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Mechanical Degradation of Experimentally Synthesized Rhizosphere with Herbaceous Plant Roots Subjected to Freeze-Thaw cycles

Mahdavi Rad, Mohammad Erfan | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58718 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Sadeghi, Hamed
  7. Abstract:
  8. Natural phenomena such as landslides claim numerous human lives annually. A portion of these landslides occurs in cold regions as a consequence of freeze–thaw cycles. Such natural freeze–thaw processes can alter the mechanical characteristics of surface soil layers, thereby leading to operational damages and to infrastructures in these regions. Accordingly, the primary objective of this study is to investigate the effects of freeze–thaw cycles on the mechanical and shear behavior of vegetated samples subjected to low normal stresses, equivalent to the shallow root-penetration zones, using a direct shear apparatus to best replicate field conditions. For this purpose, fescue, a cold-climate grass species native to many cold regions worldwide, was selected. The plants were cultivated in newly developed laboratory molds and, after one month, were removed and subjected to direct shear tests following 0, 1, 3, 5, and 10 freeze–thaw cycles. After testing, the sheared specimens were extracted to measure root biomass and to determine the ratio of root cross-sectional area to soil cross-sectional area (RAR). To achieve this, samples were bisected, and the RAR parameter was quantified through photographic imaging and image analysis. The results revealed that vegetated samples exhibited a gradual mobilization of root-reinforced resistance, leading to continuous increases in shear strength, up to 14 mm of displacement, while displaying a ductile behavior. Notably, the presence of roots induced dilative behavior during shearing, with the maximum dilation occurring within the range of 0.25–0.35% dry root mass per unit soil volume and observed predominantly at the fifth freeze–thaw cycle. Conversely, freeze–thaw cycles degraded the root–soil bonds, increased the volume of frozen water, and reorganized soil particle structures, collectively reducing the shear strength of vegetated samples by up to 29%. Nevertheless, the findings demonstrated that even after multiple freeze–thaw cycles, vegetated samples retained higher shear strength compared to unvegetated controls, with the strength increase remaining at least 1.5 times greater. This highlights the potential of using vegetative systems as a sustainable strategy to enhance the stability and geotechnical performance of slopes and green earth structures in cold climates
  9. Keywords:
  10. Vegetation ; Freeze and Thaw Cycles ; Shear Strength ; Shallow Landslide ; Root-Soil Composites ; Tall Fescue Plant

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