Page 458 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.58 THE WORK

FIGURE 9.47 Timbering for small shafts. (By permission from “Practical
Tunnel Driving” by Richardson & Mayo, McGraw-Hill.)

At first only a single set of tunnel-driving equipment may be used, as there will not be space enough
for two, and greatest efficiency will be obtained by drilling at one face while mucking at the other.
Drilling patterns may be similar to those described for shafts—wedge or burn holes, and succes-
sive rings breaking into the crushed-out area. The whole face is usually drilled and blasted in one
operation (full-face attack), but a small tunnel (drift) may be drilled full face, blasted, and cleaned
out, then enlarged by radial drilling; or the top may be kept ahead of the bottom (bench-and-heading
method). See Figs. 9.48 and 9.49.

Pilot Tunnel. Shafts may be partly or wholly replaced by a small pilot tunnel, driven parallel
and close to the main tunnel. Crosscuts are driven from this to the main tunnel, wherever new
headings are to be started. The main tunnel is opened up with a center drift, and enlargement started
after it has been cut through enough that both tunnels can be used for traffic.
The extra tunnel may be used for ventilation, both during the work and afterward. It permits a
great many operations to be performed at the same time, and may save considerable expense in sink-
ing shafts. This method has been used chiefly for long railroad tunnels through mountains where
depth was too great for shafts.