Characterization of Seasonally Frozen Soils for Seismic Design of Foundations

An experimental investigation was performed on five widespread soil types common in the United States to characterize the effects of freezing temperatures on the unconfined compressive strength ($q_u$), the modulus of elasticity ($E$), and strain at the unconfined compressive strength (${\epsilon }_{q_u}$). Soil specimens were subjected to monotonic and cyclic loading with varying strain rates at temperatures ranging from 20 to $-23°\text{C}$ (68 to $-9.4°\text{F}$). When compared with test results at 20°C (68°F), testing at $-20°\text{C}$ ($-4°\text{F}$) showed an increase in $q_u$ by a factor of 100, an average increase in $E$ by a factor of 300, and an average decrease in ${\epsilon }_{q_u}$ by 5% strain. Increase in the soil compaction, moisture content, and applied strain rate amplified the cold temperature effects on $q_u$. Additional testing at $-20°\text{C}$ ($-4°\text{F}$) resulted in an increase in ${\epsilon }_{q_u}$ with no change in $E$ when the applied strain rate was increased. Cyclic experimentation produced data trends comparable to the monotonic experimentation for the mechanical properties but allowed residual deformation as a function of cold temperature to be identified. To assist with current seismic design practice, experimental trends were incorporated into a $p\text{-}y$ curve development and the impact of observed soil response as a function of temperature is demonstrated using a series of pushover analyses on a column continued into the subsurface as a drilled shaft foundation.