This paper summarizes the results from a comprehensive laboratory experimental research program conducted at the University of British Columbia, Canada to study the mechanical behavior of natural fine-grained soils. Constant-volume direct simple shear tests were performed on natural silt samples collected from various locations in British Columbia to investigate the monotonic and cyclic shear loading response of these materials. Influencing factors such as effective confining stress, over-consolidation ratio, initial static shear bias, soil plasticity, and soil fabric/micro-structure on the mechanical behavior of silts were systematically investigated using a series of controlled laboratory tests. The key observations arising from these examinations are presented and discussed to serve as input for the development and/or refinement of design practice approaches for seismic geotechnical design. New directions to be considered to advance the current state of understanding and knowledge on the mechanical behavior of natural fine-grained soils are identified.

Introduction

Liquefaction of soils and associated ground movements could cause damage to buildings and infrastructure during earthquakes. For example, it has been reported that the damage costs from a major earthquake in the Metro Vancouver Region of British Columbia (BC), Canada could be as high as $75 billion [1], with significant damage expected to arise from soil liquefaction. Southern Ontario and the St. Lawrence Valley are the other regions in Canada where earthquakeinduced soil liquefaction is considered an engineering concern. The assessment of liquefaction susceptibility of natural fine-grained soils, particularly those with low plasticity, poses significant challenges to the current design practice. This is mainly due to the lack of understanding of the complex stress-strain-strength behavior of finegrained soils such as low-plastic silt; this knowledge gap, in turn, has led to the use of liquefaction assessment criteria that are primarily based on soil index properties such as plastic limit (PL), liquid limit (LL), plasticity index (PI), and water content. The consideration of silt with PI < 7 as “sand-like” and that with PI ≥ ۷ as “clay-like” in behavior for the purpose of liquefaction assessment by Idriss and Boulanger [2] is one such example in this regard; the approach proposed by Bray and Sancio [3], again based on index parameters, serves as another example. Fine-grained silty soils with high levels of saturation are commonly found in natural river deposits, and due to many reasons, most population centers, along with critical infrastructure, industrial plants, etc., are located near river banks on such soil deposits.