Drained Seismic Compression of Unsaturated Sand

Abstract

Seismic compression of unsaturated soils occurs due to particle rearrangement during largestrain cyclic shearing which may be resisted by interparticle stresses that depend on the matric suction and degree of saturation. Due to the high rate of shearing in earthquakes, seismic compression is expected to be an undrained phenomenon with changes in total volume, matric suction, and degree of saturation along with an evolution in soil hydro-mechanical properties during cyclic shearing. To simplify this problem and better understand the mechanisms of seismic compression, this study seeks to isolate the effect of matric suction through a series of drained cyclic simple shear tests on unsaturated sand subjected to different shear strain amplitudes. These tests were performed in a cyclic simple shear apparatus with suction-saturation control using a hanging column and suction monitoring using an embedded tensiometer. Matric suction values in the funicular regime had the greatest effects on the magnitude and rate of development of seismic compression with cyclic shearing, and values in the capillary regime were similar to those in dry and saturated conditions. The volumetric contractions also caused the soil-water retention curve and suction stress characteristic curve to shift toward higher suctions during cyclic shearing.

Undrained Seismic Compression of Unsaturated Sand

“…In the drained cyclic simple shear tests of Rong and McCartney (2020), the seismic compression of unsaturated sands in the funicular regime after a large number of cycles was observed to have a log-linear relation with matric suction from the values observed for unsaturated sand. For smaller numbers of cycles (i.e., 200 cycles), they found that specimens with an initial degree of saturation of 0.1 had the lowest seismic compression.…”

“…Although partial drainage was observed in the centrifuge shaking table tests on unsaturated sand layers reported by Ghayoomi et al (2011Ghayoomi et al ( , 2013, the trends 6 in seismic compression with degree of saturation were similar to those reported in these previous studies on drained conditions. However, direct comparisons in trends are difficult as the cyclic shear strain amplitudes in these studies are all different, ranging from values on the order of 0.06% in Le and Ghayoomi (2017) to values on the order of 5% in Rong and McCartney (2020).…”

“…Experimental studies involving the seismic compression of unsaturated soils have been performed by researchers under drained conditions (Le and Ghayoomi 2017;Rong and McCartney 2020), undrained conditions or partial drainage conditions (Sawada et al 2006;Unno et al 2008;Ghayoomi et al 2011Ghayoomi et al , 2013Kimoto et al 2011;Milatz and Grabe 2015), or without consideration of drainage conditions (Hsu and Vucetic 2004;Whang et al 2004Whang et al , 2005Duku et al 2008;Yee et al 2014). These studies used a range of experimental techniques ranging from element-scale tests in cyclic simple shear or cyclic triaxial setups to centrifuge scale model testing of soil layers.…”

“…These studies observed excess pore water and pore air pressure generation during cyclic shearing along with volume change due to compression of air voids and corresponding increases in degree of saturation. These observations are important as coupled changes in pore air and pore water pressures and degree of saturation will lead to changes in the effective stress state (Lu et al 2010;Rong and McCartney 2020). Further, the shear modulus and damping relationships with cyclic shear strain magnitude are also closely related with the void ratio and effective stress state (Khosravi et al 2010, Hoyos et al 2015Le and Ghayoomi 2017;Dong et al 2016Dong et al , 2017, which may play a major role in the evolution in seismic compression.…”

“…This information was useful to calculate the degree of saturation, matric suction, effective stress, shear modulus, and damping ratio, which are all needed to understand the mechanisms of seismic compression during undrained cyclic shearing. Specimens were tested in the funicular regime as previous drained cyclic shearing tests by Rong and McCartney (2020) indicate that the seismic compression was greatest in this regime. Specimens in the funicular regime are also expected to have a continuous air phase, which helps experimentally in monitoring the independent evolution in pore air and pore water pressures.…”