in a 2,000-meter deep hole filled with water. Energy is dissipated in the 
fluid and only a small amount is transmitted to rock. To achieve the equiv- 
alent of 30 to 50 hp requires about 13,000 10-cm spheres per hour at a depth 
of 2,000 meters. Capsules are probably expensive, and not all stone is shattered 
effectively by this method. 
Spark Drills. Spark drills use an underwater spark to produce shock 
waves. Energy stored in condensers flows through a small conducting zone, 
creating a high-temperature plasma which exerts pressure of Oeatom Oz 
atmospheres in the water confining the spark. Sparks lasting from 5 to 
50 usec produce instantaneous power release at a rate of a million horse- 
power. Energy from a 4 uf capacitor charged to 70 kv is 9,800 joules; firing 
10 shots per sec would be equivalent to 133 hp. Sparks appear to require 
about twice the energy of conventional drills for the effects produced, but 
they have a high potential because they can be operated at high frequency. 
By comparison, a large rotary drill can deliver from 20 to 50 hp. 
Maurer (1968) has proposed a spark percussion drill in which sparks 
confined behind a piston drive a conventional bit. No data are available, but 
the Soviets are believed to have tested this approach. 
Electrohydraulic Crushers. Electrohydraulic crushers use a variation 
of the spark drilling technique. Epshteyn, Arsh, and Vitort (1960) used sparks 
ina tank, with 0.19 uf at 40 kv discharging 5 times per second to reduce shale 
and chert from 7 cm to 5 mm in diameter. Bergstrom (1961) used a 0.05 to 
0.5 uf with up to 80 kv to crush limestone to 1 mm. Maroudas and Taylor 
(1964) used 0.005 pf at 30 kv to crush glass spheres to 0.3 mm. 
Mechanically Induced Stresses. Explosive drills pump 3 to 12 explosive 
capsules per minute. The capsules explode on impact with the rock. The 
Soviets have pumped twelve 50-gram capsules per minute, releasing some 66 
hp. The capsules must be spaced so that one does not detonate while the 
next is in the nozzle. Drill rate does not appear to depend on rock strength, 
but the system cannot drill soft materials which cushion the blow and are 
difficult to remove from the hole. The system as tried was limited by spoil 
removal rate, but in hard rock penetration was comparable to that of con- 
ventional drilling methods. 
The Russians also have tried a drill using liquid explosives with 
cyclical mixing of liquids by differential pressures. While the potential 
power is greater than for solid explosives, tests of the drill were unsuccess- 
ful. Flushing fluids diluted the explosive liquids, and explosives accumulated 
and destroyed the drill. 
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