Bridge Geotechnical Considerations and Designing for Scour PART II Christopher Byrum, Ph.D., P.E. Geotechnical Soil and Materials Engineers, Inc.

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1 Bridge Geotechnical Considerations and Designing for Scour PART II Christopher Byrum, Ph.D., P.E. Geotechnical Soil and Materials Engineers, Inc. Brian Barkdoll, PhD, PE Scour Michigan Technological University Geotechnical Evaluations for Bid Bridge Design USGS Topo And Bedrock Topo to estimate Depth to Rock 1

2 USDA Soil Survey For Marsh Limits Peat Marshes Detailed Soil Survey: All New Alignments Obtain Data from Existing Bridge Plans/Files 1. Existing Test Holes 2. Bridge Deck Cross Section (between beams) 3. Existing Utilities Type and Location 4. Construction History Files Issues 5. Design History Files Issues Review All This Stuff and: 1. Summarize Key Site Conditions 2. Decide if New Test Holes are Needed 3. Make the Field Geotechnical Evaluation Plans 2

3 Big Drill Rig = Deep Holes Small Truck Drill Rig Small All-Terrain Drilling Rig Remote Control for Safety 3

4 MDOT SKID drill rig mounted to Pontoon Boat SKID drill rig Set Inside a Cofferdam using a Crane 4

5 Weak Cohesive Soils- Thin Wall Shelby Tube Low Disturbance Very Weak Cohesive Soils- In-Situ Vane Shear Strength Testing 5

6 Soft-Soil Evaluation m Elev. m SPT, bpf Moisture Percent Torvane Field Vane For Design Strength, kpa Geotechnical Evaluation Case Study A Very Difficult Test Hole Existing Piers Settled and Cracked, Vibration Related I-696 over Rouge River- Bridge Widening 6

7 Pier Settled and Railing Crushed Crushed Column Big Crack 7

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10 Typical Test Hole Frequency BRIDGES 1. One per substructure Unit (<50-ft wide) 2. Two per Unit ( ft wide) 3. Three per Unit ( ) APPROACHES 1. 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 River). 10

11 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 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) 11

12 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 12

13 Widening Widening GAS PEAT Marl Fiber SEWER Core Embank 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 13

14 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 14

15 Test Locations Muck Rod Tests Deep Soil Borings: Sheet Pile and Pavement Design SPT 0 50 SPT

16 CONTINUOUS GEOGRID #2 The Anchors GWT GEOGRID #3 CONTINUOUS GEOGRID #1 Placement of EPS and Geogrid behind sheeting. View from Northwest of the Wall 16

17 View from North Hillside, Looking South View of Final Product, Looking West GEOTECHNICAL ENGINEERING PROCESS 40 ft. Abutment A Approach Embankment Pier 1 N or E Abutment B Pier 2 Pier 3 R.R. Existing Ground Superstructure-Beams, Deck, Joints, Railing. Substructures-Footings, Columns and Walls 17

18 State Route M-63 over CSX Railroad St. Joseph, Michigan PHASE I: PLANNING Span Lengths 40 ft. R.R. Min. Height for a Train Vert. Curve-Sight 50 mph PRIMARILY GEOMETRY STANDARDS vs. PROPERTY FEATURES. Existing Ground State Route M-63 over CSX Railroad St. Joseph, Michigan PHASE II: DESIGN-Acquire Geo Info 40 ft. Loose/soft sand, peat, sediment, and marl. G.W.T. R.R. Sand fill 50 ft. Loose Sands Organic Clayey Silt (OH-marl) w% = 45 to 65, LL= 60 to 70, c = 500 to 1100 psf 120 ft. Medium Dense Sands Very Dense Sands State Route M-63 over CSX Railroad Center of Rotation DESIGN-Check Stability MASS ROTATION Traffic Loads Weight 40 ft. R R.R. G.W.T. Sand fill Loose/soft sand, peat, sediment, and marl. 50 ft. Loose Sands Organic Clayey Silt (OH-marl) w% = 45 to 65, LL= 60 to 70, c = 500 to 1100 psf F.S > ft. Medium Dense Sands Soil Shear Strengths Very Dense Sands BYRUM

19 Sand Embankment Light Slag Embankment Abutment Front Slope 1:2 19

20 Light Slag Embankment Approach Side Slope 1:3 State Route M-63 over CSX Railroad DESIGN-Check Stability-SLIDING BLOCK Traffic Loads Active 40 ft. Loose/soft sand, peat, sediment, and marl. G.W.T. R.R. Passive Sand fill 50 ft. Loose Sands Organic Clayey Silt (OH-marl) w% = 45 to 65, LL= 60 to 70, c = 500 to 1100 psf F.S > ft. Medium Dense Sands Soil Shear Strengths Very Dense Sands BYRUM-2000 Sand Embankment 20

21 Light Slag Embankment State Route M-63 over CSX Railroad Lightweight slag fill (90 pcf) DESIGN REQUIREMENTS-STABILITY $$$$$$$ Geogrid Expanded Polystyrene (EPS) Ultra-Lightweight fill (10 pcf) 40 ft. p v < 3,200 psf G.W.T. p v < 2,500 psf R.R. Sand fill Loose/soft sand, peat, sediment, and marl. 50 ft. Loose Sands Organic Clayey Silt (OH-marl) w% = 45 to 65, LL= 60 to 70, c = 500 to 1100 psf 120 ft. Medium Dense Sands NOTE: For 40 of sand, p v 4,400 psf Very Dense Sands BYRUM-2000 State Route M-63 over CSX Railroad DESIGN REQUIREMENTS-SETTLEMENT Wick Drains-Accelerate Settlement 40 ft. H-piles: Major Drag-down forces R.R. G.W.T. Sand fill Loose/soft sand, peat, sediment, and marl. 50 ft. Loose Sands Organic Clayey Silt (OH-marl) w% = 45 to 65, LL= 60 to 70, c = 500 to 1100 psf 120 ft. H-piles: Drive to 110 ton, use 80 ton for bridge design Very Dense Sands BYRUM

22 State Route M-63 over CSX Railroad Other Design Services Contract Special Provisions -Wait Periods for Settlement/Strength Gain -Geotechnical Instrumentation During Const. -Wick Drains to Accelerate Settlement -Special Materials; Properties and Control Construction Services 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. 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 move sediment. 22

23 Causes of Bridge Scour Contraction scour Bed degradation 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-sectionalsectional 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 23

24 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. 24

25 Vortices Around Abutments Flood Level Normal Level Flood Channel Wake Vortex Abutment Toe Vortex Downward Flow/Front Vortex Return Flow Main Channel Vortices Around Piers PLAN A Wake Vortex Horseshoe Vortex SECTION A-A 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 bridge opening, or attack abutment. Q 25

26 Example of River Meander River Widening How can river widening lead to bridge failure???? 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 transport sediment anymore.) Deposition can form point bars. Point bars divert flow towards bank. This causes bank erosion that threatens abutment. 26

27 Widening Leads to Flow Diverted at Abutment Q Depositional Point Bar Abutment Scour Affects Pier Q Abutment 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. 27