The complete obsolescence of underground drilling and
blasting techniques does not appear to be imminent. Rock that is too . ard to cut economically with a machine must be excava ed in the convent ional manner: using drills and explosives. Tunneling machines are expensive both to purchase and to install; consequently, they are frequently uneconomical in shorter tunnels. Furthermore, improvements resulting from research into drilling and blasting may likely bring wider adoption of conventional methods.When the drilling and blasting method is used, theground is shattered by the detonation of explosives withinholes drilled into the solid rock. These holes are usually25.4 to 50.8 mm (1 to 2 in) in diameter, spaced severalcentimeters to a meter apart in a preplanned pattern, anddrilled to a depth that is dependent on the length of roundto be pulled. Pneumatic percussion drills are customarilyused. Small drills can be hand held, but larger and morepowerful drills, which are now commonly used, must bemounted on positioning devices to obtain maximum drill-ing speed with a minimum of labor. Standard practice isto mount the drills on a carriage or "jumbo" that travels to and from the face on railroad tracks or on pneumatic tires.
Explosives for tunnel blasting can be either gelatine dy-
ruwiite or ammonium nitrate. Detonation is accomplished
by use of electric blasting caps with built-in delays. Delay
refers to the short interval of time that occurs between the instant the electric current is applied and the instant of explosion, which may vary from zero to several seconds. The use of delays results in sequential loosening and ejection of the shattered rock during the blast, which starts with a "cut"shot near the center of the face and progresses in a series of shots outward to the tunnel perimeter.
Drilling and blasting is cyclical. To excavate a round re-
quires the sequential operations of drilling the holes, loading the explosive, detonating the blast, and finally removing and disposing of the broken rock, installing supports (whenneeded), and extending utilities. The aim of the tunnel engineer is to reduce the cycle time to a minimum, thereby permitting a maximum number of rounds to be "pulled" in a 24-h day and keeping the cost of tunneling as low as possible.
Since 1930 many improvements have been made in ex-
cavating tunnels by drilling and blasting. Consequently,
tunnels are today being driven twice as fast and sometimeswith less labor than they were 40 years ago. Improve-ments during these years include the drill jumbo, remov-able drill bits, tungsten carbide bit facings, better drillsteels, larger and more powerful pneumatic drills, hydrau-lically powered booms for positioning of drills, larger andfaster mucking machines, devices such as "cherry pickers," and the sliding floor (magic carpet) for faster switching ofmuck-hauling vehicles. These improvements were the products of equipment manufacturers and inventive tunnel engineers and were not publicly financed. The increased tunneling activity today and the stimulus ofpublicly financed research may bring even greater advances in conventional tunneling methods during the next decade.
Tunnel jumbo equipped with hydraulic drills (photo courtesy of Atlas-Copco).
FUTURE RESEARCH
Faster Drilling
Accelerated research into improved metallurgy of drill
bits, differently shaped bits, and faster drills could increase substantially the currently attainable drilling
speed. Rotary drills might replace percussion equipment in hard rock work. Experiments in the past with
rotary drills were discouraging largely because of the
harmful effect of noise on the human operator. Research into noise abatement might lead to discoveries that would result in a reevaluation of rotary drilling.
Automation of Drilling Mechanisms Programmed positioning of drill holes could eliminate delay now caused by human factors such as the experience, intelligence, or state of fatigue of the operator.
Mechanical Loading of Explosives
One of the factors that increase labor costs in tunnel
construction is the number of workers required to hand place and tamp the explosive charge. Machines to place the charges could result in significant savings in labor cost and loading time.
Hydraulic Drills
Manufacturers have recently introduced hydraulically
powered drills and impact breakers that give great prom-
ise of success. Perhaps further experimentation with
combinations of percussion drills and impactors of expansion breakers will lead to a new rock-breaking technique that will eliminate the need for explosives.
Smoother Excavation One of the advantages of machine excavation is the elimination of overbreak. The removal of unnecessary material beyond the required excavation lines increases the cost for labor and ground supports and especially adds to the cost of concrete lining required to fill those unwanted spaces. Further research is needed in methods to permit better control of overbreak.
Faster Muck Removal
Little improvement has been made during the past 40
6 years in mucking machines. Faster production could
undoubtedly be obtained by introduction of already
known principles to tunnel excavation.Use of hydraulically powered and more compact shovels could result in faster and less costly handling of materials in underground excavation.
Labor Efficiency
A program of education and improved relations between
employer and workers is needed to increase labor efficiency,improve safety, and reduce costs. An objective study should be made to determine the optimum number of workers required for various tunneling operations. Part of the high costs of underground excavation results from requirements of union management that more workers be employed than needed for efficient and safe operations with today's equipment.
Measuring Surface Vibrations
Vibrations at the surface of the ground are caused by blasting for rock tunnels. The character of these vibrations cannot now be accurately predicted, and a measurement method is needed, particularly when blasting occurs under densely populated areas.
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