Northumberland Strait Crossing Confederation Bridge New Brunswick, Canada Will Astley Jonathan Richards

Outline Introduction Geology Foundation Design Considerations Solution Construction Sequence

Introduction Longest bridge in Canada at 13 km – Longest bridge in the world spanning ice-covered water Crucial transport link between PEI and New Brunswick – Replaced ferry service over the strait Completed in May 1998 after 4 years of construction Total construction cost $1 bn CAN

Geology - History Developed in pre-glacial times Permo-carboniferous rock overlain by glacial and marine sediments Surfical deposits ranging from fine sands and silts to 1 m diameter sandstone boulders Glacial till the predominant deposit varying thickness of 0.5 – 12 m In post-glacial times there was a number of changes in sea level Between 13,000 – 7,000 B.P. Glacial rebound and sea level changes produced undulating land Estimated at around 5,000 B.P. the strait became a continuous body of water

Geology - Stratigraphy Extreme vertical/horizontal variations owing to the estuarine/fluvial environment of the strait Complex interlayered sequence of sandstones, mudstones, siltstones and conglomerate

Geology – Rock Types Glacial Till: varied grading, mixture of sand/silt, 193 kPa, φ’ = 34°, c’ = 95 kPa Sandstone: fine/medium grained, ranging from well to poorly cemented, 16 – 22 Mpa, oriented with land Mudstone: poor quality, indurated rock/hard clay, 3.5 Mpa, oriented with bedding planes Soft Mudstone: “silty clay”, friable/brittle, 43 – 280 kPa, φ’ = 15°, oriented with bedding planes

Foundation Design Considerations Loading – Ice, Wind, Impact Short Construction Window Overburden (up to 12m in places) Soft Mudstone Issues – Bearing Capacity / Sliding Resistance Seabed up to 40m below water surface Three main criteria – Bearing Resistance – Sliding Resistance – Settlement

Foundation Design Considerations Bearing Resistance – Rock Mass Rating (RMR) classification system used for jointed rock mass. Use Engineering experience, judgement, geological review, core analyses and case record studies Sliding Resistance – Need to resist lateral forces Settlement – Computer Analyses Conducted – 10mm predicted

Construction Sequence Dredge Overlying Material Excavate Trench Grout Bags Place Pier Tremie Concrete