Variations in optimal seismic intensity measures for shallowly buried bias loess tunnels

Uneven terrain significantly increases the seismic risk of tunnels in loess deposits. To investigate the variations in optimal intensity measures (IMs) for shallowly buried loess tunnels considering biased terrain, nonlinear dynamic analyses were conducted to obtain seismic responses validated by the actual damage pattern. Then IMs were evaluated based on the automatic calculation of the time history damage index fulfilled by a compiled Python program. Results showed that the plastic strain zone progressively developed and extended from the vault to the central slope surface with increasing seismic intensities, ultimately causing shear failure to the tunnel. For IMs at the slope top, peak ground velocity (PGV) (ζ =0.15), velocity spectrum intensity (VSI) (ζ =0.20), and peak spectrum velocity (PS_v) (ζ =0.22) were all suitable for seismic fragility assessment. The VSI (ζ =0.17) was optimal, followed by PGV (ζ =0.19) and PS_v (ζ =0.2) for those at the slope foot. Acceleration-related IMs were more sensitive to terrain variation. Comparative analyses demonstrated smaller damage probabilities for the IMs at the slope top than those at the slope foot under the same intensity level. The impact of unfavorable terrain on tunnels was accentuated as those located in uneven mountainous regions became more vulnerable to ground shaking.