16 fs^ L^ y 



g 

 is obtained from the resonant frequency, and the dynamic elastic modulus, 



Ed = ^-^^^ . 



is computed from the measured Gd and Poisson's ratio, which was assumed to be 

 0. 5 for undrained saturated clays. The plots of dynamic elastic modulus E^ obtained 

 in this manner compare fairly well with the static elastic modulus E of the triaxial 

 test. Future tests planned for the torsional apparatus will utilize samples confined 

 in thin rubber membranes, with an internal vacuum providing a more positive con- 

 fining pressure. At the low confining pressure the curves for the dynamic and static 

 moduli are almost superimposed. Perhaps this is the reason for the favorable com- 

 parison. This question will be explored further. 



f. Seismic Wave Velocity 



The observed seismic velocity determined from the sweep speed of 

 the race across the oscilloscope is used to compute the modulus either from 



-4 



E_g 



7 

 or 



\ (1 + v) (1 - 2v) V -Y > 7 



( 1 + v) ( 1 - 2v) V 7 



Because of the dynamic effects, the relationship between wave velocity and modulus 

 lies somewhere between imconstrained and fully constrained, probably closer to 

 fully constrained. Completely saturated materials have a high constrained modulus 

 because water is not able to drain and its bulk modulus is 3 x 10 psi. Other in- 

 vestigators have reported saturated constrained moduli ranging from 1 x l(r to 

 1 x 10^ psi. 



g. Relation Between Unconstrained and Constrained Moduli 



Theoretically a constrained modulus may be obtained from an un- 

 constrained modulus. 



1 - v 

 M = (E) 



( 1 + v) ( 1 - 2v) 



391 



