Page 480 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.80 THE WORK

Clay reacts most satisfactorily to compressed air, as it is so nearly impervious that it is well
supported by the air and seals it in. Primary bracing may not be required before placing the per-
manent lining.
On first exposure to compressed air, silt acts as clay, but it then tends to dry out and crumble
off at the top, and to turn to mud and flow at the bottom. The higher the tunnel, the greater the dif-
ferences between top and bottom behavior.
This is because the air pressure is the same on all parts of the tunnel rim, but the head of water
that tends to force water into the tunnel, or resists its being forced out of the lining soil, is much
less at the top than at the bottom. A partial cure for the difficulty is to excavate the upper or arch
section first under low pressure, install liner plates or other support; then increase pressure and
dig the bottom. Once a full lining is installed, the unbalanced condition becomes unimportant.
In sand the air penetrates several feet at the top, and leaves the bottom wet enough that boards
have to be stuffed with excelsior to stop sand runs. The best cure for this condition is to drive well
points ahead of and below the face, and keep the lower sand dry until lining is placed.
Air will escape in any formation except tight clay, and will reach the surface by following
porous veins, old wells, or even sewers. It is best conserved by getting the lining in immediately
after the digging. Airtightness of the lining is not automatic, however. Grouting outside it (which
is necessary for firmness also) and painting the inside of concrete with cement and water greatly
reduce leaks.
Liner plates may be made airtight by spreading wet clay along the joints. Building paper can
be used on wood lagging.
About 20 cubic feet of atmospheric air per minute is required for each square foot of face area,
with an additional allowance for losses through the locks.
Blowouts. Sometimes the compressed air in the tunnel blows out the surface. This is particu-
larly likely to occur in shallow tunneling in soft underwater mud. Any outward leak must be
immediately plugged with any material on hand, valuable or otherwise. From the outside a
blowout can be prevented or stopped by dumping enormous quantities of clay from barges.
The blowout can be disastrous in itself, hurling workers and equipment up into the water. The
immediate drop in pressure allows water and mud to enter the tunnel, threatening those in it with
drowning or suffocation. A job “lost” in this manner is expensive and tedious to resume, and
sometimes driving can more easily be done on a different route.
In addition to this below-ground work, a variable amount of surface construction is required.
The following is a list of major items of this nature:

Access roads
Construction
Maintenance
Power supply
Installation of lines
Construction of generating plant if necessary
Surface buildings
Change and washroom facilities
Blacksmith shop
Machine shop
Compressor building
Powder magazine
Cap magazine
Miscellaneous buildings
Construction camp (if needed)