experiments in which rock has been effectively cut by water jets. At the 
present state of technological development in the United States (Cooley, 1968), 
practical machines have been limited to spalling of relatively soft sandstones 
and breaking harder rock such as limestone. Present water cannon designs prob- 
ably are not the answer, but free jets working on a shaped-charge principle may 
well be. 
Down-Hole Turbine Drills. Diamond-faced cutter wheels are rotated 
at 5,000 to 10,000 rpm by a turbine on the end of a drill pipe, with the drill 
pipe itself rotating at 30 to 75 rpm. The turbines receive hydraulic power and 
are about 10 to 20% efficient. Drilling rates are comparable to rotary drills 
but always lower. Diamond cutters require twice as much energy as drag 
cutters, and present turbines are limited in power; better and more power- 
ful turbines will be required to make this concept useful. Some commercial 
models are available. 
Pellet Drills. Recirculating steel pellets are fixed against rock to 
fracture it. A drilling fluid is pumped down a drill pipe through a nozzle 
which by aspiration draws in the pellets at the bottom of the hole and fires 
them at the rock at a velocity of about 23 m/sec. A secondary nozzle 
accelerates the rock particles. Some 80% of the cuttings in the fluid are 
recirculated while the remainder are carried to the surface. The difference 
in densities tends to keep the steel pellets at the hole bottom while the 
lighter cuttings are carried off. Only about 4% of the power is delivered 
to pellets, limiting the effectiveness. Water has been found to be the optimal 
fluid. The effectiveness of some thirty-five different pellets was investigated 
before the study was discontinued. In Maurer’s (1968) opinion, a major 
change in basic concept will be required to achieve acceptable efficiencies. 
Continuous Penetrators. Continuous penetrators are hard-pointed 
metal pilings that are forced through the rock. High forces are developed 
by the combined weights of the drill collars, impact loads, and wall anchors; 
a hydraulic cylinder can be used to give additional force. The rock for some 
distance around the penetrator must be crushed to permit flow around the 
penetrator, with the diameter of the crushed region dependent upon the 
porosity of the rock. Maurer (1968) cites a case in which rock of 5% 
porosity was crushed to a diameter of 20 cm with a penetrator of 5 cm 
diameter. Because of the very high forces required for hard rock, this 
method is only used in weak rocks or unconsolidated materials. 
Implosion Drills. |mplosion drills produce rock failure from implosion 
of sealed capsules at the hole bottom. They are pumped tn a slurry to the 
hole bottom. The collapsing cavities create high pressure pulses in the fluid, 
for example, 32,600 kg/sq cm when a 50-mm-radius cavity collapses to 5 mm 
81 
