It is a generally known fact that the most massive of structures is 
potentially subject to destruction if it can be excited at or near its natural 
frequency of vibration and if its inherent damping is small. The spectacular 
failure of the main span of the Tacoma Narrows Bridge in the State of Wash- 
ington in the early 1940s when excited by wind gusts is a case in point. At 
the other end of the size scale one might hope to destroy rock with high- 
frequency excitation at the natural frequency of the very small rock particles 
as isolated, for instance, by the Griffiths cracks. A simpler system might be 
developed which would use broad-band random vibration with an upper 
frequency cutoff such that all rock would be disintegrated to very fine 
particles. Among other methods, a liquid bath might transmit this vibra- 
tional energy to the rocks. Experiments are planned to determine the degree 
of damping in rock vibrating in its first mode. Experiments would be run to 
determine if rocks can be sufficiently excited in aliquid bath. Finally, the 
effect of the broad-band random excitation would be studied. 
While the problems to be overcome appear formidable, the possible 
benefits to be gained are large compared with the modest experimental and 
theoretical program necessary to establish the necessary foundation of 
equipment development. 
Current Activity in Research and Development 
Major efforts are in progress to develop at least two systems which 
do not depend upon a cutter to disintegrate strong rock and thus avoid the 
problem of cutter replacement. Recent work at Westinghouse Research and 
Development Center, Pittsburgh, Pennsylvania (Schumacher, 1968), has made 
electron-beam heating of rock practicable in a one-atmosphere situation, with 
the further ability to work under a small (few inches) water head. Since the 
beam of electrons is rapidly scattered and absorbed except in a vacuum, the 
device must be close to the rock. The vacuum is maintained by a series of 
chambers, each of increasing pressure as the electrons approach the outlet. 
An overpressure of air in the final stage prevents the ingestion of gas and 
dust into the vacuum system and allows submergence in water. The method 
is not currently applicable to working under high pressures in the ocean, but 
may prove suitable for the subbottom tunneling of lateral chambers. The 
only requirement is for electric power, with the possible exception of a 
requirement for selective removal of noxious gases from the closed atmo- 
sphere if they are released upon heating the rock. The present state of 
development is a laboratory model of a cutting gun capable of melting or 
splitting its way through 4 to 6 inches of rock by local heating from an 
electron gun. The decelerating electrons give off X-rays, but the process 
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