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 S-u/sigma'(v) 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.