Seasonal Effects Of Frozen Soil On The Stiffness Of Bridge Piles

Abstract

In the northern regions, the upper layer of soil is frozen throughout winter months. Soil stiffness can be expected to increase several orders of magnitude as it changes from thawed to frozen. Thus, pile foundation systems embedded in frozen soils are considerably stiffer during winter months when subjected to lateral loads. This thesis explores and quantifies stiffness change for 16 inch diameter steel jacketed, reinforced concrete pilings in seasonally frozen silt. Two test piles were driven 20 feet into silty soil at a site approximately 1.5 miles from Fairbanks, Alaska. Three quasi-static lateral load cyclic tests were conducted on the piles throughout the year; one in September when the soil was thawed, the other two in January and March with frost depths of 4.5 and 7.5 feet respectively. Soil temperatures ranged from thawed to -18 degrees C. The shear demand on the piles increased by over 400 percent. Depth to fixity changed from approximately 6 pile diameters (thawed) to less then 0.75 pile diameters (frozen).