Page 453 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.53

The Delaware Aqueduct used 10,000 feet of air per minute at each heading. A standard estimate
is 1,500 feet per drill.
Shallow rocks are cool, but in deeper work an increase of 1°F for every 50 feet of depth can
be expected.
Noise is deafening, particularly during drilling, as the sound echoes back and forth in the confined
space. Diesel engines add to the uproar.
MSHA. The Department of Labor in the United States includes the Mine Safety and Health
Administration (MSHA), which oversees and regulates the conditions that could be hazardous for
workers in gassy metal and nonmetal mines. Although they were not originally intended to apply
to tunnels for highways, railroads, water pipelines, and utility lines, the MSHA rules should be
and are frequently applied in these tunnels.
MSHA defines a blowout as a sudden, violent, and unplanned release of gas or liquid due to
reservoir pressure in petroleum mines and an outburst as a sudden, violent release of occluded gases
and solids under high pressures from a geologic formation. Either a blowout or an outburst could
happen in the types of tunnels discussed in this chapter. For the sake of reference, the mine atmos-
phere is tested at any point at least 12 inches away from the face, rib, back, and floor of the tunnel.
Requirements. The MSHA standards contain operational requirements that must be satisfied
to meet the regulations for worker safety in the excavation. The standards stress tunnel ventilation
to control airborne contaminants, including natural methane gas and equipment exhaust. There are
minimum airflow quantities for ventilation. If air passing a particular input point has a mine
atmosphere with more than 0.25 percent methane, it cannot be used for ventilation. Testing for
methane must be done at least once every work shift. The volume of air ventilating each face at a
working place shall be at least 20 feet per minute multiplied by the open cross-sectional area in
square feet of the entry. According to that minimum, if it is a 10-foot-diameter tunnel, there needs
to be at least 1,570 cubic feet per minute (20 3.14 25) of ventilation air flowing to the face.
However, the required minimum at the face is 2,000 cubic feet per minute.
The tunneling equipment is to be “permissible,” that is, it does not emit gaseous exhaust or
electric sparks that could ignite a gaseous mixture. Therefore the best equipment to use is electri-
cally or compressed-air powered. Within 100 feet of the tunnel face or bench, the equipment used
must be permissible, except that nonpermissible front-end loaders and haulage trucks equipped with
methane monitors may be used at the face or bench after blasting. It is recommended that workers be
removed from a blasting area for at least 30 minutes after a blast. MSHA requires that methane-
monitoring devices be installed on continuous mining machines, and that should apply to tunnel-
boring machines also.

SHAFTS

Shafts—vertical passages between the tunnel and the ground surface over it—are required for the
majority of tunnel jobs. They are sometimes the only access. Even when there are portals, shafts
shorten the time required to do the work, as each makes it possible to work on two extra headings.
In addition, underground hauling is a headache, and runs should be kept as short as practical.
Some shafts are part of the permanent tunnel project.
The advantages of shafts must be balanced against the considerable expense of sinking and
equipping them.
Shaft location may be chosen to keep depth to a minimum, as in the troughs of valleys over the
tunnel; to take advantage of an easily worked or stable formation; or on the basis of surface condi-
tions such as good access, nearby dumping areas, cheap land, or distance of shielding from popu-
lated areas.
Size. Shaft size is highly variable, depending largely on the volume of material it must handle
and the size of objects that must be lifted and lowered. A minimum size, about 11 13 feet inside
the lining, accommodates a single hoist and supply elevator and a ladderway. The ladderway