Real-Time Bridge Scour Monitoring: Recent Technological Development

Present at Ohio Transportation Engineering Conference

Oct 23-24, 2007

Bill X. Yu and Xinbao YuAssistant Professor, Department of Civil Engineering, Case Western Reserve University, OH, USA, 216-368-6247, xxy21@case.eduResearch Assistant, Department of Civil Engineering, Case Western Reserve University, OH, USA

Background Scour

the lowering of streambed around bridge piers or abutments Types of scour

long-term degradation of the river bed general scour local scour at the piers or abutments

Background

Scour of pier or abutment poses a most severe threat to bridge service life

503,000 bridges traverse waterways, over 20,000 are classified as scour critical

1000 bridges have collapsed in 30 years in the USA and scour was responsible for 60% of those failures

Scour Caused Bridge Collapse

I-90 bridge collapse on Schoharie Creek, April 5, 1987. Courtesy of Sid Brown, Schenectady Gazette.

Purpose of Study Current scour design specifications are based models from

laboratory data. They generally doesn’t adequately describe the field condition.

Instrument of bridge scour in the field helps: Calibrate and refine the numerical models for sediment

movement and bridge scour; Describe the trend of scour evolution to help scheduling the

remediate measure; Real-time warning to prevent human or property loss due to

catastrophic failure.

This study investigates the application of electromagnetic wave technology (TDR) monitoring of scour/sedimentation process. Performance is compared with the ultrasonic technology.

Current Practice for Scour Evaluation

Yard sticking Sounding rod Sonic device Fisher bulb Maximum

scour recorder

It is fun, isn’t it?

Current Practice for Scour Evaluation

Most not sufficiently rugged for field use

Do not provide real time monitoring

Not automatic data collection and interpretation

TDR Background Use guided “radar” to identify materials properties and interfaces. Involves fast rising EM pulse of picoseconds to accurately determine

the interfaces

-1.25

-0.75

-0.25

0.25

0.75

1.25

0 1 2 3 4 5 6 7 8

Scaled Distance (m)

Rel

ativ

e V

olta

ge (

V)

1

1

f

sb V

V

CEC

2

p

aa L

LK

Vs/2

Apparent Length, La

Information from TDR signal

Vf

Lp = length of probe in soil

Dielectric constant and electrical conductivity

Soil Dielectric Constant, Ka

Soil dielectric constant, Ka <=> Young’s modulus, E Predominantly decided by water content Topp’s equation relates Ka to volumetric water content

Soil Solids

Air

Water81

1

3-5Water

Air

Soil Solids

Reflection at Interface

2,1,

2,1,

12

12

aa

aa

KK

KK

ZZ

ZZ

Air, Ka=1 Saturated soil, Ka=20~40 depending on density Water, Ka=81

Schema of TDR for scour measurement

TDR electronics

TDR Probe

L1

L2

L

Connection to computer or controller

Air/Water interface

Water/Sediment interface

End of TDR probe

Schematic of recorded TDR signal

Tra

vel T

ime

or

Dis

tan

ce

Experiment Setup and Procedure Sand deposit gradually added with the water

content kept constant Reflections can be determined

Data Acquisition Computation software was developed to automate

signal acquisition and analyses

Measured evolution of signals with sedimentation accumulation

Increasing scour depth

Typical TDR Signals

Uniform water

Water/Soil Interface

1

2

1

2

TDR signal in soil deposit inundated by water

TDR signal in water

End of TDR probe

Signal analyses Method 1: Determine internal reflections

0 50 100 150 200 250-2

-1.5

-1

-0.5

0

0.5

Time (ns)

Rel

ativ

e V

olta

ge

Identified location of reflections by the automatic signal analyses

(1)

(2)(3)

(1) Probe beginning(2) Water/sediment interface(3) End of probe

w

aLL

1,1

LLL 21

totalasw LLL ,21

where εw is the dielectric constant of water, La,1 is apparent length of probe section embedded in water, which is the measured distance between reflection points 1 and 2, and La,total is the apparent length of the whole sensor probe, which is the measured distance between reflection points 1 and 3.

Signal Analyses Method 1: Determine internal reflections

Example result

y = 0.9368x + 0.0449

R2 = 0.9968

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1Actual Sediment Thickness (m)

Mea

sure

d T

hick

ness

by

TD

R (

m)

Signal analyses Method 2: Bulk dielectric properties

Bulk dielectric properties can be relatively easily determined following common procedures

Bulk Dielectric Constant Mixing formulas for dielectric constant

Approximate linear relationship between Ka,m and deposition thickness

Deposition thickness can be estimated

1

,

,

,

,

,,,

,,2

,1

11

1

deposit) soil saturatedfor formula (mixing )1(

system) overall for the formula (mixing

LL

K

Kn

K

K

KKnKn

KKL

LK

L

L

wa

sa

wa

ma

bsawasa

mawabsa

Bulk Electrical Conductivity Mixing for electrical conductivity

Approximate linear relationship between ECb,m and deposition thickness at a given electrical conductivity of water

