Hydromechanical behavior of unsaturated soils: Interpretation of compression curves in terms of effective stress

Abstract

This state-of-the-art paper on the hydromechanical behavior of unsaturated soils focuses on the interpretation of the compression curves of unsaturated soils in terms of effective stress, with the goal of understanding the relative impacts of suction on the effective stress, net yield stress, effective yield stress and slope of the virgin compression line (VCL) during a monotonic increase in net stress. A database of compression curves was compiled for both high and low plasticity fine-grained soils under a wide range of suctions, isotropic or oedometric stress states, drainage conditions (constant suction or constant water content) and preparation techniques (impact compaction, static compaction, consolidation from slurry). Most of the compression curves plotted in terms of effective stress revealed a consistent hardening response with increasing suction and a slight suction dependency on the slope VCL. Interpretation of the compression curves in terms of effective stress led to load-collapse curves with a similar shape for a wide range of soils. Most soils evaluated had a greater rate of increase in effective yield stress with suction than the rate of increase in suction stress with suction, implying that these compacted soils may be susceptible to collapse upon wetting. Inconsistent trends were observed in some studies, which were attributed partially to natural variability but also experimental issues and limitations on the range of conditions investigated. Accordingly, recommendations are provided for future studies on the compression curves of unsaturated soils to ensure that results can be clearly interpreted in terms of effective stress.

Compression Response of Sedimented Unsaturated Soils

“…After yielding, the compression curves for unsaturated soils are expected to converge with the compression curve for saturated soils at mean stresses greater than 10 MPa (Mun and McCartney 2017). However, the review of compression response of unsaturated soils reported by McCartney and Behbehani (2021) indicates that the compression curves are highly nonlinear and depending on the maximum stress applied the curves may show a diverging trend (e.g., Maatouk et al 1995) or a parallel trend (e.g., Sharma 1998;Rampino et al 1999).…”

“…The modified model showed that the yield stress of soils that are initially compacted will have higher yield stress 7 with increasing suction than soils consolidated from a slurry. McCartney & Behbehani (2021) collected and analyzed compression curves from 25 studies in the literature for compacted and sedimented unsaturated soils to understand the impacts of suction on the suction stress at yielding, the apparent yield stress, and slope of the vertical compression line. In general, both sedimented and compacted soils showed an increase in yield stress with suction, and that suction generally had a greater impact on the yield stress than on the suction stress, but it was not possible to compare the role of soil preparation techniques on these variables for a single soil.…”

“…In general, both sedimented and compacted soils showed an increase in yield stress with suction, and that suction generally had a greater impact on the yield stress than on the suction stress, but it was not possible to compare the role of soil preparation techniques on these variables for a single soil. McCartney & Behbehani (2021) noted that it was important to measure the degree of saturation during compression to interpret the compression curves in terms of effective stress. Mun and McCartney (2017) proposed a general hypothetical representative of the drained hydro-mechanical compression response of compacted soils to high stresses, which was adapted in Figure 1 for the case of soils sedimented from a slurry, unloaded, desaturated to different suction, then loaded to high stresses.…”