Numerous studies have explored the influence of strain rate on the small- and large-strain shear resistance of nonplastic soils, most using conventional drained and undrained (or constant volume) laboratory tests. To supplement the initial states, soil gradations, and modes of shear examined in earlier studies, the authors used direct simple shear (DSS) tests to evaluate the effect of shear strain rate on the shear resistance of three nonplastic soils with fines contents that varied from 0% to 60%. In addition, the effect of stress path on the critical state and uniqueness of the critical state line (CSL) were explored. Results from the DSS tests on these soils indicated that irrespective of the nonplastic fines content: (i) peak undrained shear strength increased by up to approximately 9% for every order of magnitude increase in strain rate; (ii) strain rate had little to no impact on shearing resistance at the critical state regardless of stress path; and (iii) the CSL was independent of strain rate. In addition, the authors postulate that the effect of strain rate on peak undrained shear strength may be related to differences in inertia during shearing as well as particle rearrangement.

This experimental study investigated the characteristics of nonplastic silt subjected to multiple freeze-thaw cycles. The study used a novel, open test system developed to better represent field conditions in seasonal frost areas than can be achieved with conventional laboratory test setups. Various sensors were used to measure changes in temperature, water content, surface displacement, and electrical conductivity in the soil during five cycles of freeze-thaw testing. The test system featured a transparent window for visual observations of the soil (high resolution photographic images) throughout the duration of testing. The experimental results showed that volumetric water contents in the active layer of the soil sample decreased during the freezing period whereas they increased again when thawing started, reaching water content values closer to the initial values at the end of the thawing period. However, the electrical conductivity in the active layer became much greater than the initial value after freeze-thaw cycles, indicating changes in the pore structure of the soil in the active layer. High-resolution images of the soil sample taken during the freeze-thaw cycles and from soil samples exhumed after completing the five cycles of freeze-thaw confirmed that the nonplastic silt in the active layer became more porous after freeze-thaw cycles, whereas no visible changes in pore structure occurred in the soil beneath the active layer. The amount of the thaw settlement was greater than the amount of the total frost heave for each cycle, indicating a decrease in sample height after freeze-thaw cycles. The experimental results further showed that the frost depth increased after multiple freeze-thaw cycles.

A series of strain-controlled cyclic triaxial tests were performed on Firoozkuh sand-silt mixtures with wide ranges of fines content (FC), relative density (Dr), and effective confining stress ( sigma c ') to investigate their liquefaction resistance in terms of capacity energy (Wliq). Also, several cyclic test results from previous studies were collected and reanalyzed. The results showed that Dr could be used as a proper parameter to define soil density state for predicting Wliq of clean sands and sand-silt mixtures when FC is greater than dispersing fines content (FCdis). However, due to the complicated role of FC in coarse-fine assemblies, no universal relationship between Wliq and FC has been reported for the soil mixtures when FC is less than the threshold fines content (FCth). Therefore, the concept of equivalent granular void ratio ( e & lowast;) was used to capture the coarse-fine interactions in such mixtures. It was also found that the fines contribution factor (b), which is the fraction of fines participating in load transfer, is dependent on Dr, as well as particle size disparity ratio (chi) and FC, neglected in previous studies. Finally, a new model was proposed for the prediction of the b value and also a unique relationship between e & lowast; and Wliq was obtained for all mixtures of specified sand and silt where FC <= FCth.

Incorporation of nonplastic fines can dramatically affect the liquefaction resistance and stiffness of sands. This study aims to evaluate the influence of nonplastic fines on the liquefaction resistance and small-strain shear modulus of calcareous sand under cyclic loading. Forty-seven sets of undrained cyclic triaxial tests and companion bender element tests are conducted on reconstituted specimens. The cyclic behavior of clean sand and silty sand with varying fines content is examined with respect to the global void ratio, relative density, and granular skeleton void ratio. The findings demonstrate that the microscopic contacts between coarse and fine grains have a significant impact on the macroscopic behavior of sand-fines mixtures. The experimental findings are evaluated using the equivalent granular skeleton void ratio, which has been recognized as a suitable parameter to describe the overall effect of fines. The findings on calcareous sand with fines are supplemented and compared with published data in accordance with the semiempirical simplified approach for liquefaction triggering based on shear wave velocity.