Estimating the undrained strength of sand: a theoretical framework

A framework for estimating the ultimate undrained steady state shear strength of sand (S u ) from in situ tests, which combines the theory of critical state soil mechanics with shear wave velocity measurements, is presented. For a particular direction of undrained loading, samples of a given sand at a constant void ratio will reach the same S u , despite the magnitude of the initial effective confining stresses. Unique S u /p′ or [Formula: see text] ratios for a given direction of loading exist for a particular sand only if state parameter is constant throughout the deposit. Normalized shear wave velocity, V s1 , can be correlated with void ratio and is therefore used to estimate S u for a given initial state and direction of loading. Strengths in triaxial compression are examined in this paper; however, the same framework can be used to estimate strengths under other directions of loading. The S u –V s1 relationship is shown to be relatively sensitive and should be used more as a screening tool rather than an accurate means of predicting S u . V s1 is converted to equivalent values of SPT (N 1 )60 and CPT q c1 , and the results are compared with the current methods of estimating S u .