Bridge Geotechnical Considerations and Designing for Scour ...
Transcript of Bridge Geotechnical Considerations and Designing for Scour ...
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Bridge Geotechnical Considerations and Designing for Scour PART IIChristopher Byrum, Ph.D., P.E. GeotechnicalSoil and Materials Engineers, Inc.
Brian Barkdoll, PhD, PE ScourMichigan Technological University
Geotechnical Geotechnical Evaluations for Evaluations for B id D iB id D iBridge DesignBridge Design
USGS TopoAnd
Bedrock Topoto estimate
Depth to Rock
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USDA Soil SurveyFor Marsh Limits
Peat Marshes
Detailed Soil Survey: All New Alignments
Obtain Data from Existing Bridge Plans/Files
1. Existing Test Holes2. Bridge Deck Cross Section (between beams)3. Existing Utilities Type and Location4. “Construction History” Files Issues5 “Design History” Files Issues5. Design History Files Issues
Review All This Stuff and:1. Summarize Key Site Conditions2. Decide if New Test Holes are Needed3. Make the Field Geotechnical Evaluation Plans
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Big Drill Rig = Deep Holes
Small Truck Drill Rig
Small All-Terrain Drilling RigRemote Control for Safety
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MDOT “SKID” drill rig mounted to Pontoon Boat
“SKID” drill rig Set Inside a Cofferdam using a Crane
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Weak Cohesive Soils- Thin Wall Shelby Tube “Low Disturbance”
Very Weak Cohesive Soils- In-Situ Vane Shear Strength Testing
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m
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Soft-Soil Evaluation
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0 25 50 75Moisture Percent
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0 5 10 15 20SPT, bpf
Elev
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0 25 50 75Strength, kPa
TorvaneField VaneFor Design
Geotechnical Evaluation Case Study
A Very Difficult Test Hole
Existing Piers Settled and Cracked, Vibration Related
I-696 over Rouge River- Bridge Widening
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Pier Settled and Railing Crushed
Crushed Column
Bi C kBig Crack
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Typical Test Hole Frequency
BRIDGES1. One per substructure Unit (<50-ft wide)2. Two per Unit (50-100 ft wide)3. Three per Unit (100-150)………
APPROACHES1. Fill over Marsh (50-ft Spacing, muck rods)2. Fill over Good Dirt (250-ft Spacing)
Artesian Drilling Protocol:
1. Do not poke a hole into the artesian within the proposed cofferdam limits.
2. Do not poke a hole into the artesian where you cant seal it (Barge Test Hole performed in the middle of a ( g pRiver).
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Artesian Drilling Protocol:
3. Use Special Drill Rigs with ability to grout and drive casing (Water Well Rigs).
4. Do poke a few holes into the Artesian in select areas that can be sealed and measure:
A. the static head and flow rate.B. the cover soil type/thickness.C. the artesian soil type and thickness.D. depth to hard bottom below the artesian soil.
Artesian conditions were close to the Ground Surface and intense at this site
Flow out of SB5 Casing
Soil Sampling
Deep Test Holes with SPT Testing:•SPT Needed for Soil Strength Design Info.•Sewer Trench and Sheet Piling Designs.•Global Staility Analayses for Embankments and Walls.•Shelby Tube or Vane Shear Tests Very Weak Soil Deposits. •Drill Rigs get 75 to 150 linear feet of sampling per dayDrill Rigs get 75 to 150 linear feet of sampling per day.
SCOUR•Do Particle Size Gradation (Sieve/Hydrometer) tests/plots•Define Layers and Gradation to 8 meters below footing•Sometimes we get Channel Surface/Bed Samples (The Armor)
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Initial Questions/Data Before Setting the Geo Scope
1. Are we trying to save the existing pavement?No = No Coring Required, just thickness and type data.Yes = Get Core Samples of Existing Pavement and Samples of
Existing Base Materials for Gradation Analyses.
2. Are there new/relocated utilities? Data for Shoring/Dewatering
3. Unusual Existing Utilities needing special attention?Yes = Special Stuff May Be Needed.
4. Weak Soils/Swamps with Major Grade Increases/Widening?Yes = Deeper Borings and Special Stuff.
5. Do we need Retaining Walls or New Traffic Signal Poles?Yes = Deeper Borings and Special Stuff
Initial Questions/Data Before Setting the Geo Scope
1. Are we trying to save the existing pavement?No = No Coring Required, just thickness and type data.Yes = Get Core Samples of Existing Pavement and Samples of
Existing Base Materials for Gradation Analyses.
