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SCOUR: EVALUATION and RIPRAP John G SCOUR: EVALUATION AND RIPRAP John G. Delphia, P.E. TxDOT Bridge Division Geotechnical Branch 2017 BRIDGE WEBINAR JULY 2017 IMPORTANCE OF SCOUR • The most common cause of bridge failures is from floods scouring bed material from around bridge foundations (from pg 1-1, Evaluating Scour at Bridges- 5th Edition, FHWA 2012) • There is a need to ensure public safety and minimize the adverse effects resulting from bridge failures/closures 2017 BRIDGE WEBINAR JULY 2017 2 IMPORTANCE OF SCOUR Scour Program • 1991 – The FHWA initiated a Scour Evaluation Program. • The program required state DOT’s to http://www.timesunion.com/local/article/30-years-ago- Bridge-collapse-kills-10-11045976.php#photo- evaluate all existing structures for scour 12655627 vulnerability. • All subsequent new bridge designs are required to include a scour evaluation. https://dspace.library.colostate.edu/bitstream/handle/10 217/86173/Lesson02.pdf?sequence=11&isAllowed=y 2017 BRIDGE WEBINAR JULY 2017 3 IMPORTANCE OF SCOUR PURPOSE OF PERFORMING A SCOUR ANALYSIS To ensure that a bridge can withstand the effects of scour without failing. PURPOSE OF CODING A BRIDGE FOR SCOUR To identify the current status of the bridge regarding its vulnerability to scour PURPOSE OF DOCUMENTATION OF SCOUR FOR BRIDGES To document the current status of the bridge regarding its vulnerability to scour and to indicate what to do during /after flooding events 2017 BRIDGE WEBINAR JULY 2017 4 BRIDGE SCOUR EVALUATION PROCEDURES SCOUR EVALUATION PROCEDURES 2017 BRIDGE WEBINAR JULY 2017 5 BRIDGE SCOUR EVALUATION PROCEDURES BRIDGE SCOUR BRIDGE TYPE NEW EXISTING BRIDGES BRIDGES KNOWN BRIDGE CLASS KNOWN BRIDGE CLASS UNKNOWN FOUNDATION CULVERT FOUNDATION CULVERT FOUNDATION 2017 BRIDGE WEBINAR JULY 2017 6 BRIDGE SCOUR EVALUATION PROCEDURES: New Bridges BRIDGE SCOUR BRIDGE TYPE NEW BRIDGES KNOWN FOUNDATION FOLLOW TxDOT GEOTECHNICAL MANUAL 2017 BRIDGE WEBINAR JULY 2017 7 BRIDGE SCOUR EVALUATION PROCEDURES: New Bridges 2017 BRIDGE WEBINAR JULY 2017 8 BRIDGE SCOUR EVALUATION PROCEDURES: New Bridges TxDOT GEOTECHNICAL MANUAL – 2012 SCOUR ANALYSIS METHODS CONTRACTION PIER SCOUR SCOUR ROCK NEAR THE MATERIAL SUSCEPTIBILITY TABLE SURFACE ANNANDALE’S ERODIBILITY INDEX METHOD COHESIONLESS FROEHLICH’S EQ. SOILS HEC-18 EQ. HEC-18 EQ. (sands, gravels) FLORIDA DOT PIER SCOUR EQN. COHESIVE SOILS HEC-18 EQ. HEC-18 EQ. WITH CLAY (clay) D50 LIMIT = 6.56 X 10-3 REDUCTION FACTOR in. SRICOS METHOD ANNANDALE’S ERODIBILITY INDEX METHOD LAYERED SOIL - CONDUCT SCOUR ANALYSIS LAYER BY LAYER - SRICOS METHOD - ANNADALE’S ERODIBILITY INDEX METHOD 2017 BRIDGE WEBINAR JULY 2017 9 BRIDGE SCOUR EVALUATION PROCEDURES: Cohesionless Soil TxDOT GEOTECHNICAL MANUAL – 2012 SCOUR ANALYSIS METHODS CONTRACTION PIER SCOUR SCOUR ROCK NEAR THE MATERIAL SUSCEPTIBILITY TABLE SURFACE ANNANDALE’S ERODIBILITY INDEX METHOD COHESIONLESS FROEHLICH’S EQ. SOILS HEC-18 EQ. HEC-18 EQ. (sands, gravels) FLORIDA DOT PIER SCOUR EQN. COHESIVE SOILS HEC-18 EQ. HEC-18 EQ. WITH CLAY (clay) D50 LIMIT = 6.56 X 10-3 REDUCTION FACTOR in. SRICOS METHOD ANNANDALE’S ERODIBILITY INDEX METHOD LAYERED SOIL - CONDUCT SCOUR ANALYSIS LAYER BY LAYER - SRICOS METHOD - ANNADALE’S ERODIBILITY INDEX METHOD 2017 BRIDGE WEBINAR JULY 2017 10 BRIDGE SCOUR EVALUATION PROCEDURES: Cohesionless Soil D50 and D95 ONLY USEFUL FOR COHESIONLESS SOILS Critical Velocity Equation 1/6 1/3 Vc = Ku y D50 Ku = 11.17(English) ( threshold velocity for material size < D50) 2017 BRIDGE WEBINAR JULY 2017 11 BRIDGE SCOUR EVALUATION PROCEDURES: Cohesionless Soil UNIFORM SAND PROFILE 60.00 50.00 100 YEAR WSEL 51.5' NORMAL POOL 39.8' 40.00 Sand 30.00 Channel 20.00 Contraction Scour: Elevation (ft) Ysc = 7.52 ft 10.00 Different Methods for the Pier Scour: HEC-18 Sand Scour Calculations HEC-18 Eq. Ysp = 8.25 ft 0.00 Contraction Scour: Ysc = 7.52 ft Sheppard’s Eq. Ysp = 7.35 ft Pier Scour: Ysp = 8.25 ft Froehlich’s Eq. Ysp = 7.84 ft -10.00 Total Scour: Ystotal = Ysc + Ysp = 15.77 ft -20.00 13980 14000 14020 14040 14060 14080 14100 14120 14140 14160 14180 14200 14220 14240 14260 14280 14300 14320 14340 14360 14380 Roadway Station 2017 BRIDGE WEBINAR JULY 2017 12 BRIDGE SCOUR EVALUATION PROCEDURES: Cohesive Soil TxDOT GEOTECHNICAL MANUAL – 2012 SCOUR ANALYSIS METHODS CONTRACTION PIER SCOUR SCOUR ROCK NEAR THE MATERIAL SUSCEPTIBILITY TABLE SURFACE ANNANDALE’S ERODIBILITY INDEX METHOD COHESIONLESS FROEHLICH’S EQ. SOILS HEC-18 EQ. HEC-18 EQ. (sands, gravels) FLORIDA DOT PIER SCOUR EQN. COHESIVE SOILS HEC-18 EQ. HEC-18 EQ. WITH CLAY (clay) D50 LIMIT = 6.56 X 10-3 REDUCTION FACTOR in. SRICOS METHOD ANNANDALE’S ERODIBILITY INDEX METHOD LAYERED SOIL - CONDUCT SCOUR ANALYSIS LAYER BY LAYER - SRICOS METHOD - ANNADALE’S ERODIBILITY INDEX METHOD 2017 BRIDGE WEBINAR JULY 2017 13 BRIDGE SCOUR EVALUATION PROCEDURES: Cohesive Soil D50 and D95 - ONLY USEFUL FOR COHESIONLESS SOILS DEFAULT VALUE FOR D50 and D95 FOR CLAY & SILT IS 0.2 mm or 6.56 x 10-4 ft 2017 BRIDGE WEBINAR JULY 2017 14 BRIDGE SCOUR EVALUATION PROCEDURES: Cohesive Soil SRICOS METHOD PIER SCOUR Re = Vd/v 2 τmax = KwKspKshKa 0.0094ρV {1/log(Re) – 1/10} Zri 0.635 Zmax = 0.18Re Clay Sample Zt = t /(1/ Zri + t / Z) 1 0.1 0.01 0.001 0.0001 Erosion Rate (ft/hr) 0.00001 0 0.2 0.4 0.6 0.8 1 1.2 1.4 She a r Stre ss (psf) CONTRACTION SCOUR Rh =A/P 2 2 -1/3 τmax = KRKLKWKθ ρg n V Rh Zri -1/2 1/2 -1/3 Zmax = Kθ KLx1.90{1.49VHEC(g H1) – (τc/ρ) (g n H1) } H1 Zt = t /(1/ Zri + t / Z) 2017 BRIDGE WEBINAR JULY 2017 15 BRIDGE SCOUR EVALUATION PROCEDURES: Cohesive Soil Scour Rate in Cohesive Soils (SRICOS) METHOD Sound Limestone 0.0001 Erosion Rate (in/hr) 0.00001 0.01 0.1 1 10 Shear Stress (psf) 2017 BRIDGE WEBINAR JULY 2017 16 BRIDGE SCOUR EVALUATION PROCEDURES: Cohesive Soil From HEC-18 Manual 2017 BRIDGE WEBINAR JULY 2017 17 BRIDGE SCOUR EVALUATION PROCEDURES: Cohesive Soil NCHRP Project 24-43 “Relationship between Erodibility and Properties of Soils” Objective: To determine relationships between erodibility and geotechnical properties that can be used as cost-effective means to assess site-specific, surficial erosion resistance of cohesive and cohesionless materials. Example for Cohesive Soils: 1.8 Erosion Rate = α C (τ – τ ) d 1 b c b αd 2017 BRIDGE WEBINAR JULY 2017 18 BRIDGE SCOUR EVALUATION PROCEDURES: Cohesive Soil UNIFORM CLAY PROFILE 60.00 50.00 100 YEAR WSEL 51.5' NORMAL POOL 39.8' 40.00 Clay 30.00 Channel 20.00 Elevation (ft) 