Influence of Moisture-Temperature-Vegetation on Expansive Soils and their Implications on the Slope Stability

Abstract

The annual economic losses due to expansive soils problems are estimated to be several billion dollars worldwide, and these damages are expected to further increase with climate changes associated with global warming. Climate-induced processes such as freeze-thaw (FT) cycles markedly alter the thermo-hydro-mechanical (THM) behavior of expansive soils, leading to significant degradation of their strength and, consequently, to the failure of geotechnical infrastructures. Therefore, a comprehensive understanding of the mechanical behavior of expansive soils for addressing the influence of climatic conditions extending constitutive models for capturing their complex responses, is of paramount importance. Moreover, effective mitigation strategies are crucial for ensuring the long-term serviceability and resilience of infrastructures founded on expansive soils.

During the past two decades, significant advances have been made based on experimental and theoretical studies related to the macro- and micro-mechanical behaviors of soils subjected to FT cycles. However, a quantitative framework that bridges microstructural evolution and the macroscopic mechanical response remains limited. Likewise, only a few constitutive models are available that can adequately represent the degradation mechanisms induced by FT cycles. Recent investigations suggest vegetation reinforcement has proven to be an environmentally sustainable and effective measure for mitigating the adverse effects of climate change on soil slope stability. Nevertheless, constitutive models that can rationally describe the mechanical behavior of vegetated soils remain scarce.

The focus of this PhD thesis is to investigate the THM behavior of expansive soils subjected to FT cycles, with emphasis on their strength and deformation characteristics. The study also examines the reinforcing effects of vegetation on soil shear behavior. The overall objective is to establish a comprehensive understanding of the degradation and reinforcement mechanisms under environmental loading and to develop practical constitutive models capable of simulating complex stress-strain responses. The major research contents and findings are summarized as follows:

(1) The stress-strain responses of two FT-impacted expansive soils (Ningming and Nanjing clays) were systematically investigated through unconfined compression tests. Complementary mercury intrusion porosimetry (MIP) tests were performed to characterize the evolution of microstructural pore networks. A quantitative relationship between the shear strength and the surface fractal dimension (Ds) derived from MIP tests was established and referred to as the fractal strength criterion. Building on this criterion, an improved statistical damage constitutive model incorporating strain-softening effects was developed to describe the stress-strain behavior of FT-impacted soils.

(2) A hyperbolic decay model was proposed to characterize the strength degradation of soils subjected to FT cycles. Building upon this degradation law, a novel constitutive framework extending the Disturbed State Concept (DSC) was developed, with particular emphasis on post-peak strain-softening behavior. The proposed DSC-based model incorporates five physically meaningful parameters derived through a systematic calibration procedure. The model's predictive capability was successfully validated using experimental data for four different soils reported in the literature.

(3) The volumetric behavior of various soils during FT cycles and subsequent one-dimensional compression was investigated through experimental and theoretical studies. A new model extending the DSC was proposed to predict the compressive response of FT-impacted soils. Empirical relationships were established to link key model parameters with the soil state variable, thereby simplifying the prediction process. The model's applicability to various soil types, including low- and high-plastic clays and organic soils, was verified through successful validation against published datasets.

(4) The influence of plant roots on the full-range shear stress-shear displacement (SS) behavior of vegetated soils was examined through a combination of experimental testing and theoretical modeling. A new DSC-based constitutive model was developed to describe the entire SS response of vegetated soils. The model effectively predicts the full range SS response of vegetated soils considering the influences of various factors such as the vegetation type, age, root distribution pattern, and testing conditions. In addition, it reliably captures the peak strength and strain-softening behavior associated with root breakage and slippage deformation.

(5) A comprehensive study was conducted on a seasonally frozen expansive soil embankment slope in Regina, Canada, to evaluate its long-term stability under FT cycles and climate warming. Numerical simulations from 2025 to 2100 were performed using temperature fields and factors of safety (FOS) corresponding to three representative climate scenarios (RCP 2.6, 4.5, and 8.5). The results show that slope stability is higher during freezing periods and lower during thawing periods. More importantly, the combined effects of FT cycles and progressive warming lead to a gradual reduction in FOS, with the largest decline occurring under the high-emission scenario (RCP 8.5). -

In summary, this research integrates experimental evidence, fractal analysis, and constitutive modeling within a unified DSC-based framework to elucidate the degradation mechanisms of expansive soils under FT cycles and the reinforcement effects of plant roots. The developed models provide a robust theoretical basis for predicting soil behavior considering climate changes for designing sustainable and resilient geotechnical infrastructures.