Geotechnical Assessment of the 1930s Jacques Cartier Bridge

ABSTRACT: As part of the seismic performance assessment of the Jacques Cartier Bridge in the Montréal region in Québec, a detailed geotechnical investigation was carried out. The bridge represents a lifeline bridge as defined in CAN/CSA-S6-14 and is an important transportation link between the Island of Montreal and the South Shore of the St. Lawrence River with a total span of more than 3 km. The bridge is supported on a range of different foundation types, including large piers supported directly on bedrock, large piers supported on over 140 wood piles, small piers supported directly on soils and small piers supported directly on wood piles. The bridge is supported on 60 piers and 2 abutments covering range of spans from about 30 m to 332 m and includes île Sainte-Hélène Pavilion structure within its overall length. The ground conditions cover a wide range of bedrock types, from breccia intrusions to shale sedimentary rock, and soils ranging from silty clay to loose sands to dense glacial tills. Deep fills were also present in several locations, resulting from the original bridge pier work to the construction of the St. Lawrence Seaway. The field investigation included eight traditional boreholes with standard penetration tests (SPTs), six downhole geophysics shear wave velocity profiles, and four cone penetration tests (CPTs). A series of five test pits were also excavated on the Montreal shore to expose and assess some of the wood piles dating back to the 1930s. The results of the field investigation were used to assess site response relative to seismic loading, the potential for seismic liquefaction of certain loose soil horizons, and to model the dynamic axial and lateral load capacity and stiffness for the multitude of foundation types present. This case study reveals several interesting aspects relative to the seismic performance of the structure from the potential for seismic liquefaction, the dynamic response of specific sections of the bridge to seismic loads, the lateral dynamic stiffness and capacity of the existing foundation systems. Recommendations for additional field investigations as part of the next phase of bridge assessment were also provided.