INTERNAL FRICTION IN ROCKS 



625 



Carrara marble, described above, except that the lateral resistance 

 to movement was increased by increasing the thickness of the walls 

 of the steel tube inclosing the marble from a thickness of 0.25 

 centimeter to 0.33 centimeter. In these the amount of material 

 moved is identical with that in the series of experiments just 

 described, while the internal friction is increased by the increased 

 thickness of the steel tube. 



A series of additional experiments were also made to determine 

 the resistance offered by such tubes when filled with soft tallow. 



Fig. 8. — Longitudinal section through steel cylinder with pistons inserted and 

 inclosing a deformed column of rock— showing the obtuse shearing cones which advance 

 into the deforming rock. 



In this way another series of curves were obtained for each 

 material and another "true curve" for the deformation of a stand- 

 ard column of Carrara marble under conditions identical with those 

 of the former experiments, except that the resistance to deforma- 

 tion offered by the steel tube was much greater. The " true curve " 

 for the deformation of the marble in a steel tube having walls o . 33 

 centimeter thick is shown in Fig. 10. 



An inspection of this curve will show that while, as before, 

 starting from the limit of elastic expansion the rising load at first 

 induces a relatively small amount of movement in the rock, the 



