Frozen and Unfrozen Shear Wave Velocity Seismic Site Classification of Fairbanks, Alaska: Fid-Bau Portal

Frozen and Unfrozen Shear Wave Velocity Seismic Site Classification of Fairbanks, Alaska

Cox, Brady R. / Wood, Clinton M. / Hazirbaba, Kenan

Relationships between seismic site classification and surficial geology for the city of Fairbanks, Alaska, for both frozen (winter) and unfrozen (summer) surface conditions are investigated in this article. The work is based on 59 shear wave velocity ( $V_{s}$ ) profiles collected using the spectral analysis of surface waves (SASW) method. Forty-three $V_{s}$ profiles were collected during the summer when the ground surface was fully thawed, and 16 $V_{s}$ profiles were collected by reoccupying some of the test locations during fully frozen winter ground surface conditions. Relationships between mean $V_{s}$ over the top 30 m of the subsurface ( $V_{s 30 m}$ ) and surficial geologic units in Fairbanks were developed, permitting estimates of seismic site classification throughout the city for both frozen and unfrozen conditions. Much of Fairbanks was determined to have been founded on NEHRP Site Class D material when unfrozen. When frozen, the $V_{s}$ of the near-surface material increased significantly (by 700% on average). However, this drastic increase in stiffness was typically limited to the top 1 to 2 m of the subsurface and only resulted in an increase in site class from NEHRP D to NEHRP C at three of the 16 sites retested during the winter.