Seismic Analyses for the Fourth Bore of Caldecott Tunnel

This paper presents a brief summary of seismic analyses for Caldecott Tunnel Improvement Project. It includes seismic hazard analysis, wave scattering analyses to evaluate soil structure interaction (SSI) for the proposed 4^th bore, and development of seismic coefficient for slope stability design. Probabilistic Seismic Hazard Analysis (PSHA) was used to establish the rock motion criteria; 1,500-year and 300-year return periods were adopted for the Safety Evaluation Earthquake (SEE) and Functional Evaluation Earthquake (FEE), respectively. The wave scattering analyses were performed using the spectrum-compatible time histories to account for localized geologic features including site specific topography and soil properties specific to the current project. The analyses enable evaluation of the degree of ground distortion that contributes to racking and ovaling of the tunnel liner. Seismic coefficients for several critical slopes near portals were also derived considering the concept of wave propagation within the slope mass.