2. Are there new/relocated utilities? Data for Shoring/Dewatering
3. Unusual Existing Utilities needing special attention?Yes = Special Stuff May Be Needed.
4. Weak Soils/Swamps with Major Grade Increases/Widening?Yes = Deeper Borings and Special Stuff.
5. Do we need Retaining Walls or New Traffic Signal Poles?Yes = Deeper Borings and Special Stuff
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PEATM l
GAS
Widening Widening
Core Embank
Marl
FiberSEWER WATER
Initial Questions/Data Before Setting the Geo Scope
1. Are we trying to save the existing pavement?No = No Coring Required, just thickness and type data.Yes = Get Core Samples of Existing Pavement and Samples of
Existing Base Materials for Gradation Analyses.
2. Are there new/relocated utilities? Data for Shoring/Dewatering
3. Unusual Existing Utilities needing special attention?Yes = Special Stuff May Be Needed.
4. Weak Soils/Swamps with Major Grade Increases/Widening?Yes = Deeper Borings and Special Stuff.
5. Do we need Retaining Walls or New Traffic Signal Poles?Yes = Deeper Borings and Special Stuff
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Initial Questions/Data Before Setting the Geo Scope
1. Are we trying to save the existing pavement?No = No Coring Required, just thickness and type data.Yes = Get Core Samples of Existing Pavement and Samples of
Existing Base Materials for Gradation Analyses.
2. Are there new/relocated utilities? Data for Shoring/Dewatering
3. Unusual Existing Utilities needing special attention?Yes = Special Stuff May Be Needed.
4. Weak Soils/Swamps with Major Grade Increases/Widening?Yes = Deeper Borings and Special Stuff.
5. Do we need Retaining Walls or New Traffic Signal Poles?Yes = Deeper Borings and Special Stuff
SWAMPS: Embankment Widening Example
On-Going Slope Movements
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Test Locations
Muck Rod Tests
Deep Soil Borings:Sheet Pile and Pavement Design
SPT
SPT0 50
0 50
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The AnchorsCONTINUOUS GEOGRID #2
GWT
GEOGRID #3
CONTINUOUS GEOGRID #1
Placement of EPS and Geogrid behind sheeting.
View from Northwest of the Wall
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View from North Hillside, Looking South
View of Final Product, Looking West
40 ft.
R.R.
GEOTECHNICAL ENGINEERING PROCESS
Abutment AAbutment B
Pier 1 Pier 2 Pier 3
Approach Embankment
N or E
Existing Ground
Substructures-Footings, Columns and Walls
Superstructure-Beams, Deck, Joints, Railing….
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40 ft.
R.R.
State Route M-63 over CSX RailroadSt. Joseph, Michigan
PHASE I: PLANNINGSpan Lengths
PRIMARILY GEOMETRY“STANDARDS” vs. PROPERTY FEATURES.
Min. Height for a Train
Vert. Curve-Sight Distance @ 50 mph Existing Ground
40 ft.
R.R.
State Route M-63 over CSX RailroadSt. Joseph, Michigan
G.W.T. Sand fill
PHASE II: DESIGN-Acquire Geo Info
Very Dense Sands
Medium Dense Sands
Organic Clayey Silt (OH-marl)w% = 45 to 65, LL= 60 to 70, c = 500 to 1100 psf
Loose/soft sand, peat,sediment, and marl.
120 ft.
50 ft. Loose Sands
40 ft.
R.R.
State Route M-63 over CSX Railroad
G.W.T. Sand fill
DESIGN-Check Stability – MASS ROTATIONTraffic Loads
Weight
Center of Rotation
R
Very Dense Sands
Medium Dense Sands
Organic Clayey Silt (OH-marl)w% = 45 to 65, LL= 60 to 70, c = 500 to 1100 psf
Loose/soft sand, peat,sediment, and marl.
120 ft.
50 ft. Loose Sands
BYRUM-2000
Soil Shear Strengths
F.S > 1.3
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Sand Embankment
Light Slag EmbankmentAbutment Front Slope 1:2
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Light Slag EmbankmentApproach Side Slope 1:3
40 ft.
R.R.
State Route M-63 over CSX Railroad
G.W.T. Sand fill
DESIGN-Check Stability-SLIDING BLOCK
Traffic Loads
Active
Passive
Very Dense Sands
Medium Dense Sands
Organic Clayey Silt (OH-marl)w% = 45 to 65, LL= 60 to 70, c = 500 to 1100 psf
Loose/soft sand, peat,sediment, and marl.
