A series of isotropically consolidated undrained triaxial tests (CU) were conducted on a clean sand and a well-graded silty sand (with 10 % silt content) to investigate the effect of silt

void ratio and confining stress on the instability of saturated sands. The test results show that both clean sand and well-graded silty sand exhibit strain softening characteristics under undrained condition (i.e. instability). Under the same confining pressure

the brittleness index increases with increasing void ratio

but the stress ratio of the instability line decreases with increasing void ratio. If equivalent intergranular void ratio (ege) is used to interpret the critical state data of the silty sand

clean sand and silty sand possess similar critical state line in ege-ln p? plane. Based on the critical state theory and equivalent intergranular void ratio

clean sand and well-graded silty sand exhibits similar instability under the same modified state parameter (?ge).