Excavation technology
Drilling and blasting
All underground excavation works are done using drill and blast technique. For drilling handheld pneumatic drill is used. Here, main drilling and blasting parameters are presented.
Explosive | |
Density: | 1.15 g/cm^{3} |
Velocity of detonation: | 4200m/s |
Patronne diameter: | 28mm |
Type: | PETN |
Rock properties | |
Tensile strength: | 5MPa |
Poisson’s ratio: | 0.20 |
Drilling parameters | |
Hole dimeter: | 32mm |
Hole length: | 1.6m |
Drilling speed: | 0.2m/min |
Burden is determined using expression:
$$ B=\frac{0,17\ast P_h\ast r_h}{k\ast\sigma_t} $$
Where:
B – burden (m),
Ph – borehole pressure (GPa),
rh – borehole radius (m),
σt – Tenisle strength of rock (MPa),
k – coefficient based on Poisson’s ratio,
$$ k=\frac{(1-\mu)}{(1+\mu)(1-2\mu)}=1.11 $$
For the explosive with density higher than 1g/cm^{3} blasthole pressure is calculated as follows:
$$ P_d=\frac{\rho_e\ast D^2}{8} $$
Where:
$\rho_e-$explosive density (g/cm^{3}),
$ D- $velocity of detonation (km/s).
Therefore:
$$ P_d=\frac{1.15\ast{4.2}^2}{8}=2.5GPa $$
In case when explosive patron is smaller size than the borehole, blasthole pressure is reduce by following expression:
$$ P_h=P_d\ast\left(\frac{d_e}{d_b}\right)^3 $$
Where:
d_{e} – diameter of explosive patron,
d_{b} – diameter of borehole.
Therefore:
$$ P_h=2.5\ast0.67=1.68GPa $$
And finally, burden is determined:
$$ B=\frac{0.17\ast1680\ast0.016}{1.11\ast5}=0.85m $$
Blasting pattern for the stopes and drifts at other levels consists of 15 borehole of 1.6m length. Drilling is done using pneumatic handheld drill. Main drilling parameters in this case are:
Number of boreholes | 15 |
Length of borehole | 1.6m |
Drilling speed | 0.2m/min |
Explosive patrons per hole/face | 4 / 60 |
Amount of explosive per hole | 850g |
Amount of explosives per blast | 12.75kg |
Specific amount of explosive | 1.06kg/m3 / 0.35kg/t |
Initiation | 500ms electric detonators |
Figure 22 Blasting and initiation pattern for stopes and drifts at levels L+1602.5m, L+1605m, L+1607.5
Drilling time is:
$${{T}_{drill}}=\frac{15\cdot 1.6m}{0.2m/\min }\approx 120\min $$
Charging time is:
$${{T}_{ch}}=15\cdot 2\min =30\min $$
Total drilling and charging time is 150 minutes.
Table 1 Gantt chart for drilling and blasting for stopes and drifts at levels L+1602.5m, L+1605m, L+1607.5