L

Ln

EC

EC

nEC

ECF

ECL

LEC

L

LEC

f

w

mb

f

w

bsb

mbwbsb

1,

,

,21

,

11

Factor Formation :Law sAkie'

Results and Analysis

Estimate water level from location of surface reflection

4.4

4.5

4.6

4.7

4.8

4.9

5

5.1

5.2

0 50 100 150 200 250 300 350

water thickness(mm)

refl

ec

tio

n p

oin

t p

os

itio

n(m

)

TDR Software value

New algorithm value

Results and Analysis Example of TDR measured dielectric constant

versus deposition thickness (water level kept constant)

Results and Analysis Example of TDR measured electrical

conductivity versus deposition thickness

Results and AnalysesApplication procedures using design equations

1) Estimate scour depth; 2) estimate electrical conductivity of water; 3) estimate density of sediment

Results and Analysis

0

0. 2

0. 4

0. 6

0. 8

1

1. 2

0 0. 2 0. 4 0. 6 0. 8 1 1. 2Measured sand thi ckness/ Total thi ckness of water and

sand

Measured sand thickness/Total

thickness of water and sand 1: 1+5%-5%

Estimated sediment thickness

Results and Analysis

Estimated electrical conductivity of water

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140

TDR measured water conductivity(ms/m)

Est

imat

ed w

ater

co

nd

uct

ivit

y(m

s/m

)

1:1

+5%

-%5

Results and Analysis

Estimated density of sediments

Results and Analysis Estimation of Pore Water Electrical Conductivity

0 2 4 6 80

5

10

15

20

25

Experimental Stage

Ele

ctric

al C

ondu

ctiv

ity o

f Wat

er (m

S/m

) Actual pore water electrical conductivity

Ultrasonic system

Example ultrasonic signal

-0.5 0 0.5 1 1.5 2 2.5

x 106

-1000

-800

-600

-400

-200

0

200

400

Time(ns)

Vot

age(

mv)

Pulse signal1st reflection at the water and sediment interface

Round trip time from water surface towater and sediment interface

Set up for Comparing TDR and Ultrasonic Method

Example of TDR signal

Example of Ultrasonic Signal

-1 0 1 2 3 4 5 6

x 105

-1000

0

1000

Time(ns)

Vol

tage

(mv)

thickness of water layer: 30.5cm

-1 0 1 2 3 4 5 6

x 105

-1000

0

1000

Time(ns)

Vol

tage

(mv)

thickness of water layer: 23cm

-1 0 1 2 3 4 5 6

x 105

-1000

0

1000

Time(ns)

Vol

tage

(mv)

thickness of water layer: 15.9cm

-1 0 1 2 3 4 5 6

x 105

-1000

0

1000

Time(ns)

Vol

tage

(mv)

thickness of water layer: 9cm

Comparison of TDR and Ultrasonic Results

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2

Ruler measured water thickness(Normalized)

Sen

sor

mea

sure

d w

ater

thi

ckne

ss (

Nor

mal

ized

)

TDR method 1

1:1

Ultrasonic method

TDR method 2

TDR electrical conductivity

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35

Sediment thickness(cm)

EC

b,w

(ms/

m)

Measured by TDR

Measured by Ec Meter

TDR estimated sediment dry density

0

0. 5

1

1. 5

2

2. 5

0 5 10 15 20 25 30 35

Sedi ment thi ckness(cm)

Dry

dens

ity(

g/cm̂

3)

Predi ctedMeasured

Comparison of TDR and Ultrasonic Method Both TDR and ultrasonic methods accurate measure scour depth. TDR system:

Inexpensive and automatic=> real time scour monitoring and surveillance system.

Information on sediment status (density) and water conditions (electrical conductivity) are obtained simultaneously. These could be used to enable a mechanistic understanding of scour phenomena.

Accuracy of TDR can be affected by the electromagnetic interference and signal attenuation in the cable length.

TDR sensor only measures scour at a given point. Multiple TDR probes will be needed to map the scour hole shape. This requires the designed field TDR probes to be rugged and inexpensive. The deployment of the TDR probes also needs to be well planned.

Ultrasound method: post-event scour measurement. Coupling the ultrasonic transducer with water is needed which requires the

ultrasonic transducer to be maintained below the water level. Ultrasonic method is also a local measurement. However, as it is a non-intrusive

technology, ultrasonic transducer can be moved to determine the shape of river bed after scour event.

The interpretation of ultrasonic signal can be challenging especially for complex river bed territories. Experience from this research indicated that there could be significant amount of background noise in the ultrasonic signal. Experience in ultrasonic signal analyses is needed to ensure a sound interpretation of measurement results.

Summary and Recommendations

Instrument of bridge scour/sedimentation is critical for bridge safety

A new approach has been developed for TDR Measure a variety of information related to scour (water

level, scour/sedimentation depth, sedimentation status (density&water content), electrical conductivity of water)

Data acquisition and analyses can be can be automated to provide real time surveillance

TDR and Ultrasonic method has similar performance for riverbed determination

Combined TDR and Ultrasonic method for within flood and post-flood survey are recommended

Thank You!