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