120 ft.
50 ft. Loose Sands
BYRUM-2000
Soil Shear Strengths
F.S > 1.3
Passive
Sand Embankment
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Light Slag Embankment
Expanded Polystyrene (EPS) Ultra-Lightweight fill (10 pcf)
40 ft.
R.R.
State Route M-63 over CSX Railroad
G.W.T.
Geogrid
Sand fill
DESIGN REQUIREMENTS-STABILITY
pv< 2,500 psfpv< 3,200 psf
Lightweight slag fill (90 pcf)
$$$$$$$
Very Dense Sands
Medium Dense Sands
Organic Clayey Silt (OH-marl)w% = 45 to 65, LL= 60 to 70, c = 500 to 1100 psf
Loose/soft sand, peat,sediment, and marl.
120 ft.
50 ft. Loose Sands
BYRUM-2000
NOTE: For 40’ of sand, pv≈ 4,400 psf
40 ft.
R.R.
State Route M-63 over CSX Railroad
G.W.T. Sand fill
H-piles: Major Drag-down forces
DESIGN REQUIREMENTS-SETTLEMENTWick Drains-Accelerate Settlement
Very Dense Sands
Organic Clayey Silt (OH-marl)w% = 45 to 65, LL= 60 to 70, c = 500 to 1100 psf
Loose/soft sand, peat,sediment, and marl.
120 ft.
50 ft. Loose Sands
BYRUM-2000
H-piles: Drive to 110 ton, use 80 ton for bridge design
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Other Design ServicesContract Special Provisions
-Wait Periods for Settlement/Strength Gain-Geotechnical Instrumentation During Const.-Wick Drains to Accelerate Settlement-Special Materials; Properties and Control
State Route M-63 over CSX Railroad
Geotechnical Instrumentation during Const.- Possibly Allow Contractor to Proceed Early-Identify Nature of Unusual Movements-Justify (or not) Extra Expense Recommended-Justify (or not) Design Procedure Used.
Construction Services
BYRUM-2000
TYPICAL BRIDGE FOUNDATIONS
Bridge Scour Hydraulics
• Reasons why bridges fail• Scour caused by many flow patterns• In general water velocity high enough to• In general, water velocity high enough to
move sediment.
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Causes of Bridge Scour
• Contraction scour• Bed degradation• Vortices• Vortices• Out-flanking• River widening???• Abutment scour causes pier failure!
Contraction Scour
• For some bridges the width of the river has been narrowed to reduce span length.
• This smaller flow cross-sectional area leadsThis smaller flow cross sectional area leads to higher velocity (V=Q/A)
• If increased velocity is high enough, then the sediment will start to erode.
Contraction Scour Schematic
• Original riverbanks• Reduced flow area• Bridge Abutments• Bridge Abutments
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Riverbed Degradation
• Some rivers have beds that are naturally degrading due to conditions upstream or downstream.
• Any bridge piers or abutments built will need to have a deeper foundation.
Degradation Failure,Ariz.
Riverbed Aggradation
• Some rivers have beds that are naturally aggrading due to conditions upstream or downstream.
• Higher riverbed leads to increased flow depth and bridge over-topping.
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Vortices Around Abutments
Flood Level Normal Level
Flood Channel Wake VortexAbutment
Toe VortexDownward Flow/Front Vortex
Return Flow Main Channel
Vortices Around Piers
• PLAN SECTION A-AA Wake Vortex
Horseshoe Vortex
A Downward Roller
River Out-Flanking Bridge Opening
• Some rivers continue to meander and migrate in plan view.
• River may go around (out-flank) the bridgeRiver may go around (out flank) the bridge opening, or attack abutment.
Q
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Example of River Meander
River Widening
• How can river widening lead to bridge failure????
• Widening river should reduce velocity!Widening river should reduce velocity!
Widening Out-Flanking
• Widening leads to decreased velocity.• Decreased velocity can lead to sediment
deposition (water not fast enough to p ( gtransport sediment anymore.)
• Deposition can form point bars.• Point bars divert flow towards bank.• This causes bank erosion that threatens
abutment.
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Widening Leads to Flow Diverted at Abutment
Q Depositional Point Bar
Abutment Scour Affects Pier
QAbutment Scour
Threatened Pier
Environmental Concerns
• Any changes made to the river can cause harm to fish and wildlife.
• Some fish feed off the bottom sediments of a river. When we change the river characteristics, we change the sediment size on the river bed.
• If we don’t change the river, then nobody can blame us for environmental damage.