LATERAL RESPONSE OF TEST PILES IN FROZEN AND THAWED GROUND

• Main Authors: J. Leroy Hulsey, Horazdovsky, Jacob, Marx, Elmer, Zhaohui Yang
• Format: Conference Object
• Language: unknown
• Published: Network for Earthquake Engineering Simulation (NEES) 2014
• Subjects: Fairbanks; permafrost; Alaska
• Online Access: https://dx.doi.org/10.4231/d3r20rx1b; https://nees.org/resources/11549

Consider that seasonal frost will have a significant effect on soil-structural stiffness (lateral seismic response) and therefore, these phenomena should be accounted for when designing structures located in northern states. As early as 1973, Stevens reported that soil stiffness can increase during freezing by as much as two orders of magnitude. This paper provides the findings from three laterally loaded field tests on 16 inch diameter steel jacketed reinforced concrete pilings embedded in seasonally frozen silt with the pile tips approximately 1-ft above permafrost. Two test piles were driven 20 feet into Fairbanks silt about 1.5 miles from Fairbanks, Alaska at a CREEL test site. A 24 inch steel jacketed reinforced concrete reaction pile was located between and in line with the test piles. Both test piles were embedded in Fairbanks silt. Except for 1-ft gravel layer atop the ground at the north pile, the subsurface soil conditions at both test piles was essentially the same. 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. Where possible, the recommended FHWA guidelines for conducting tests on laterally loaded piles were followed for this research. During thawed conditions, displacement control was used for conducting the cyclic load tests. However, during winter months, it was quickly determined that load control was required for conducting the cyclic load tests. Soil temperatures for these tests ranged from thawed to – 0.4 F (-18 C). Shear demand on the piles increased by over 400 percent. The results of these tests showed that the test pile behaved similar to the same pile idealized as fix cantilever beam with the location of the fixed end being just below the ground surface. Depth to fixity was less than 0.75 pile diameters for the frozen condition. In this case, a 16-inch steel jacketed reinforced concrete pile may be considered fixed when the pile is embedded in frozen Fairbanks silt in interior Alaska. For the thawed condition, since the pile experienced short foundation behavior, the depth of fixity for a long foundation still must be determined by other methods. In this study, the depth of pile embedment for the thawed condition acted like a short pile and this precluded finding an equivalent depth of fixity. The soil springs used to model the pile in the frozen condition are significantly stiffer in the frozen condition in comparison to the unfrozen condition. The spring stiffness needed to model the pile increased by two orders of magnitude, from thawed to frozen. A soil spring representing the top few inches of soil and a 1 inch long pile section, the force required to displace a soil spring 0.5 inches went from 310 pounds in summer (when the surface was thawed) to over 27,500 pounds in winter (when the surface was frozen).