Shear wave velocity and stiffness of sand: the role of non-plastic fines

<jats:p> Current knowledge on the shear wave velocity (V<jats:sub>s</jats:sub>) and associated stiffness (G<jats:sub>0</jats:sub>) of sand is built mainly on the results of extensive laboratory studies on clean quartz sands. Often natural sands are not clean, but contain a certain amount of fines. The role of fines in altering the stiffness of sands is a matter of great concern, yet remains poorly understood. This paper presents an investigation into the problem through well-controlled laboratory experiments in conjunction with analysis and interpretation at the macro and micro scale. The laboratory experiments were conducted for a sequence of mixtures of clean quartz sand and crushed silica fines under saturated conditions, by the simultaneous use of the resonant column (RC) and bender element (BE) techniques. A broad range of states in terms of void ratio, confining stress and fines content was covered so as to obtain a comprehensive view on the effect of fines and the possible interplay with other factors. Both the RC and BE tests showed that G<jats:sub>0</jats:sub> tends to decrease continuously as the quantity of fines is increased and the reduction rates are similar; a similar stress dependence is also obtained for G<jats:sub>0</jats:sub> from both types of testing. Nevertheless, G<jats:sub>0</jats:sub> values obtained from BE tests are notably greater than those obtained from RC tests, and this effect of testing method is shown to be coupled with the sample reconstitution method. A new approach that allows unified characterisation of G<jats:sub>0</jats:sub> values for both clean sand and sand–fines mixtures is developed in a sound theoretical framework, thereby providing important insights into the various empirical correlations that involve G<jats:sub>0</jats:sub> (or V<jats:sub>s</jats:sub>) in geotechnical engineering practice. A new micro-scale mechanism is also suggested for the observed effect of fines, which attributes the reduction of G<jats:sub>0</jats:sub> caused by fines to the decrease in the coordination number at an approximately constant void ratio. </jats:p>