Blast Design; Limestone Quarry
CSMM140 Surface Excavation Design Coursework
640046848
Dr A. Wetherelt
March 2015
1. Introduction
This report details the design for the creation and operation of a limestone quarry, with the
primary goal of producing 15,000 tonnes per week of rock for use in the aggregates
industry. The site has been specified as ‘greenfield’, with the only prior work conducted
being 2 meters of overburden stripping – leaving 58m of workable ground before reaching
the maximum specified depth.
1.1. Overall Quarry Layout
While the original quarry design would employ benches of a standard size, the detection of
a 3.5m weak layer, encountered at a depth of 35m, demands an adaption to ensure the
layer is accounted for in blast design to produce a satisfactory blast, taking into account
influenced factors such as vibration and flyrock. The bench containing the weak layer has
been allocated a height of 11m to provide enough space for additional stemming within the
bench to suppress the more unpredictable rock within the weak layer.
An alternative overall design has also been outlined - regarding the prospect of using 5
larger benches (3x11m, then 2x12m) which would align the weak layer to the top of the
third bench. The unstable weaker rock could then either be excavated via stripping or
suppressed using blast covers –
however analysing the comparative
cost and efficiency of these methods is
beyond the scope of this report. It
should be noted that this layout would
leave a 1m layer of unused rock at the
base of the quarry, as bench heights
are limited to 12m – potentially
incurring a large revenue loss.
Each non-critical bench has a specified
width of 5m to ensure sufficient
capacity to act as a ‘rockfall’ trap, with
a 10.5m haul road, to satisfy the
requirement of being 3.5 times the
width of the largest road vehicle, in this
case being a Bell B25E truck – holding a
width of 3m. In combination with
bench face angles of 71.57o (equal to
the 3:1 blast column inclination) this
gives an overall slope angle of 47.6o. The
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F IGURE 1 – SLOPE GEOMETRY
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overall slope is shown in Figure X.
1.2. Stereographic Analysis
Using the software DIPS 6.0, a stereographic analysis
has been conducted to investigate three sets of
joints identified from core logging. The results, of
which some key cases have been highlighted and
are shown in Figure X, indicate that multiple TABLE 1 - PREDICTED FAILURES ; BENCH
possible failure modes are likely. Separate analysis
FACE
have been carried out for both the overall slope
angle, X, and the bench face angle, 80o. The angle of friction for the limestone has been
assumed as 30o The resultant expected failure mechanisms and the corresponding
orientations are shown in Table X. Risk of failure was only found when considering the
bench face angle – the overall slope does not appear to incur any failures, however a planar
failure is within close proximity, shown in the left diagram of Figure X, and could be possible
depending on the actual friction angle.
All three failure types; planar, wedge and flexural, have been identified as being likely
depending on orientation. While these should not substantially impede progress, they
should be monitored – with operations adapted as necessary. Bench slopes have been given
a minimum width of 5m to act as rockfall traps should a failure occur on a non-critical
bench, however failure of a haul road bench could be critical; reinforcement should then be
investigated. While this is outside the scope of this report, it is recommended that the use
of dowels or rock bolts, in addition to drainage, should be considered.
F IGURE 2 STEREOGRAPHIC A NALYSIS
SLOPE ANGLE PLANAR FAILURE ON LEFT , REMAINING ORDERED ACCORDING TO T ABLE X
2. Blast Setup
The following sections describes to the setup of a typical column blast for the standard 9m
high bench. Data for both 11m bench types (standard and weak layer) have been tabulated
at the end of this section.
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2.1. Individual Column Geometry
The column has been inclined at a ratio of 3:1 (71.56o) to reduce the back break, and the
amount of boulders from the upper part of the blast. The blast hole depth used in design
has then been extrapolated to accommodate this angle (via trigonometry). Burden has been
taken as 3.2m for the two 11m benches, however it has been increased to 3.5m for the 9m
benches. While this increase does seem counter intuitive, as there is then less space
available for the column charge, the burden is proportional to the column spacing – and
thus a larger burden corresponds to a greater amount of limestone blasted. To ensure this
assumption is realistic, values for minimum and maximum burden have been derived using
the equations below, which give values of 2.63m and 5.3m respectively. Stemming has been
taken as equal to the burden. This should consist of sand or gravel, with a particle size of 4
to 9mm for optimal confinement of the explosive gases.
