Resistance of partly saturated sand to liquefaction with reference to longitudinal and shear wave velocities

In order to examine the liquefaction resistance of partially saturated sand, a cyclic triaxial test apparatus was manufactured, with which it was possible to first measure the longitudinal and shear wave velocities and then to conduct cyclic triaxial tests on the same specimens under p-constant conditions. A series of the tests indicated that the longitudinal wave (P-wave) velocity could be used to identify the degree of saturation of near-saturated samples, and its effect on the liquefaction resistance could be clarified by conducting cyclic loading tests immediately following measurement of P-wave velocity. The results of the P- and S-wave measurements indicated that the velocity of P-wave propagation, V-p, tends to increase from about 500 m/sec to about 1800 m/sec when the B-value increases from 0 to 0.95 corresponding, respectively, to the saturation ratio S, of about 90 and 100%. Simple theoretical consideration indicated, on the other hand, that V-p could be expressed as a function of two factors, that is, the B-value and what may be termed skeleton Poisson's ratio which is defined for the small-strain level of soil deformation, based on the measured V-p and the propagation velocity, V-s, of the shear wave (S-wave). The theoretically derived simple formula as above was shown to be in good coincidence with the relations obtained in the tests. For the cyclic phase of triaxial tests, the cyclic resistance to liquefaction was shown to increase with a decrease in the B-value and hence with a drop in the saturation ratio S-r. It was also shown that when the B-value drops practically to zero with a V-p-value of about 500 m/sec, the cyclic strength of the specimen becomes twice as much as that at full saturation with B = 0.95 and V-p 1800 m/sec.