Page 450 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.50 THE WORK

There are several problems that occur for the microtunneling machine. Because of its sealed head
arrangement it can not deal with obstacles in the way of the advancing pipe the way a horizontal
boring machine can. But the boring machine needs to be operated with manpower inside and other
complications. Also there is a limit to the toughness of the ground that the pipe is driven through using
microtunneling. Generally, if the ground is no harder than 5,000 psi resistance, then the microtun-
neling machine can operate.
A microtunneling machine performs better as the moisture content of the soil increases and the
soil type changes from a stiff clay to a more granular material. In good conditions this method may
be able to advance from 20 to 80 feet per day.

LARGE TUNNELS

Twenty- to 30-foot-diameter tunnels are big, yet they provide a floor width that would be consid-
ered skimpy for a haul road on top.
Equipment to be managed at and near a tunnel heading may include a drill jumbo (a movable
frame almost as big as the tunnel, carrying a battery of drills), a machine for loading muck (the below-
ground name for spoil) and railcars or rubber-tire trucks to remove it; the same or other cars or
trucks to bring drill steel, bits, explosives, and other supplies to the heading; a locomotive to push and
pull cars; and a switching or passing device to permit hauling units to get past each other, although
often there is room only for a single width of track or roadway.
There will be high-pressure air pipes to supply the drills, and often large low-pressure ducts for
ventilation. Overhead wires or ground cables carry electricity for light, power, and blasting juice.
Water under pressure may be supplied for wet drilling. A system of drainage, pumping, or both
may have to handle tremendous volumes of water.
In addition to the regular equipment there may be need for a diamond drill to make test and
grouting holes, grouting equipment to seal off leaks and solidify wet ground, and/or a movable
buffer to confine rock throw from blasts.
If the tunnel is to be timbered for support, or lined for support or for permanent use, the crews
and materials for this work may follow the digging closely, and in any event will have to work in
and over the single entranceway.
If driving is from a shaft, its bottom is another crowded point. Haulage equipment may be lifted
to the top to dump, or it may dump into containers at the bottom. Supplies must be unloaded from
the elevator cages and reloaded for hauling to the face. Crews leaving work, and their supervisors
and inspectors, wait here for transportation to the surface. Pumps, compressors, and even drill and
repair shops may be located in skimpy quarters excavated near the shaft.
Sequences are very exacting. The tunnel cycle (the succession of drilling, shooting, and mucking)
must keep the largest possible number of workers and machines usefully employed, and the time inter-
val from any operation to its repetition should not vary. Whenever possible, two or more operations
should be performed simultaneously, as drilling the top of a face while digging the bottom, and
installing lining a few feet back at the same time.
When two headings driven from one shaft are close together, one may be drilled while the
other is shot and mucked. With increasing distance, the advantages of this arrangement are reduced.
Most tunnel crews are of the universal type, and perform all operations in the cycle. This saves the
contractor from paying a crew waiting time because of a delay in a prior operation.

Speed. Under favorable conditions, tunneling may progress very rapidly. The Owens River Gorge
Power Tunnels in the Los Angeles water system were driven as fast as 104 feet in a day, and 2442
feet in the best 31-day period. Other tunnels excavated by using tunnel-boring machines, called
TBMs, working in fairly ideal ground conditions have made even better progress.
Gold mines at Kimberley, South Africa, hold the depth record at 9000 feet. These tunnels must
be air-conditioned, as otherwise the heat would make it impossible to work in them.
The 12-mile Simplon tunnel in the Alps is 7,000 feet beneath the surface at one point.
Temperatures up to 131°F were encountered in drilling it.
A record for its time was established in twin power tunnels at Niagara Falls in Canada. These are
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each 51 feet in diameter and 5 ⁄ 2 miles long. Together they required over 5,000,000 yards of excavation.