FRICTION AND LIMITING STRENGTH OF ROCKS 655 



APPENDIX 



In order to study the experimental data on the flow of rocks in the light 

 of a theory of internal friction, the data reproduced in Tables I and II were 

 obtained from the original large-scale curves obtained by Adams and Bancroft 

 connecting the end load on the steel pistons with the bulge of the nickel-steel 

 jacket. Each of the curves represented the mean of two, three, or more com- 

 plete sets of observations. The first row of numbers for each specimen is the 

 total load W (in thousand pounds) on the steel pistons required to bulge 

 the nickel-steel jacket by the amounts entered under the various columns. 

 The second row gives the pressure-intensity P — W irbl in thousand pounds per 

 square inch exerted on the end of the specimen of radius b . The third line 

 gives the average pressure-intensity P = zz in thousand pounds per square inch 

 at the central portion of the specimen in the direction of the axis, correcting 

 for the effect of the bulge from formula (20). It will be noticed that the 

 average longitudinal pressure at the center is somewhat less than that over 

 the ends by amounts which increase considerably with the harder rocks. 

 The final results given in Tables I and II are shown graphically in Plates I 

 and II, respectively. Against the lateral pressures (given by the experiment 

 on tallow) are plotted the longitudinal pressures required to bulge the nickel- 

 steel jacket to the same extent. For such of the rocks as give curves approxi- 

 mating to straight lines we may say that a definite modulus of plasticity and 

 coefficient of internal friction exist. Rough estimates of these constants as 

 determined for the soft rocks from large-scale curves are given in Table III, 

 in which the first entry corresponds to the 0.25-centimeter wall nickel-steel 

 jacket and the second to the 0.33-centimeter wall. It will be noticed that the 

 two sets of results are in poor agreement for K, and are only in rough agreement 

 for /*, the difficulty arising from attempting to fit a straight line to a series of 

 points which are only approximately colinear. 



In the case of the harder rocks no definite coefficient of friction can be said 

 to exist. In the case of dolomite and Belgian black marble it is noticed that 

 the coefficient of friction tends to diminish with increasing longitudinal and 

 lateral stress. Slate gives a very irregular curve due to the development of 

 cracks while the material is stressed. The sudden bend in the curves for 

 diabase and granite is attributed to the actual breakdown of the rock material. 

 From the curves of Plates I and II it will be noticed that this occurs in the 

 neighborhood of 22=150,000, xx = 25,000, corresponding to a stress-difference 

 of 125,000 pounds per square inch. In a general way this result confirms the 

 conclusion already arrived at from a discussion of the experiments of Adams 

 on the pressure required to close up small cylindrical cavities in specimens of 

 Westerly granite. In the writer's paper already mentioned (p. 641, n. 1) it 

 was pointed out that the stress-difference required to break down the rock 

 material in the neighborhood of small cavities amounted to as much as 



