The Journal of Explosives Engineering - July/August 2022 - 8

Table 1. Mechanical properties of the rock at a mine with a similar geology.
limestones in Southwestern Pennsylvania. Named for exposures
along the Loyalhanna Creek in Westmoreland County,
the Loyalhanna Limestone is considered the basal member of
the Mauch Chunk Formation throughout much of Southwestern
Pennsylvania. It is described as light gray to grayish-green
sandy limestone to calcareous sandstone that contains conspicuous
large to medium scale cross-beds. The Loyalhanna
Limestone reaches a thickness of approximately 85 ft (26 m)
in Southwestern Westmoreland and Fayette Counties (Adams,
1970). The geologic unit below the Loyalhanna Limestone
is the Burgoon Sandstone. Described as a buff, medium to
coarse-grained, cross-bedded quarzitic sandstone, the Lower
Mississippian Burgoon Sandstone is equivalent to the Pocono
Formation found in Northeastern Pennsylvania. It contains
minor siltstone, shale, and some very thin coaly horizons. Its
maximum thickness is about 300 ft (91 m)(McElroy, 2001). The
Loyalhanna occurs closest to the surface along the anticlinal
ridges. The current mine site lies along the Chestnut Ridge
Anticline, as do other Loyalhanna mine sites. Inherent discontinuities
occur as joints, fractures and localized faulting.
The footprint of the mine holds over 200 acres (0.8 km2
),
and until recently, was accessed by only two portals situated
120 ft (36.5 m) apart. This layout of limited portals left the operation
vulnerable to safety concerns and interruption in operations.
Mine evacuation was limited to the required minimum
of two separate escape routes, and ventilation was resigned to
one portal for fresh air intake and one portal for exhaust. The
addition of portals addresses these concerns for the current
layout and future development. Local geology and surface topography
limit the possible locations for portal development.
The proposed portals at the mine are situated where the dip
of the stone is manageable, overburden was minimal, and the
location serves as a gateway towards future reserves. The high
wall for portal entry was newly constructed in 2019 in an area
where overburden is less than 100 ft (30 m).
Establishing a Vibration Limit for
Blasting
Although not studied as extensively as vibration damage to
residential structures, the U.S. Bureau of Mines and other researchers
have studied the effects of the vibration generation
from blasting in underground works and the possible damage
to underground structures from nearby large surface blasts.
These studies have mainly been in response to safety concerns
and the potential for blast-induced damages (e.g., tunnel collapse,
subsidence, damage to rock mass) in mines and tunnels.
Damage to rock mass would include spalling, crack extension,
Equation 2.
cific gravity, and VP
where σc is the rock compressive strength (Pa), SG is speis
in m/s. Substituting the appropriate parameters
into equation 2:
and block sliding. The research conducted on the vibration
risks in underground structures points to the general observation
that major failures such as roof collapse and pillar failure
would require vibrations with a peak particle velocity (PPV)
greater than 12 in/sec (305 mm/s) (Engineering Research Associates,
1953; Langefors and Kihlström, 1973; Sakurai and
Kitamura, 1977). In some cases, loose pieces were dislodged
at lower levels of 1.2 to 5 in/sec (Jensen et al., 1979; Oriard,
1982). The occasional falling of loose stones on slopes in open
pit operations occurs in the 2 to 4 in/sec range (50-102 mm/s).
The consensus in the literature is that adverse effects have not
been observed at vibration levels below 1 in/sec (25.4 mm/s).
The generalized scale of effects given here should be considered
to be very crude due to the wide range of conditions that
can occur such as the variations in the physical characteristics
of the rock/soil and varying frequency characteristics of the
vibration source.
A vibration limit was proposed prior to initiating the blasting
program. Rock strength parameters were acquired from a
nearby mine in the same geologic formation. Table 1 lists the
mechanical properties of the red shale and sandstone as well
as the Loyalhanna limestone (i.e., the mining horizon). The
tensile strength (σt
mary (VP
), compressive strength (σc
) and shear (VS
), density (ρ), pri)
wave velocities were experimentally
measured.
Two considerations were taken into account in establishing
a vibration limit. First, to avoid the development of new cracks
in the roof, and second, to avoid the propagation of the existing
crack. With respect to the former, an allowable PPV was
established using mechanics of wave propagation:
Equation 1.
And with regards to latter, Calder (1977) proposed the following
equation to estimate the vibrations required to propagate
an existing crack:
8
The Journal of Explosives Engineering
July/August 2022
The Journal of Explosives Engineering - July/August 2022

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