Ground freezing impact on laterally loaded pile foundations considering strain rate effect

Evidences from past winter earthquakes and field experiments have shown that seasonally frozen ground can have significant impact on the lateral performance of bridge pile foundations. This paper proposed a modified von Mises model to account for the effects of strain rate, temperature and dry density in simulating the mechanical behavior of frozen Fairbanks silts and presented a procedure for material model updating with strain rate evolution. A FE model was calibrated by using data from field experiments of large-scale steel-jacketed reinforced concrete piles during lateral loading and was then used to conduct inelastic pushover analyses to assess the impact of ground freezing on the laterally loaded pile performance. Modeling results from both the frozen and thawed conditions including lateral load-deflection response, the soil-pile interface behavior including gap size and distribution, yielding and rupture of the steel jacket and rebars, crack/crush of the core concrete, plastic hinge length, displacement capacity, and soil yielding zone were presented and compared. Favorable agreement between the FE modeling results and field experimental data shows that the modified von Mises model and the FE model are capable of capturing the essential features of the frozen soil behavior and the soil-pile interaction during lateral loading. Results reveal that the strain rate within the seasonally frozen layer generally decreases with increasing depth and distance from the soil-pile interface, and variation of three to four orders of magnitude in the strain rate can occur within the seasonally frozen layer. Such large variation in the strain rate can have significant impact on the performance of laterally loaded piles at the frozen condition and its effects should be accounted for. It was also found that ground freezing can transform a soil-pile system from ductile behavior to brittle one, and it is imperative to consider its impact in the seismic design of bridge pile foundations in cold regions.