𝜌𝑒𝑥𝑝
√𝑅𝑒𝑙. 𝐵𝑢𝑙𝑘 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ
𝑑𝑖𝑎
𝑀𝑎𝑥 𝐵𝑢𝑟𝑑𝑒𝑛 =
33 (𝑐 + 0.05) × 0.95 × 𝑆𝑝𝑎𝑐𝑖𝑛𝑔
𝐵𝑢𝑟𝑑𝑒𝑛
𝜌𝑒𝑥𝑝
𝑀𝑖𝑛 𝐵𝑢𝑟𝑑𝑒𝑛 = ℎ𝑜𝑙𝑒 𝑑𝑖𝑎 × (2 ×
+ 1.5)
𝜌𝑟𝑜𝑐𝑘
Where dia is the blast hole diameter in mm, and ρ refers to the density of both the explosive
(exp - 1.2kg/m3) and the limestone (rock – 2.5kg/m3). C is the rock constant, which has been
assumed as 0.4kg/m2.
2.2. Column Layout and Timings
Using the tonnage per column (341tonnes) and the required weekly amount (15,000tonnes)
– the total number of columns required has been calculated and rounded up to 45. To
enable a good grid arrangement, this has been increased to 48 which can then be split into
two 24 column blasts each setup in a 3x8 grid. Each column holds two NONEL detonators
within a cast pentolite primer to ignite the charge. Each primer is set to a standard 475ms
delay utilising surface connectors of interval timings 17, 25, 42 and 67ms delays. A minimal
gap of 8ms is maintained between the detonation of each charge, with connecting rows
being a minimum of 50ms apart to prevent the creation of volatile fly rock. Full layouts for
both standard (two free faces) and front (one free face) blasts can be seen in Appendix A,
along with separate setups for specialised situations – which are detailed below.
2.3. Specific Columns
2.3.1. Decked blast
It has been specified that a residential property lies 350m from the quarry, and could be
affected by blast vibrations. Further detail on this is covered in a later section, however a
separate column design has been established to reduce blasting via the inclusion of
‘decking’ within the centre of the column – which has been set to 12 times the 89mm
diameter. Requiring an increased burden of 3.7m, the decked blast uses an additional
detonator above the decking to fire the charges 25ms apart to reduce the total vibration.
This increased burden also allows for less columns to be used – utilising a setup of 2x10 with
separate timings for the additional detonators.
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2.3.2. Weak Layer bench
A separate design has been established to reduce the negative impact the weak layer holds.
This includes the stemming of the weak layer, which has been additionally extended to 0.5m
above and below the layer. While the layer is outlined as 3.5m thick, this has been
extrapolated to 3.69m to account for the incline of the charge passing through more than
3.5m of the layer. The burden has been decreased to allow more space for charge, with the
hole diameter subsequently being increased to compliment this. The column grid setup
remains the same as standard.
2.4. Explosive
As the ground is expected to be below the water table – a
typical ANFO explosive would not be appropriate. Instead,
the selected explosive, ‘Centra Gold’ - a pumped bulk
emulsion from Orica – has been selected for all blast
designs - in part due to its specific design for use in wet
blast holes, it also holds good strength and VOD values;
further details can be seen in Table X.
2.5. Charge Setup
TABLE 2 – E XPLOSIVE
PROPERTIES
The column charge is split into two sections; bottom and column charges. This is to account
for the lower amount of constriction to the upper column, and thus the charge
concentration has been reduced by half to minimise fly rock chances but still affect the
same volume of limestone. The height of the bottom charge is taken as 0.3 times the
burden. Sub-drilling has also been included, equal to 0.3 times the maximum burden, to
ensure the avoidance of the creation of stumps above the theoretical grade.
The amount of explosives in each section can then be derived, via the loading rate – which
has been calculated using the equation below – multiplied by the length of the bottom
charge section, plus 0.6 times the length of the column charge.
