Shear strength degradation of fine-grained frozen soils due to a rise in temperature
Date
2025-05
Authors
Emami Ahari, Hossein
Major Professor
Advisor
Ajmera, Beena
Rutherford, Cassandra
Trinidad, Yuderka
Arenas Amado, Antonio
Xia, Wenjie
Committee Member
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Abstract
As climate change causes a rise in the temperature of the permafrost, there is an increase in the unfrozen water content, leading to a reduction in the shear strength of the underlain soils. This makes infrastructure more vulnerable to damage and failure. In this study, the shear strength of fine-grained soils composed of kaolinite, montmorillonite, and quartz is examined. A temperature-controlled direct shear device was developed to measure the shear strength of frozen soils under different temperatures and normal stresses. This device includes a custom direct shear box, with hollow components within the top half, bottom half, and shear cap. The box is connected to a chiller system that circulates glycol at a specific temperature within the hollow spaces to either freeze or thaw the specimen. Additionally, the temperature-controlled direct shear apparatus is insulated using insulating sheets and a high-density polystyrene enclosure. Statistical methods, specifically the Design of Experiment (DoE) approach, were employed to assess the effects of temperature, normal stress, and plasticity characteristics. The results indicate that an increase in temperature moving the soil from the frozen to the thawed state causes the shear strength of the frozen soils to reduce. However, after thawing, a slight increase in peak shear strength was observed due densification of the soil under the applied normal stress. The study also found that both the cohesion intercept and the friction angle of the frozen soils decrease with an increase in temperature, with cohesion showing a more pronounced reduction. Moreover, an increase in the liquid limit resulted in a reduction of both these shear strength parameters. Specimens with kaolinite as the dominant clay mineral exhibit a marked decrease in cohesion and friction angle as kaolinite content increases, though the effects stabilize at higher proportions of the clay mineral. In contrast, specimens with montmorillonite as the dominant clay mineral demonstrate a more complex interaction, where increased montmorillonite content did not impact the cohesion intercept significantly but the friction angle decreased due to finer particles and unfrozen water content acting as a lubricant. The DoE approach provided useful insights into the significance of different parameters, where temperature was the dominant factor influencing peak shear strength, while soil type and normal stress had smaller direct effects. In addition, the predictive capabilities of DoE were limited by an inability to account for critical boundaries and fundamental differences between the behavior of frozen and unfrozen soils. The DoE approach also found that an increase in the temperature from -10℃ to the thawing point of specimens resulted in a decrease in the peak shear strength. Further increases in temperature from the thawing point to +4℃ densified the soil, leading to a slight increase in the peak shear strength.
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dissertation