Monotonic and Cyclic Behavior of Unsaturated Sandy Soil Under Drained and Fully Undrained Conditions

Abstract

Changes in air pressure during monotonic and cyclic loading are in some cases important for the behavior of unsaturated soil. For example, in order to investigate the stability of embankments and slope failure during earthquakes, it is necessary to consider the eŠect of the pore air or the pore gas pressure as well as the pore water pressure and the interaction between the soil and the pore ‰uids. In the present study, we carried out a series of monotonic and cyclic loading tests on sandy soil used for the improvement of river embankments. The eŠects of the initial suction, the conˆning pressure, and the degree of compaction under fully undrained conditions, namely, constant water and constant air shearing tests, as well as under drained conditions for both air and water, were studied. For the stress variables of the unsaturated soil, the skeleton stress was used to describe the experimental results and was deˆned as the diŠerence between the total stress tensor and the average pore pressure of water and gas (Oka et al., 2010). From the monotonic and cyclic test results, we found that the stress-strain behavior of unsaturated sandy soil strongly depends on the initial suction, especially under fully undrained conditions, due to the diŠerence in pore pressures. In the cyclic loading tests under fully undrained conditions, the mean skeleton stress decreased due to the increase in air pressure and led to the failure of the specimen in the case of a lower level of initial suction. In addition, the test results exhibited the strain rate eŠect on the stress-strain behavior during cyclic loading under fully undrained conditions.

Investigating the use of Boyle's Law to relate pore air pressures and volume changes in unsaturated triaxial samples

“…The pore water and air pressure responses tended to be very similar and as a result, only small suctions developed in the samples (where suction is the difference between the pore air and water pressures). These observations are consistent with the typical pore pressure responses observed in unsaturated samples of silty sand that were tested under undrained conditions, at relatively high degrees of saturation above 60% and at low suctions in other liquefaction studies [2,5,9]. Fig.…”

“…The samples tested in this study had a diameter of 100mm, an approximate height of 190mm and therefore a volume of 1,492,000mm . These samples were much larger than the unsaturated samples of silty sand tested in [2,3,5,9,10] and the associated volume changes of the larger samples were significantly greater than the volume changes measured in smaller samples. Therefore, the volume changes of the unsaturated samples tested in this study were measured through monitoring the volume of water that flowed directly in and out of the triaxial cell and volume corrections that accounted for the volume measurement errors from the expansion of the cell due to creep, and due to water being absorbed into the cell wall, were applied.…”

“…Image processing methods have also been used to track the changes in volume from photographs taken of the sample [7,8]. Alternatively, the volume changes in the cell water surrounding the sample have also been used to measure the volume change of the sample [2,3,5,9,10]. Monitoring the volume changes of the cell water is the more common approach, as irregularities in the sample's shape from the formation of shear planes and bulging, can make it difficult for internal instrumentation to accurately measure the global volume changes of the sample.…”

Cyclic Triaxial Test to Measure Strain-Dependent Shear Modulus of Unsaturated Sand

“…Although researchers have increasingly explored the effect of degree of saturation on the dynamic soil modulus and damping, they have mostly focused on the small-strain shear modulus for various suction values or degrees of saturation using bender element or resonant column tests (Wu et al 1984;Qian et al 1991;Marinho et al 1995;Cho and Santamarina 2001;Mancuso et al 2002;Mendoza et al 2005;Alramahi et al 2008;Ng et al 2009;Khosravi et al 2010;Hoyos et al 2015). In addition, in some recent studies, researchers investigated the effect of suction on the strain-dependent shear modulus, soil volume change, stress path, pore pressure, and drainage conditions in cyclic triaxial systems (Cui et al 2007;Craciun and Lo 2010;Biglari et al 2011;Kimoto et al 2011;Cary and Zapata 2016). However, the effect of the degree of saturation on soil stiffness for medium to large strain levels still requires further examination, especially in relation to dynamic analysis.…”

The effective stress concept and the evaluation of changes in pore air pressure under jacketed isotropic compression tests for dry sand based on the 2‐phase porous theory

“…On the other hand, if m ≠ 1 and/or C s /C b ≠ 0, Terzaghi effective stress equation Δσ ′ = Δσ − Δu does not hold exactly because the volumetric strain of the soil skeleton is not 0 in Equation 24 under undrained conditions. It is worth noting that the mean effective stress increment is 0 under undrained isotropic compression conditions and the associated strain is 0 from the perspective of Terzaghi effective stress concept.Substituting Equation 19 into Equation 24, we have…”