928 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



bearing strength, compaction, rigidity and general stability of foundations, 

 seismic resistance of foundations and dams, effectiveness of compaction in 

 earth dams in horizontal and vertical direction, solidity of road beds and 

 road surfaces, determiiiation of the thickness of cement slabs, and the like. 



C. Strain Gauging 



Strain gauging, in the general sense, involves the measurement of small 

 displacements of mechanical parts under static or transient loading. 

 Secondarily, strain gauging devices are useful for the recording of vibrations 

 and for a comparison of the dimensions of machined parts with those of 

 standards. Strain gauging finds -wddespread application in the fields of 

 automotive, railroad, pipe-line, highway, and related fields of transporta- 

 tion engineering ; in the testing of all kinds of industrial machinery requir- 

 ing an analysis of performance under transient loads and measurements of 

 ambient pressures; and in automatic dimension control in the machining 

 of matched parts. 



Application in the mechanical engineering field includes the measure- 

 ment of strain (and usually vibrations) in airplanes, airships, locomotives, 

 steamships; the testing of railway and trolley tracks, railway and highway 

 bridges and beds, and transmission towers; the determination of pressure 

 and vibrations in pipe lines; and measurement of impact and vibration 

 stresses in all kinds of industrial machines, such as punch presses, rolling 

 mills, steam and water turbines, internal combustion engines, elevators, 

 mine hoists, and the like. 



Although most of these applications of stram gauging are rather remote 

 from our field, geophysical problems arise where mechanical structures are 

 tested in relation to, or in connection with, their foundations. Examples 

 are railway and highway road beds and bridges, irrigation and flood con- 

 trol dams, foundations for industrial machines, and similar structures. 

 Another application of a purely geological nature is the measurement of 

 rock displacements in faults, shear zones, active earthquake areas, muie 

 workings, and railroad and drainage tunnels, and the investigation of rock 

 bursts and the subsidence of the surface of the ground and the roofs of 

 mines. 



Methods and instruments for the measurement of strains or displace- 

 ments cover the entire range from ordinary length and elevation measure- 

 ments to measurements as precise as one one-millionth of an inch. Ordi- 

 nary procedures for measuring lengths and the regular methods of leveling 

 are often satisfactory for observing the changes on very active faults, 

 particularly in earthquake areas, for checking the movement of forma- 

 tions along major faults or fissures underground, and for keeping track of 