𝐿𝑜𝑎𝑑𝑖𝑛𝑔 𝑅𝑎𝑡𝑒 = 7.85 × (
𝑑𝑖𝑎 2
) × 𝜌𝑒𝑥𝑝
100
For the 9m benches, with a diameter of 89mm, the loading rate comes to 7.47kg/m – with
the total weight summing to 57.3kg. This is accounting for the column charge being 100% of
the initial loading rate, not the reduced concentration to be physically input. As the
concentration is reduced under the assumption that the lower concentration will affect the
same volume of rock per kg, it should then subsequently be taken as the full concentration
in design - i.e. despite having 60% of the concentration, it is expected to affect as much rock
as the calculated concentration within the bottom charge and thus included as the 100%
value.
To check the adequacy of the planned explosive amount, the volume of rock required has
been calculated using the blast ratio, derived from dividing the tonnage of limestone per
column (spacing x burden x blast hole depth x ρexp ) by the blast hole ratio – taken as 6t/kg
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for the medium limestone rock, multiplied before the explosive’s weight strength relative to
ANFO; 112%. This produced a requirement of 51kg – indicating an acceptable overcharge of
6.5kg.
2.6. Vibration
A residential property has been
outlined as being in the proximity
of the quarry with regards to
which, any blast vibration must
remain below 6mm/s as required
by UK law. The expected blast from
the most shallow 9m benches –
which contain the greatest amount
TABLE 3 – BLAST VIBRATION DATA
of explosive and will be of the
nearest proximity – has been analysed to assess the resultant PPV using the equation below.
To obtain the coefficients of A and B, data was collected via an assessment by MIST (2003)
on the effects of blast vibration from several UK sites. Three limestone sites were identified,
with their coefficients averaged as shown in Table X.
𝑃𝑃𝑉 = 𝐴 × (
𝑃𝑟𝑜𝑝𝑒𝑟𝑡𝑦 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 (𝑚)
√𝐸𝑥𝑝𝑙𝑜𝑠𝑖𝑣𝑒 𝑊𝑒𝑖𝑔ℎ𝑡
)𝐵 = 498.3 × (
350𝑚
√57.34𝑘𝑔
)−1.34 = 2.89𝑚𝑚/𝑠
The resultant value of 2.89mm/s is then substantially less than the 6mm/s limit, and is
deemed sufficient. However, a further analysis of the site-specific coefficients should be
conducted before this can be confirmed – utilising the decked blast design if the PPV
increases significantly. Re-analysis should also be considered if any factor related to the
quarry ground conditions change, or the location of the quarry boundary itself.
Aperture. Vibration affected by joint sets; perpendicular & parallel minimum effect, but
otherwise would act to lessen vibration
2.7. Rock Fragmentation
In order to efficiently process the blasted limestone, the fragments producted must fit
within a reasonable size range. A PF Impact Crusher from GALIN has been selected to
simulate a reasonable range, working particularly well with limestone due to its lower
strength (crusher takes up to 360MPa, as opposed to the rock’s 155MPa). With a maximum
feed size of 350mm, the optimal size was taken as 200mm, with fines less than 10mm
assumed as being too small to be processed efficiently. The elastic modulus has been taken
as 10GPa, as limestone tends to range between 3-17GPa. As with vibration, a specific value
should be obtained for a more accurate analysis before the confirmation of any designs.
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The fragmentation was predicted using the Kuz-Ram fragmentation analysis model, with the
resultant percent passing graph shown in Figure X. The model predicts that 5% of the
fragmentation will be above the maximum size of 0.35m, and 0.5% being below the
minimum 0.01m. It should be noted that the oversize passing increases to 8% with the two
11m benches – however this is still deemed acceptable as the chosen crusher’s maximum
size is specified for up to 360MPa rock. Considering the joint sets , sets 1 and 2 do not hold a
particularly significant dip with regards to joint blocking, and though set 3 does have a dip of
49o and low spacing, it’s low persistence suggests that its influence could be significantly
lower as it is not as consistently present as the others. Set 2 has then been input into the
model as an approximate ‘average’ of the sets. Alternatively, the larger PFW crusher of the
120%
100%
Percent Passing
80%
60%
40%
20%
0%
0
0.2
0.4
0.6
0.8
1
1.2
Size (m)
F IGURE 3 – KUZ-RAM F RAGMENTATION A NALYSIS
same range can take up to 500mm fragments.
