The cyclic direct simple shear (CyDSS) device of the NGItype
was used because of its ability to simulate initial in situ
consolidation conditions and subsequent cyclic loading conditions
corresponding to those in horizontally-layered soil deposits
subjected to vertically-propagating shear waves. The
vertically-propagating shear wave components are generally
considered to be the main contributor to ground surface damage
during earthquakes. Consequently, the NGI CyDSS device
has been often used in important soil dynamics studies (e.g.,
Finn 1985; Dobry and Vucetic 1987; Woods 1991, 1994). This
has been a practice in spite of the nonuniform stress-strain
conditions in certain parts of the DSS specimen, in comparison
with the desirable ideal uniform pure-shear stress conditions
(Lucks et al. 1972; Shen et al. 1977). The results of theoretical
(Shen et al. 1977) and experimental studies (Wood and Budhu
1980; Vucetic and Lacasse 1982) show that the difference between
the nonuniform stresses and the average horizontal shear
stress in the DSS specimen is relatively small, and that the
test is therefore adequate for engineering practice (Vucetic and
Lacasse 1984).