Page 474 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.74 THE WORK

Traveling forms of various types are used. For fast schedules, it is essential to have telescop-
ing forms that can be folded up and moved through other forms supporting more recently poured
sections. On other jobs, forms are used that can be collapsed just enough to break away from the
concrete surface so as to be moved ahead to the next section. In either case the forms are carried
on carriages that may move on steel wheels and tracks or on rubber tires, depending largely on the
muck haulage method used.

Breakthroughs. Sometimes, in spite of precautions, there will be a sudden rush of water or mud
into the tunnel. This most often occurs at the face immediately after a blast. Sometimes the source
is a limited underground pocket which will give no trouble after it has once drained off. At other
times a stream or large body of water will keep up a continuous flow. If the water is muddy, or
the flow is partly or wholly mud, an unstable soil formation has been reached which may give
increasing rather than diminishing trouble.
In any case the first step is to seal off the face with a bulkhead (wall) as quickly as possible. Timber,
sandbags, or sandbags with timber may be used. Occasionally timber may be backed with concrete.
The bulkhead must not be used as a dam while being built. Pipes should be built into it large
enough to take the water flow until the structure is complete. Otherwise water pressure will tend
to destroy the bulkhead as it is being erected, and conditions will be very dangerous to personnel.
With water discharged through pipes, the structure can be properly and strongly made and keyed
into the tunnel rim. The water can then be controlled by valves on the pipes.
The bulkhead should also be fitted with pipes for grouting and concrete placement. After the
water has been shut off, grout can be injected into the space between the bulkhead and the break,
and will sometimes work back along the water seam and stop or reduce the flow. Grouting may
also be done through exploration holes drilled through the bulkhead and into the rock beyond.
Over 90,000 bags of cement have been used to control one water pocket.
Further tunneling through such a spot is first in the form of drifts (small tunnels) each of which
serves as a base for further grouting, until the ground is consolidated enough to drive the big tunnel.
Tunneling machines, as described in Chap. 20, have an important effect on methods and costs of
solid rock tunneling.

SOFT GROUND TUNNELING

Soft ground is divided roughly into the following subclasses, description of which is abbreviated
from Practical Tunnel Driving by Richardson and Mayo, McGraw-Hill.
Running ground: Must be instantly supported. May be dry sand or gravel, quicksand, silts, and muds.
Soft ground: Roof must be instantly supported, but walls will stand vertically for a few minutes.
Firm ground: Roof will stay up unsupported for a few minutes, and the sidewalls and face for
an hour.
Self-supporting ground: Will stand unsupported while the entire tunnel is driven a few feet
ahead of the timbering.

The standard methods of driving through soft ground are forepoling with wood or steel, or
working in a shield. The plenum method is keeping out soil and water with air pressure, with
either forepoling or shield.
Forepoling. The use of plank forepoles was formerly the standard method of driving a tunnel
through soft ground. While this technique has been largely replaced by steel liner and poling
plates, it is still widely used on jobs too small to justify obtaining steel.
In forepoling, the tunnel is protected by timbering, and by breast boards set against the face.
Planks are driven through slots cut in the breast board and supported cantilever fashion to make
a temporary roof, under which dirt can be dug and permanent supports installed.