10.00 HEC-18 Sand Scour Calculations SRICOS Method for the Channel 0.00 Contraction Scour: Ysc = 7.52 ft Contraction Scour: Ysc = 3.45 ft Pier Scour: Ysp = 8.25 ft Pier Scour: Ysp = 5.20 ft Total Scour: -10.00 Total Scour: Ystotal = Ysc + Ysp = 15.77 ft Ystotal = Ysc + Ysp = 8.65 ft -20.00 13980 14000 14020 14040 14060 14080 14100 14120 14140 14160 14180 14200 14220 14240 14260 14280 14300 14320 14340 14360 14380 Roadway Station 2017 BRIDGE WEBINAR JULY 2017 19 BRIDGE SCOUR EVALUATION PROCEDURES: Cohesive Soil TxDOT GEOTECHNICAL MANUAL – 2012 SCOUR ANALYSIS METHODS CONTRACTION PIER SCOUR SCOUR ROCK NEAR THE MATERIAL SUSCEPTIBILITY TABLE SURFACE ANNANDALE’S ERODIBILITY INDEX METHOD COHESIONLESS FROEHLICH’S EQ. SOILS HEC-18 EQ. HEC-18 EQ. (sands, gravels) FLORIDA DOT PIER SCOUR EQN. COHESIVE SOILS HEC-18 EQ. HEC-18 EQ. WITH CLAY (clay) D50 LIMIT = 6.56 X 10-3 REDUCTION FACTOR in. SRICOS METHOD ANNANDALE’S ERODIBILITY INDEX METHOD LAYERED SOIL - CONDUCT SCOUR ANALYSIS LAYER BY LAYER - SRICOS METHOD - ANNADALE’S ERODIBILITY INDEX METHOD 2017 BRIDGE WEBINAR JULY 2017 20 BRIDGE SCOUR EVALUATION PROCEDURES: Rock ROCK AT OR NEAR THE SURFACE Material Susceptibility to Scour Material Subtype Texas Cone Susceptibility Penetrometer Rock Hard (Granite, < 4”/100 blows Not susceptible Limestone, Shale) Soft (Shale) < 12”/100 blows Mildly susceptible, but not considered over time span of one flood event Clays Hard (Redbed, Shaley < 12”/ 100 blows Mildly susceptible, Clays, Very Stiff Clays) but not considered over time span of one flood event Soft to Medium > 12”/ 100 blows Susceptible to scour at a moderate rate Sands All All Very susceptible 2017 BRIDGE WEBINAR JULY 2017 21 BRIDGE SCOUR EVALUATION PROCEDURES: Rock ROCK PROFILE 60.00 100 YEAR WSEL 51.5' 50.00 NORMAL POOL 39.8' 40.00 50(1”)502(2”) 30.00 50(1”)50(1”) 20.00 Elevation (ft) Elevation 10.00 LIMESTONE HEC-18 Sand Scour Calculations Other Methods For the Channel 0.00 L EGEND Rock Eq.: Ystotal = 0.55 ft For the Channel DATE LINE Ystotal = Ysc + Ysp = 15.77 ft 1956 TxDOT : Ystotal = 0.00 ft -10.00 non-scourable rock Rock at the Surface -20.00 use Ystotal = 1 to 2 diam. example – DS diam = 3 ft : 13980 14000 14020 14040 14060 14080 14100 14120 14140 14160 14180 14200 14220 14240 Ystotal14260 14280 = 3 - 14300 6 ft 14320 14340 14360 14380 14400 Station 2017 BRIDGE WEBINAR JULY 2017 22 BRIDGE SCOUR EVALUATION PROCEDURES: Rock Wetting and drying cycles make Clay Highly weathered and fractured rock and Shale susceptible to scour can be susceptible to scour https://www.slideshare.net/Vyankyo/river-erosion-and-its-associated-fetures 2017 BRIDGE WEBINAR JULY 2017 23 BRIDGE SCOUR EVALUATION PROCEDURES: Programs PROGRAMS AVAILABLE TO EVALUATE SCOUR: Cohesionless Soil: - HEC-RAS contains the HEC-18 sand contraction and pier scour equations, as well as Froehlich’s equation - Florida DOT has a spreadsheet available to calculate pier scour (http://www.fdot.gov/roadway/Drainage/Florida-Scour-Manual-Training-Course.shtm) - FHWA Hydraulic Toolbox (https://www.fhwa.dot.gov/engineering/hydraulics/software/toolbox404.cfm) Cohesive Soil: - HEC-RAS contains the HEC-18 sand contraction and pier scour equations.
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