Page 467 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.67

FIGURE 9.56 Cutting crown for self-support.

the majority of tunnels outside of mines are more or less permanent in nature, and except in very
firm rock, will require lining to prevent deterioration and to reduce or eliminate maintenance.
Support or lining may be wood timber, steel ribs, plates or bolts, or concrete. Concrete is frequently
placed inside one of the other types of support.
Timber is the oldest material used, and is found in ancient tunnels. Concrete was used to some
extent by the Romans, and has become the standard for permanent installations. Steel liners and
roof bolts are quite modern developments, and are rapidly replacing timbering.

Timbering. Figures 9.57 to 9.59 show some designs for timbering. The square-set framing is con-
fined to small tunnels, and various forms of arch construction can be used in quite large ones. The
arch may be supported on posts supported in the floor, or rest on a springline shelf (hitch) cut in the
sidewalls. Support may vary between these methods with changes in ground, or in shape of the edges.
Posts should be fastened to the wall plate by dowels, lag bolts, or scabs (nailed-on pieces) so
that they cannot fall if relieved of weight.
The weight of timbering varies with expected ground pressure. Sometimes it is merely a light
roof to catch light rockfalls, at other times a high-strength lining designed to resist squeeze from
all directions, including the bottom.
Where timbering ends at a portal, or at an enlarged shaft base, it must be securely braced by diag-
onal beams, as in Fig. 9.60, so that any compression developing in the tunnel will not squeeze it out.

Packing. In rock or soil that tends to push in, it is important not to leave any space between the
lagging and the wall or roof, as any inward movement will increase the instability of the ground, and
may cause it to exert tremendously more pressure than if it had been held in its original position.
An exception is found in swelling or squeezing ground that is allowed a limited space for
movement.