Mining_Methods_UnderGround_Mining - Mining and Blasting

Plan of pre-undercut holes.
el TenienTe, CHile
the undercut level difficult. The effective
extraction rate defined for the Es-
meralda sector was 0.14 to 0.44 t/day/
sq m at the initial caving stage, and
reached 0.28 to 0.65 t/day/sq m at the
steady-state caving stage. The height
of primary ore column to be exploited
is around 150 m, relatively low if compared
with Teniente 4 Sur, where the
height is over 240 m.
At Esmeralda, 7 cu yd LHDs working
on the production level load and tip
into 3.5 m-diameter ore passes. Here,
teleremote controlled hydraulic breakers
positioned above 1 m x 1 m grizzlies
break any oversize rock before it goes
Drilling patterns in andesite and breccia.
Standard drill plan with 4 hole-fans
through the ore pass and into the loading
bin. On the haulage level, the
mineral is loaded into trains featuring
Automatic Train Protection (ATP) and
consisting of a locomotive with eight 50
t cars. These trains, which were retrofitted
with an Automatic Train Operation
(ATO) system, tip into storage bins
which feed a 5.0 m-diameter orepass
to the main transport level Teniente 8.
Trains with 90 t electric locomotives
and 18 cars each of 80 t capacity carry
the mineral out to the Colón concentrator.
The main haulage level at Teniente
8 was recently upgraded, incorporating
new technology similar to Esmeralda.
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Drill plan with 3 x 3 hole-fans Drill plan with 4 x 4 hole-fans
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Basic concepts
In the conventional panel caving and the
pre-undercut variant, the same basic con-
cepts apply. The main difference is the
sequence of each of the operational ele-
ments. In the conventional panel caving
method, the sequence of activities is:
development of tunnels on each level
for production and undercut; drawbell
opening; undercut blasting; and extraction.
In the pre-undercut variant, the un-
dercut is excavated first, and the production
level is developed subsequently
within the stress-relieved zone: development
of the undercut level; undercut
blasting; development of the production
level; drawbell opening; and extraction.
The main challenge associated with
this variant involved the undercutting.
Several alternatives were tried, with
the current preference being a flat, low
height 3.6 m undercut. The undercut is
blasted some 80 m ahead of the actual
production zone, with the production
level and drawbell development following
around 22.5 m behind the undercut,
and 57.5 m ahead of the production
zone.
The undercut comprises drives, 3.6
m-wide by 3.6 m-high, developed parallel
to each other on 15 m centres. The
excavation of the undercut is achieved by
blasting three- or four-hole fans, some
7 m to 10 m length, drilled into the sidewall.
The drill holes are fanned slightly,
to ensure an undercut height equal to
the height of the drives. Swell material
from each undercut blast is removed by
LHD to provide a free face for the next
blast. The production haulage level is
developed 18 m below the undercut,
giving a crown pillar thickness of 14.4
m through which the drawbells are then
developed straight into the pre-blasted
undercut. The production level requires
substantial support, with fully grouted
2.3 m rebar installed in a 0.9 m x 1.0
m pattern immediately behind the face,
followed by chain mesh and shotcrete.
Permanent support is added around 15 m
behind the face, and consists of fully
grouted long cable bolts, with additional
reinforcement at drawpoints. One of the
challenges of this method is that two
mining fronts have to be managed, one
on the undercut level, and the other located
on the production level, and these
86 underground mining methods