Pile foundation behavior in unsaturated frozen soil

Abstract

This study objectives to examine the deformation pattern of deep foundations in unsaturated frozen soils during seasonal periods of freezing and thawing, with a focus on the Astana soils of Kazakhstan. This study seeks to deepen the understanding of the pile foundations behavior in soils with unsaturated and frozen states, which are simultaneously present and affected by matric suction and temperature changes. Laboratory tests are performed to define the index properties of the chosen soil sample, including the liquid limit, plastic limit, and characteristics from the compaction test of the soils. Unsaturated soil tests are then performed to establish the Soil-Water Characteristic Curve (SWCC) of the soils. Utilizing established relationships, the SWCC obtained from the High Suction Polymer Sensor is correlated with the Soil Freezing Characteristic Curve (SFCC) using the principles of cryogenic suctions and the Clapeyron equation. A numerical model was developed for pile foundation behavior under representative climatic conditions for Astana using a coupled thermo-hydro-mechanical numerical model built-up in PLAXIS software with the constitutive model considering frozen and unfrozen soils. Suction-dependent hydraulic properties based on the SWCC, local freezing depth, temperature variations, groundwater table conditions, and surface evaporation effects were included in the numerical model. The numerical model was run for different suction conditions, and it was observed that an upward deformation was induced due to frost heave, and settlement and deformation occurred due to thawing after completing the thermal cycle. It was also observed that with a rise in suction due to groundwater table (GWT) fluctuations, an increase in displacement was observed at all stages of temperature conditions, verifying the impact of unsaturated conditions on effective stress distribution. For Astana soil, it was observed that an upsurge in suction causes an increase in final displacement from 6.094 mm at suction of 48 kPa to 7.163 mm at suction of 100 kPa, and further increase in deformation was observed due to inclusion of evaporation at 7.508 mm. The results show that the behavior of a pile foundation in unsaturated frozen soils is influenced by the combined action of thermal loading, suction variation, groundwater state, and climatic drying. The research will help better fathom the behavior of pile foundations in cold geotechnical problems and will provide a useful tool for including unsaturated and freezing effects in numerical calculations for foundation behavior.