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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170

175

180

170

175

180

m

170

175

180

Soft-Soil Evaluation

160

165

0 25 50 75Moisture Percent

160

165

0 5 10 15 20SPT, bpf

Elev

. m

160

165

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








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PEATM l

GAS

Widening Widening

Core Embank

Marl

FiberSEWER WATER








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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



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.


G.W.T. Sand fill

DESIGN-Check Stability-SLIDING BLOCK

Traffic Loads

Active

Passive

Very Dense Sands

Medium Dense Sands



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.


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



120 ft.

50 ft. Loose Sands

BYRUM-2000

NOTE: For 40’ of sand, pv≈ 4,400 psf

40 ft.

R.R.


G.W.T. Sand fill

H-piles: Major Drag-down forces

DESIGN REQUIREMENTS-SETTLEMENTWick Drains-Accelerate Settlement

Very Dense Sands



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


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.

bridge geotechnical considerations and designing for scour

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