Page 452 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.52 THE WORK

FIGURE 9.42 Underground exploration and tunnel detour. (By permission from “Practical Tunnel Driving” by
Richardson & Mayo, McGraw-Hill.)

with depth. Below 500 feet even apparently firm rock may creep, and break off slabs with explosive
violence. There is always danger of loose pieces falling.
Caving and breaking off are combatted with compressed air, timbers, steel and concrete linings,
and holding rock bolts.
If the soil will not stand at all without support, bracing must be installed ahead of the digging;
or the heading protected by a movable shield.
Water, with or without accompanying soil, may break into a tunnel in such volume as to flood it
completely within minutes. Escape of workers may be difficult, machinery is apt to be abandoned,
and an expensive and tedious job of sealing off the water and pumping out the tunnel is often
required before work can be resumed.
Fire must be carefully guarded against, particularly on jobs using compressed air and/or timbers.
Air conditions are difficult to keep healthy. Drills produce rock dust, and most air-powered
machines have foul, oil-charged exhausts. Explosives produce fumes. Some clay and rock formations
give off unpleasant or poisonous vapors. The increasing use of internal combustion engines
underground makes tremendous demands on ventilation systems that try to clear out dangerous or
irritating exhaust gases.
Silicosis, a lung disease caused by breathing dust from rock drills over long periods, was for-
merly one of the greatest health dangers of tunnel work. It now can be almost entirely avoided by wet
drilling and good ventilation.