was carried out. This is not the case now.
Errors in blasthole angle can be compounded by the blasthole not being drilled
in the correct orientation. Figure 2 shows an example of a plan of blastholes
at a quarry where the driller had incorrectly orientated the drill. The toe
of the blastholes varied by up to 2m from the design position and as could
be expected the blast did not move forward and diggability was very poor. This
result was due entirely too poor control of the drill orientation whereas the
initial conclusion of the Customer was that the explosive had not performed
to its full potential.
Figure 2. Example of resultant deviation due to incorrect drill orientation.
Black squares represent blasthole collars and red lines represent the bore-track
of each blasthole.
A common parameter for drill bits is 5% wear, such that a 102mm bit will be
resharpened and reused until it is 97mm, an 89mm bit will be used to 84mm.
Experience has shown however blasthole bits will be used outside this range
if no procedure is in place to specify what is acceptable. The effect of
this is to reduce the explosive energy within the design. The reduction in
kilograms loaded per metre for a 5% reduction in a 102mm blasthole is 11%.
If this occurs it will significantly affect the blast result. It is suggested
that 5% variation in bit size is too great and a 3% variation is a more realistic
and feasible target.
Blasthole deviation is a well known phenomenon however what is acceptable
deviation and what are the methods of reduction are not well defined. Commonly
guide tubes are used on front row blastholes to ensure blastholes do not deviate.
However a common practice, due to their cost, is to only use them on front
rows with subsequent blast-rows are usually drilled without guide tubes. Add
to this that the majority of subcontractor drillers are paid to some extent
on a meters per hour basis can lead to a less than desirable result. It is
not common to bore-track back row blastholes for deviation however Figure 3
shows an example of a section of a blast that was bore-tracked with the results
being of grave concern. The results indicated burdens and spacings varying
as well as the toe line which in this particular case was the back row of blastholes.
The variation in some areas of the wall was over two meters or a 60% variation
in burden. It must be remembered that this row of blastholes represents the
face of the next blast.
Figure 3. Example of a plan showing blasthole deviation. Black squares represent
blasthole collars, red lines represent bore-track results, blue lines are
design. The Black dashed line represents the new toe line of the back face.
It varies by up to 60% from design.
Technical Services
Technical Services, Laser Profiling and Bore-tracking, of blasts are commonly
used in the modern quarry industry and provides a tool to evaluate the deviation
from design of blastholes. When blastholes are found that deviate from the
design they are evaluated as to the degree of deviation and its likely effect
on blast outcome. If it is considered to be significant then the blastholes
must be redrilled or the explosive loading criteria changed. As has been shown
this situation is more common than should be expected with modern drilling
rigs. The value of Technical Services in Quarry blasting can not be underestimated
when ensuring blast designs are executed to acceptable standards. Too many
incidents and questionable blast outcomes have occurred within the industry
that could have been avoided with smart use of this tool. When concerns about
a blast result occur the whole process needs to be investigated to ensure true
root causes are found. Unfortunately the bulk of the information is usually
not available to investigators as little information regarding drilling is
recorded unless profiling of free faces and bore-tracking of blastholes is
carried out and analysed in combination with explosives loading and firing
sequences.
Article appeared in the Quarry Magazine, March 2005.
Written by Shane Slaughter (UEE Drill and Blast Engineer)