Laboratory investigation of dynamic properties of sand with fines

Abstract

The soil response under a dynamic load, such as that caused by earthquakes, traffics, blasting and machine vibration is governed by dynamic properties of soil including soil stiffness and damping. Over the past years extensive laboratory experiments have been carried out to study these two properties for clean quartz sand, yielding a well-round framework covering a variety of factors, such as packing density, stress condition, strain level, gradation and particle shape. Complications are, however, brought into the application of natural sands which are often not clean but contain some amount of fines of silt or clay size. In particular, what is the effect of fines on the dynamic properties still remains inconclusive and even controversial. From both practical and academic perspectives, it is therefore a matter of concern to enhance the current understanding of the dynamic properties on silty sand and clayey sand. To this end, a series of laboratory experiments were carried out in this study in a state-of-art resonant column apparatus, also incorporating bender element (BE) and torsional shear (TS) functions. Manufactured mixed soils were adopted covering a wide range of fines content. In brief, clean sands of similar particle shape and parallel gradations were selected as the host sand. Accordingly, different types of commercial fines with distinct particle shape were added. Comparisons were made among well-controlled test groups, enabling a stepwise understanding of the influence of fines content, particle size disparity, and particle shape. Besides, empirical models taking into account the coupled effects between the fines content and other factors were also proposed, yielding satisfied predictive performance. From a perspective of earthquake geotechnical engineering, the condition of partial saturation may produce two major impacts: estimates of ground motion and analysis of soil liquefaction susceptibility. While efforts have been made to establish a relationship between compression wave velocity and degree of saturation, attempts were mainly made on clean sand sometimes without consistent success. In this study, the compression wave velocity was also measured, demonstrating a remarkable influence of pore-space distribution arising from the coupled effects of sample reconstitution method and fines content. Gaining relevance from the silty sand, on the other hand, kaolinite clay was added as a counterpart into the clean host sand, creating a distinct feature of particle interactions. The influence of clayey content and plasticity on soil stiffness was thus investigated collectively at both dry and saturated conditions. Of more interest, distinct features were also found on clayey sand using different reconstitution methods and a hypothesis was made accordingly. Owing to a nonlinear characteristic of soil properties, the strain dependent dynamic properties were also studied. A synthesized investigation of the influence factors was made on silty sand and clayey sand. The results indicated that fines content plays an essential role in modulus reduction curve, whereas the influence on damping ratio appears to be less prominent.