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Geology Presentation - Full
Citation preview
Suspension Bridge
Sadia KhanTerryn Kuzyk
Brogan Gordon-CooperPaul Coutts
Presentation Outline
• Introduction• Geological
Setting• Bridge
Components• Engineering
Solutions• Maintenance• Conclusion Tacoma Narrows Bridge
Introduction
Structure• Suspension Bridge
Geological Setting
• Bedrock is granite• Bedrock is beneath
300m of glacial till & 200m of silt/mud
• Ends must be anchored in highly-fractured shale that dips towards the water
Geological Settings
1. Shale2. Glacial Till3. Granite Bedrock 4. Unconsolidated Silt and Mud
Shale
• Lithified Mud• Highly Bedded & Fissile• Weak Rock• Fractured Shale is
Permeable & Susceptible to Fluid Migration
Problems with Fractured Shale
• Water can creep through the cracks increasing weathering of the rock
• Clay within the shale can expand and contract resulting in slope failure
• Shale rapidly scours• Excavation causes stress release• Since it dips towards the water, hydrostatic pressure
develops forcing bedding planes apart
• Silt: fine or intermediate-sized particles from various mineral
• Mud: silt and clay
Problems with Silt and Mud
Silt and Mud
• Mass movements can occur from saturation• Development of quick clays (clays originating from
marine environments)
Glacial Till
• Poorly sorted • Primarily angular in
nature• Contains almost all rock
sizes• Result of glacial
movement
Problems with Glacial Till
• Unsorted sediment introduces risk of hitting large boulders, which can interfere with construction
• Mass movements can occur when overriding silt and mud layer starts to slide
Granite Bedrock
• Extrusive igneous rock consisting of mostly quartz, orthoclase and biotite
• Relatively hard rock therefore ideal for placing foundation of structures
• 500m below surface (our geological location)
Problems with Granite Bedrock
• Depth may not be feasible for constructing anchors and piers
• Excavating hard rocks such as granite may abrade expensive machinery
Suspension Bridge Components
• Main Cables • Hangers • Deck • Piers • Anchors
Engineering Solutions Outline
Problems Solutions
Solutions for Anchoring into Fractured Shale
• Anchor directly into granite bedrock instead of fractured shale
• Locate anchor in areas with minor fractures in shale
• Place concrete slab over fractured shale • Use rock bolts to restrain fractured rock
Anchoring into Granite Bedrock
• Granite is an ideal rock to anchor into due to its high compressive strength and lack of bedding
• Unreasonable solution due to its location at 500m depth and highly costly (35%-50% increase in cost)
• Project descriptions specifies anchoring into fractured shale
Locating Ideal Placement • Fractures in Shale are of interest to the oil and gas industry• Vertical Drilling is commonly used but many bore holes maybe
required • Horizontal drilling is a relatively new method and it can cover a
larger area • Horizontal drilling is more expensive• Drilling and closing the bore hole maybe difficult and time
consuming
Locating Ideal Placement
Fracture zones compared to host rock have:• Lower resistivity (higher conductivity)• Lower seismic velocity • Cause scattering of seismic and Ground
Penetrating Radar (GPR) waves• Topographic depressions due to fracturing and
weathering
Placement of Concrete Slap Over Shale
• Ends of the main cables need to be hooked into the concrete slab which will carry all of the loading from the bridge
• Need to ensure shale is consolidated under the concrete slab • Concrete slap cannot be placed parallel to the shale bedding• Concrete slab cannot be placed parallel to the dipping angle of shale • Rock bolts and tendons used to keep shale intact
Placement of Concrete Slap Over Shale
• Post-tensioned Concrete will be used
• Concrete slab will withstand the tension from the cables and the compressive loads from the weight of the anchor
• Expansion and contraction from underlying rock are supported by the concrete without significant flexure
Anchors of the Golden Gate
• The Golden Gate is also anchored into shale using a concrete slab
• Over one million tons of concrete was used to build the anchors that hold the cables in place
Scour is one of the top three causes of bridge failure.Approximately 60% of bridge failures are caused by scour at the abutments and the piers.
Problems with Fractured Shale
Bridge Maintenance
• Initial Inspection• Intermin Inspection• Damage (Emergency)
Inspections
• Underwater Bridge Inspection
• Fracture Critical Inspection
Bridge Maintenance Underwater Bridge Inspection
Fracture Critical Inspection
Frequency of Inspection: Every Five Years Every Five Years
Primarily Looks for: Scour Visual Cracks, Rust, Weld Termination, Arc Strikes/Scars & Cross Section Change
Methods/Devices: Black & White FathometerColor Fathometer
Dye Penetrant, Magnetic Particle, Ultrasound
Structure Monitored: Piers Any area with non-redundant tensile stress