Critical state soil mechanics (CSSM) framework have been developed to describe soil behaviors. The feature of CSSM is defining a unique stable-state boundary surface (SSBS) as a yielding surface to characterize elastoplastic behaviors of soil. When soil state is inside SSBS, only elastic strain is induced in soil elements. However, when soil state transverses SSBS both elastic and plastic strains are induced. The critical state line representing all possible failure states of soil elements, is an important component of SSBS at which only plastic strain is induced. In mean effective stress-deviator stress space for triaxial test or vertical effective stress-shear stress space for direct simple shear test, this line goes through the origin with a slope M called as critical state stress ratio, the subject of this investigation. A set of monotonic drained direct simple shear (DSS) tests was performed on silty sands to study the effects of fines content on critical state stress ratio. This kind of testing can replicate continuous principal stresses rotation and Ko condition which occur to in-situ soils. The mist pluviation was used to prepare samples to prevent severe particle segregation. The results showed that the critical state stress ratios behaved distinctively for different fines contents ranges. The increase in fines content of sand-like soils and silt-like soils decreased critical state stress ratios while unstable behavior was found for mixed soils in the transitional zone.

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