3. Production
3.1. Equipment
To excavate the blasted rock, an ‘R 970 SME’ crawler excavator from LIEBHERR has been
selected. Using a 4.5m3 backhoe bucket meets the specification desired in the brief, and
with a 7m mono boom the excavator has a maximum reach of 11.65m with the backhoe
bucket – enabling it to sufficiently reach the full height of all benches. Assuming a swell
factor of 80%, bucket fill factor of 90%, a cycle time of 40 seconds and excavating 7
hours/day (8 hour shift minutes general time lost), 5 days a week (254 days/year including
public holidays) – with mechanical availability and utilisation taken as 85% - a single
excavator could move up to 16,500tonne of limestone per week. Calculations are shown
below.
𝑀𝑎𝑥 𝑡𝑜𝑛 𝑝𝑒𝑟 𝑠ℎ𝑖𝑓𝑡 =
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𝑇𝑟𝑢𝑐𝑘 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 (24𝑡)
× 𝑇𝑖𝑚𝑒 𝑝𝑒𝑟 𝑠ℎ𝑖𝑓𝑡 (60 × 7 ℎ𝑜𝑢𝑟𝑠) = 4,536 𝑡𝑜𝑛𝑛𝑒𝑠
𝐿𝑜𝑎𝑑𝑖𝑛𝑔 𝑡𝑖𝑚𝑒 (2 𝑚𝑖𝑛𝑠)
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The 4,536 tonnes is then further adapted by multiplying through both the mechanical
availability and utilisation of 85% - giving 3277 maximum tonnes per shift and a weekly
maximum of just under 16,500 tonnes.
To get closer to the 15,000tonne target would require a smaller excavator bucket size,
however this setup has been kept to satisfy the original size request while remaining close
enough to the target to setup a small stockpile in case of a prolonged operation shut down.
The equation used to calculate the excavation time is shown below.
𝑆𝑤𝑜𝑙𝑙𝑒𝑛 𝑏𝑙𝑎𝑠𝑡𝑒𝑑 𝑟𝑜𝑐𝑘 (4013𝑚3 )
𝑥 𝐶𝑦𝑐𝑙𝑒 𝑡𝑖𝑚𝑒 (30𝑠)
𝐵𝑢𝑐𝑘𝑒𝑡 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 (4.5𝑚3 )
𝐸𝑥𝑐𝑎𝑣𝑎𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 =
= 8.23 ℎ𝑜𝑢𝑟𝑠
𝐴𝑣𝑎𝑖𝑙𝑎𝑏𝑖𝑙𝑖𝑡𝑦 (0.9)
While the brief requests the use of four 80 tonne dump trucks, analysis has proven this to
be a substantially inefficient design. Assuming a swell factor of 80%, it would take 14 passes
of a 4.5m3 bucket to fill a truck – taking over 8 minutes assuming a cycle time of
approximately 35 seconds. Assuming an average travel speed of 20km/h an hour (obeying
standards (MiningInfo, 2015) and assuming a 10km/h loaded average speed), in addition to
a 90 second dump manoeuvre, the trucks would take approximately 5 minutes to travel the
1.1km round trip, including dumping the material. This would then cause a roughly 3 minute
downtime, equivalent to 37.5%, assuming another truck began loading directly upon
departure. Considering that only 1 excavator is needed to produce 22,680 tonnes per week
(shown below), this would be extremely inefficient both logistically and financially.
Instead, an alternative setup utilising smaller articulated dump trucks is recommended –
with a Bell B25E suggested, which holds a payload of 24 tonnes. Assuming the same values
as before, but with a shortened dump manoeuvre of 1 minute, and a slightly higher
mechanical availability of 90%, the amount of trucks required comes to 2.7, implying 3,
trucks, due to a overall travel + unload time of 6 minutes with a time to fill from the
excavator of 4 passes (2 minutes) aligns the trucks to be constantly in use. While these are
timings are on a tight schedule, the setup is deemed acceptable due to the conservative
estimations involved – coupled with the expected weekly overproduction of limestone.
4. Conclusions
It should be outlined that this report outlines a recommended setup of the proposed open
pit quarry. This has been largely designed based on certain assumptions, which should be
further investigated before the project is set in motion. The design is also not all
encompassing or the only option available – an attempt to allude to possible variations has
been included where possible.
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