Numerical modeling, characterization and monitoring of the seasonal behavior of expansive clays

Abstract

Seasonal ground movements associated with the swelling and shrinking of expansive clays represent a major cause of damage to buildings, roads, and pipelines. The processes that give rise to these movements involve complex ground-atmosphere interactions (due to combined effects of infiltration and evaporation), combined with highly non-linear hydraulic and mechanical properties of partially saturated clay. This thesis presents an integrated study to measure and interpret long-term ground movements at a greenfield test site in Mustang Ridge (MR), Texas, adjacent to toll road SH130. The site is underlain by almost 12 m of high plasticity (montmorillonite-rich) clay. We designed and installed an autonomous field station that measures local weather conditions together with sub-surface water contents, using WCR reflectometers, and deformations, through a novel system of string-pot potentiometers.

We have obtained online data from the site for more than 3 years and observed a seasonal range of 50 mm in ground surface movements. We have investigated the engineering properties of the MR clay using samples extracted during site investigation and lab tests on compacted specimens of blended/reconstituted clay. These data are then used to calibrate constitutive models of suction-water content (SWCC), hydraulic conductivity and 1D compressibility for the MR clay. Numerical analyses of non-linear coupled flow-deformation in the partially-saturated soil column has been carried using a customized finite difference and finite volume framework (MPME) implemented within MATLAB. The MPME framework enables the representation of specified atmospheric boundary conditions (either as specified fluid pressures or fluxes) and hence, can simulate periods dominated by rainfall-induced infiltration or net drying by evapotranspiration.

Parametric MPME analyses are used to interpret field measurements at the test site to explain seasonal fluctuations in average strains and water contents and hence, to interpret the active depth in the MR clay. The results show that surface cracks influence the transient response of ground movements by significantly increasing the hydraulic conductivity of the medium. This feature affects the response rate of soil deformations to atmospheric phenomena. This behavior was simulated through the use of a constitutive model proposed by Stewart et al. (2016a) that quantifies the development of desiccation cracking. By improving our understanding of sources of seasonal ground movements, the research provides the basis for a more robust design of foundations and roadbeds in areas underlain by expansive clays.