Page 466 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.66 THE WORK

FIGURE 9.55 Exploration and grouting holes.

The pump or pumps at the base of the shaft are sometimes placed in a sealed room, with power
and control directly from the shaft top. In other cases the pumps are in the open, but are of the
submersible type. These arrangements permit use of the units along with emergency pumps if the
tunnel should be flooded.

Grouting. Water inflow can often be checked by grouting. This may be done by drilling deep
into the rock in the direction of the supposed source of the water, sealing in pipes with cement,
and then pumping cement and water grout through them, either straight for seepage or mixed with
sawdust or shavings for gushing flow. This may be done in advance of the tunnel driving in very
wet areas, by fanning the grout holes out from the face and edges of the heading, as in Fig. 9.55.
Grouting is also done through completed linings, either to check water or to fill in spaces
between it and the wall. Grout pipes may be cemented into a concrete lining when it is poured.
Successful grouting of a wet seam sometimes merely diverts the water so that it enters the tunnel at
another point that was previously dry. This also may be grouted, but a point may be reached where the
contractor either installs a complete concrete lining or gives up the effort to seal off and relies on pumps.
The aboveground uses of grouting were discussed in Chap. 6.

ROCK SUPPORT

Ground pressure in rock tunnels is difficult or impossible to estimate. In firm formations there will
be little or no pressure until depths of greater than 500 feet are reached.
However, there are soft, joined, or laminated formations that will scale off or fall from a flat or
moderately curved roof, until a Gothic or pointed arch develops, after which it will be self-supporting.
If bracing is done only to support the roof, it is a question whether it will be more economical to cut
up to a stable roof line, and avoid placing of supports. See Fig. 9.56.
In any roof problem, width is a very important factor, as wide spans will drop pieces or fall in
much more readily than narrow ones.
Many rock tunnels are perfectly safe without any bracing. Others get by without accidents. But
very often is is necessary to place supports directly after the digging, or within a few days. Also,