Time Domain Reflectometry for Evaluation of Simulated Sedimentation

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Real
-
Time Bridge Scour Monitoring: Recent
Technological Development

Present at Ohio Transportation Engineering Conference

Oct 23
-
24, 2007

Bill X. Yu and Xinbao Yu

Assistant Professor, Department of Civil Engineering, Case Western Reserve University, OH, USA,
216
-
368
-
6247,
xxy21@case.edu

Research 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)
Relative Voltage (V)










1
1
f
s
b
V
V
C
EC
2









p
a
a
L
L
K
V
s
/2

Apparent Length,
L
a

Information from TDR signal


V
f

L
p

= length of probe in soil


Dielectric constant and electrical conductivity

Soil Dielectric Constant,
K
a


Soil dielectric constant,
K
a

<=> Young’s modulus,
E


Predominantly decided by water content


Topp’s equation relates
K
a
to volumetric water content

Soil Solids

Air

Water

Reflection at Interface

2
,
1
,
2
,
1
,
1
2
1
2
a
a
a
a
K
K
K
K
Z
Z
Z
Z








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

Travel Time or Distance

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)
Relative Voltage
Identified location of reflections by the
automatic signal analyses

(1)

(2)

(3)

(1) Probe beginning

(
2) Water/sediment interface

(
3) End of probe

w
a
L
L

1
,
1

L
L
L


2
1
total
a
s
w
L
L
L
,
2
1




where ε
w

is the dielectric constant of water,
L
a,1

is apparent length of probe
section embedded in water, which is the measured distance between
reflection points 1 and 2, and
L
a,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
R
2
= 0.9968
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
Actual Sediment Thickness (m)
Measured Thickness by TDR (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
1
1
1
deposit)

soil

saturated
for

formula

(mixing

)
1
(
system)

overall

for the

formula

(mixing

L
L
K
K
n
K
K
K
K
n
K
n
K
K
L
L
K
L
L
w
a
s
a
w
a
m
a
bs
a
w
a
s
a
m
a
w
a
bs
a


















Bulk Electrical Conductivity


Mixing for electrical conductivity








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





L
L
n
EC
EC
n
EC
EC
F
EC
L
L
EC
L
L
EC
f
w
m
b
f
w
bs
b
m
b
w
bs
b
1
,
,
,
2
1
,
1
1
Factor
Formation

:
Law

s
Akie'








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)
reflection point position(m)
TDR Software value
New algorithm value
Results and Analysis


Example of TDR measured dielectric constant
versus deposition thickness (water level kept
constant)

0.0
0.2
0.4
0.6
0.8
1.0
0.5
0.6
0.7
0.8
0.9
1.0
Tap water
250ppm
500ppm
750ppm
Linear Fit of All Data
sqrt(Ka)/sqrt(kw)
Sand thickness/Total thickness of water and sand
Y = 1.00354 -0.43321 * X
Results and Analysis


Example of TDR measured electrical
conductivity versus deposition thickness

0.0
0.2
0.4
0.6
0.8
1.0
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Tap water
250ppm
500ppm
750ppm
Linear Fit of All Data
ECb/ECb,w
Sand thickness/Total thickness of water and sand
Y = 1.02032 -0.66686 * X
Results and Analyses

0.0
0.2
0.4
0.6
0.8
1.0
0.5
0.6
0.7
0.8
0.9
1.0
sqrt(Ka)/sqrt(kw)
sand thickness/Length of probe below water surface
Y = 1 -0.43 * X
Measured Ka
Sand thickness ratio
x
r
Step a)
0.0
0.2
0.4
0.6
0.8
1.0
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
ECb/ECb,w
Sand thickness/Final sand thickness
Y = 1 -0.67 * X
Sand thickness ratio
Conductivity of water
x
r
Step b)
Application 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
1.2
Measured sand thickness/Total thickness 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)
Estimated water conductivity(ms/m)
1:1
+5%
-%5
Results and Analysis


Estimated density of sediments

1.4
1.5
1.6
1.7
1.2
1.3
1.4
1.5
1.6
1.7
1.8

Predicted_Tap water

Predicted_750ppm

Predicted_500ppm

Predicted_250ppm
1:1
+5%
-5%
TDR Estimated Sediments Dry density(g/cm
3
)
Actual Dry density(g/cm
3
)
Results and Analysis


Estimation of Pore Water Electrical Conductivity

0
2
4
6
8
0
5
10
15
20
25
Experimental Stage
Electrical Conductivity of Water (mS/m)
Actual pore
water electrical
conductivity

Ultrasonic system


ediment
ater
Ultrasound
pulse generator
Oscilloscope
Ultrasonic
transducer
Example ultrasonic signal


-0.5
0
0.5
1
1.5
2
2.5
x 10
6
-1000
-800
-600
-400
-200
0
200
400
Time(ns)
Votage(mv)
Pulse signal
1st reflection at the water and sediment interface
Round trip time from water surface to
water and sediment interface
Set up for Comparing TDR and Ultrasonic Method


Example of TDR signal


0
2
4
6
8
-1.5
-1.0
-0.5
0.0
0.5

Thickness of water layer: 30.5cm

Thickness of water layer: 23cm

Thickness of water layer:15.9cm

Thickness of water layer: 9cm

Thickness of water layer: 2.5cm

Thickness of water layer: 0cm
Voltage(V)
Length(m)
Decreas in thickness
of water layer
Example of Ultrasonic Signal

-1
0
1
2
3
4
5
6
x 10
5
-1000
0
1000
Time(ns)
Voltage(mv)
thickness of water layer: 30.5cm
-1
0
1
2
3
4
5
6
x 10
5
-1000
0
1000
Time(ns)
Voltage(mv)
thickness of water layer: 23cm
-1
0
1
2
3
4
5
6
x 10
5
-1000
0
1000
Time(ns)
Voltage(mv)
thickness of water layer: 15.9cm
-1
0
1
2
3
4
5
6
x 10
5
-1000
0
1000
Time(ns)
Voltage(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)
Sensor measured water thickness (Normalized)
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)
ECb,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
Sediment thickness(cm)
Dry density(g/cm^3)
Predicted
Measured
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!