Undrained Fragility of Clean Sands, Silty Sands, and Sandy Silts

Abstract

In this paper, intergranular (\Ie\dc\N) and interfine (\Ie\df\N) void ratios and confining stress are used as indices to characterize the stress-strain response of gap-graded granular mixes. It was found that at the same global void ratio (\ie) and confining stress, the collapse potential (fragility) of silty sand increases with an increase in fines content (FC) due to a reduction in intergranular contact between the coarse grains. Beyond a certain threshold fines content (FC\Dth\N), with further addition of fines, the interfine contact friction becomes significant. The fragility decreases and the soil becomes stronger. The value of FC\Dth\N depends on \ie and the characteristics of fines and coarse grains. At FC FC\Dth\N), fine grain friction plays a primary role and dispersed coarse grains provide a beneficial, secondary reinforcement effect. At the same \Ie\df\N, the collapse potential decreases with an increase in sand content. Beyond a certain limiting fines content, the soil behavior is controlled by \Ie\df\N, the collapse potential decreases with an increase in sand content. Beyond a certain limiting fines content, the soil behavior is controlled by \Ie\df\N only. An intergranular matrix diagram is presented that delineates zones of different behaviors of granular mixes as a guideline to determine the anticipated behavior of gap-graded granular mixes. New equivalent intergranular contact void ratios, (\Ie\dc\N)\Deq\N and (\Ie\df\N)\Deq\N, are introduced to characterize the behavior of such soils at FC FC\Dth\N, respectively.