Adams and Colter — Elastic Constants of Rocks. 103 



Application of the Method of Simple Compression to the 

 Determination of the Compressibility of Rocks. 



As has been noted, it was found in the case of marble when 

 subjected to bending stress, that the strain as exhibited by the 

 deflection of a point of the bar increases with the time, and 

 the strength under shear produced by a torque was also so 

 small that a determination of the strain was very difficult to 

 measure. 



These difficulties have been noted by Nagaoka,* who states 

 that " Preliminary experiments on granite showed that Iiooke's 

 law does not hold even for very small flexure and torsion, and 

 that the after effect is very considerable when the prism is 

 sufficiently loaded or twisted ; the deviation from the direct 

 proportionality between strain and stress was incomparably 

 great as compared with that observed in common metals. 

 This must be chiefly due to the low limit of elasticity, so 

 that it is necessary to experiment only within narrow limits 

 of loading and twisting. These limits are widely different for 

 different specimens of rocks, and the modulus of elasticity as 

 well as that of rigidity was always determined with such 

 stresses as will approximately produce strains proportional to 

 them. The deviation from Iiooke's law was prominent in 

 certain specimens of sandstone, and it was more marked in 

 torsion than in flexure experiments; in certain rocks it is 

 indeed doubtful if anything like a proportionality between 

 stress and strain can be found, even for extremely small change 

 of shape: on releasing these rocks from stress the return 

 toward the former state is extremely small, showing that the 

 elasticity of the rocks is of very inferior order." 



If, however, the rock be subjected to direct compression, 

 strains in which the time effect is small, and the lag of the 

 strain is also small, are almost invariably obtained. This is 

 especially the case if, before the actual experiment is carried 

 out, the material be several times subjected to a range of 

 stresses at least as great as those employed in the experiment 

 itself. This preliminary stressing brings the material to a 

 " state of ease," and is also commonly adopted when the 

 elastic constants of metals are determined. 



It is evident therefore that this Direct Compression Method 

 may with confidence be applied to the measurement of the 

 cubic compression of rocks, although as mentioned below the 

 accuracy of the results obtained will differ with different 

 classes of rocks. If the rocks be massive, compact and crys- 

 talline (or glassy), the method can be safely employed and good 

 results will be obtained. If, on the other hand, the rock is 



* Elastic Constants of Bocks and the Velocity of Seismic Waves ; Nagaoka, 

 H.— Phil. Mag., vol. l, 1900, p. 58. 



