Load Transfer and Creep Behavior of Pile Foundations in Frozen Soils

Pile foundations have long been used in cold region to support structures and infrastructure situated on frozen grounds. The main issue associated with the present design approaches are that they were developed based on data from limited observational studies on performance of pile foundations in field and lack important information on soil type, ground temperatures, and normal stress conditions. Furthermore, there is a lack of clear guidance for design of some of the commonly used types of pile foundations such as helical piles in frozen ground. Such short comings contributed to a wide range of uncertainties associated with predicting the load carrying capacity and creep behavior of piles installed in ice-poor and ice-rich soils. Ultimate load carrying capacity, for instance, has been often correlated to the shear strength of the surrounding frozen soils using a surficial roughness factor "m". This roughness factor is different for various pile materials (e.g., steel, concrete, timber), but often assumed constant for any soil type, ground temperature, and stress condition. This study investigates the load transfer mechanism and creep behavior of concrete piles, open-ended steel pipe piles, heliacal piles, and grouted shaft helical piles installed in frozen ice-rich and ice-poor soils in the field. The evolution of unfrozen water content and the associated thermally induced normal stress and adfreez strength of the pile-soil interfaces following to soil freeze-back after pile driving was also studied using laboratory experiments. The effects of strain rate on load carrying capacity of model steel piles in ice-poor and ice-rich soils were also investigated. The results showed that the roughness factor "m" may not be considered constant for a given pile material but rather changed as a function of ground temperature. In addition, the roughness factor was significantly different for a given pile installed in different soil types. The load carrying capacity of pile foundations in frozen ground was influenced significantly by the loading rate, showing lower values under slower loading condition. Modified forms of previously proposed theoretical models for predicting load carrying capacity and creep behavior of piles in frozen ground were introduced.