Scour at Bridge Foundations on Rock:

Status of NCHRP Project 24-29Jeffrey R. Keaton, Principal Investigator

MACTEC Engineering and Consulting, Inc.Los Angeles, CA

Su K. Mishra, Co-Principal InvestigatorAyres Associates, Inc. (now with HDR)

Sacramento, CA

Project No. 24-29: Scour at Bridge Foundations on Rock

Geohazards In TransportationIn the Appalachian  Region04 Aug 2010

NCHRP 24-29 Objectives

• Develop a methodology for determining– Design scour depth– Time-rate of scour

• Develop design and construction guidelines for applying the methodology

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"Far better an approximate answer to the right question, which is often vague, than an exact answer to the wrong question, which can always be made precise."

John W. Tukey, 1962, The future of data analysis: Annals of Mathematical Statistics, v. 33, p.13

Bridge Sites

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OregonDOT;

OregonState U

CaliforniaDOT

UtahDOT

FloridaDOT;Universityof Florida

New YorkState ThruwayAuthority;New YorkStateDOT;USGS

Chipola River – I-10

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Oligocene marine dolomitic limestone

Rotating Erosion TestApparatus (RETA)

Mill Creek – OR SR-22

5Oligocene marine siltstone

WaterFlow

downstream

WaterFlow

Schoharie Creek – I-90

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Paleozoic marine sandstone

Quaternary ice-contact stratified drift

Schoharie Creek Gage

Montezuma Creek – UT SR-262

7September 2003

Average annual precipitation = 8.43”100-yr, 24-hr precipitation = 2.62”

Gage 1 mile downstream15 yrs of data 1963 – 93(Source: HDR 2003 report)

Montezuma Creek – UT SR-262

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Headcut > 2.5 m

Jurassic fluvial interbeddedsandstone and claystone

Sacramento River – CA SR-273

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Cretaceous marine siltstone

Rock Scour Flow Diagram

Modes ofScour

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Quantify modes that applyDismiss modes that don’t

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Soluble Rock

Bridges are not founded on rocks that dissolve in engineering time (halite, sylvite, gypsum)

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Cavitation

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Degradable Rocks

October 1, 2008

1:36 pm

August 6, 200812:03 pm

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Durable Rocks

Erik Bollaert, AquaVision Engineering

Degradable Rock Scour Procedure

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Rock scour is a“rock-water interaction “

problem

Gage & Cross Section Data

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Unit stream power= d S v = v

3/1

2

486.1 dv n

3v

22 ftslbft

sft

ftlb

Gage Data; No Cross Section

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Cross Section; No Gage Data

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No Gage or Cross Section Data

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Stream Gage Data

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Repeated Cross Sections

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Cretaceousmarinesiltstone

Cumulative Stream Power

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Stream power can be accumulated over time;Neither velocity nor shear stress can be accumulated

“Excess” > 2-yr Qpeak

Probability Weighted Scour

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AnnualFrequency

(yr-1)

ReturnPeriod

(yr)

PeakDischarge

(ft3/s)

Excess Stream Power

(ft-lb/s / ft2)

EventScour

(ft)

0.5 2 30,300 43.00 0.27290.2 5 53,500 66.56 0.17370.1 10 71,300 83.03 0.11070.04 25 96,200 104.55 0.05730.02 50 116,300 120.98 0.03390.01 100 137,600 137.70 0.0197

0.002 500 192,000 177.95 0.0054

0.00

0217

ft p

er u

nit

of e

xces

s po

wer

Qp

>= 2

-yr E

vent

Empirical Scour Number

Average Annual Scour

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33.8 yr * 0.156 ft/yr = 5.27 ft33 yr * 0.51 ft/yr = 16.8 ft

Time Rate of Scour

Modified Slake Durability

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Slake durability equipment used to perform continuous abrasion test modified from Dickenson and Baillie (1999)

Start

1 hr

5 hrs

ASTM D4644 -08 Standard Test Method for Slake Durability of Shales and Similar Weak Rocks

Geotechnical Scour Number

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Geotechnical Scour Number

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Implied Threshold Condition

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Abrasion test results cannot be converted to equivalent velocity or shear stress;Stream Power is the best hydraulic parameter for rock scour

Geotechnical Scour Number

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Predicted Scour Depth

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Rock scour is a “rock-water interaction “ problem

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SCOUR AT BRIDGE FOUNDATIONS ON ROCK

NCHRP Project No. 24-29

Jeffrey R. Keaton, Principal InvestigatorMACTEC Engineering and Consulting, Inc.

Los Angeles, CASu K. Mishra, Co-Principal Investigator

HDR, Inc.Folsom, CA

04 August 2010