Suction Stress in Unsaturated Soils Considering Hydraulic Hysteresis

Aims: To develop a flow-moisture model that allows determining the variation of suction over time, as well as the suction stresses, using the finite element method in a two-dimensional model of unsaturated soil through an analogy with a transient thermal problem.

Study Design: The variables used in this study were soil suction, hydraulic conductivity, diffusivity and degree of saturation which was represented as the parameter of the Bishop’s effective stress equation.

Place and Duration of Study: Graduate Engineering Department, Universidad Autónoma de Querétaro, between November 2019 and August 2020.

Methodology: To establish the model, experimental Soil-Water Retention Curve was taken from Galaviz (2016). With this information, the curves of hydraulic conductivity and diffusivity were calculated with the methods of Fredlund et al. (2012) and Li (1996). In ANSYS 19.2, an analogous transient thermal analysis was run to determine suction changes over time in a 12 x 2.4 meters two-dimensional medium with an impermeable membrane at the center of its surface which was 4.8 meters long. Through these suction changes, the hydraulic hysteresis algorithm presented by Zhou et al. (2012) was used to calculate the respective degrees of saturation, which were considered as the parameter to obtain the suction stresses.

Results: The changes in soil suction, degree of saturation and suction stress were properly modeled.

Conclusion: When considering the hydraulic hysteresis cycles, both spatial and temporal variations behaved in a similar way in the parameters as well as in the suction stresses. Such stresses depended on the analysis period, increasing in the dry season, according to the precipitation-evapotranspiration model, and decreasing in the wetting season. A time lag was observed between the maximum and minimum stresses as greater depths were studied. Along the horizontal axis, considering the same depth, the stresses varied more in the areas adjacent to the impermeable membrane, while at the center this variation was practically null.