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(c) The SMARTRAIL Consortium 2012 1
Smart Maintenance, Analysis and Remediation of
Transport Infrastructure
Deliverable 1.1 Selection of Sensors to be used at SMARTRAIL
test sites
Project funded by the EU 7th Framework Programme under call
SST.2011.5.2-6 Cost-effective improvement of rail transport
infrastructure. Grant agreement no: 285683
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(c) The SMARTRAIL Consortium 2012 2
Project Information Project Duration: 01/09/2011 31/08/2014
Project Coordinator: Dr. Kenneth Gavin ([email protected])
School of Civil, Structural and Envrionmental Engineering
University College Dublin Newstead Building Belfield, Dublin 4
Ireland
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Document information
Version Date Action Partner
01 22.05.2012 1st draft UCD
02 26.11.2012 Final UCD
Title: SMARTRAIL DEL 1.1 Specification of Sensors to be used at
SMART Rail test sites
Authors: The SMARTRAIL Consortium
Reviewer: Kenneth Gavin (UCD) Copyright: Copyright 2011 2014.
The SMARTRAIL Consortium
This document and the information contained herein may not be
copied, used or disclosed in whole or part except with the prior
written permission of the partners of the SMARTRAIL Consortium. The
copyright and foregoing restriction on copying, use and disclosure
extend to all media in which this information may be embodied,
including magnetic storage, computer print-out, visual display,
etc.
The information included in this document is correct to the best
of the authors knowledge. However, the document is supplied without
liability for errors and omissions.
All rights reserved.
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Contents
1 Background
..............................................................................................................
7
2 Bridge Scour
.............................................................................................................
9
2.1
Introduction...............................................................................................
9
2.2 Instrumentation used to monitor bridge scour
........................................... 9
2.3 Single use Buried Devices / Float-Out Devices
............................................ 9
2.4 Pulse / Radar Devices:
..............................................................................
12
2.5 Piezo-electric Film Sensor Devices
............................................................ 15
2.6 Buried / Driven Rods:
...............................................................................
15
2.7 Superstructure Monitoring
......................................................................
19
2.8 Sound Wave Monitoring
..........................................................................
21
2.9 Electrical Conductivity Devices
.................................................................
23
2.10 Discussion on Scour Monitoring Equipment
............................................. 24
2.11 Instrumentation of choice for Scour
......................................................... 25
3 Slope Monitoring
...................................................................................................
26
3.1
Introduction.............................................................................................
26
3.2 Slope Monitoring Instrumentation
........................................................... 26
3.3 Positive Pore Water Pressures
.................................................................
33
3.4 Negative Pore Water Pressures
................................................................
35
3.5 Soil Moisture
...........................................................................................
38
3.6 Summary
.................................................................................................
39
4 Laboratory Study
...................................................................................................
40
4.1 Overview
.................................................................................................
40
4.2 Use of Accelerometers to investigate bridge scour
................................... 40
4.3 Experimental
Apparatus...........................................................................
42
4.4 EXPERIMENTAL METHODOLOGY
..............................................................
47
4.5 Results
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49
4.6 Discussion
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52
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4.7 Conclusions and Recommendations
......................................................... 53
5 Laboratory measurement of soil suction
...............................................................
54
5.1
Background..............................................................................................
54
5.2 Experimental Procedure
..........................................................................
55
5.3 Test Results
.............................................................................................
58
5.4 Discussion
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60
5.5 Summary
.................................................................................................
61
6 Conclusions
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62
7 References
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63
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Executive Summary
The vision of SMART Rail is to provide a framework for
infrastructure operators to ensure the safe, reliable and efficient
operation of ageing European railway networks. This will be
achieved through a holistic approach which will consider input from
state of the art inspection, assessment and remediation techniques,
whereby this data will be used to consider what if scenarios using
whole life cycle cost models. Key to achieving the cost-effective
monitoring of complex infrastructure elements such as bridges and
embankments will be the achievement of a step-change in monitoring
techniques. The development or relatively low-cost and high
precision sensors offers the opportunity to provide a real-time
monitoring of infrastructure. Climate change is resulting in
increased scour of bridges and rainfall-induced landslides on
transport networks. This report discusses the methods available to
monitor bridge scour and slope stability. In keeping with the theme
of cost-effective methods which can deliver rapid and continual
feedback on the performance of structures, the use of
accelerometers for bridge scour monitoring and water content and
suction probes for slope stability is favoured for full-scale trial
testing in the latter stages of the project. Initial performance of
the chosen instruments in laboratory testing is briefly
presented.
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1 Background Several European countries boast highly advanced
rail networks whereby their primary area of concern in relation to
infrastructure performance is related to achieving ever higher
network speeds. In several new EU countries, accession states, and
some long-term EU members, an historic lack of investment in rail
infrastructure had resulted in a situation whereby some elements of
the network are in very poor condition. In these countries, parts
of the rail infrastructure would be deemed to have reached the end
of its useful life when analysed using conventional assessment
methods. Climate change effects are further increasing the burden
on ageing transport networks with the incidence of infrastructure
failure increasing.
Irish railways were amongst the first constructed in Europe, and
the 180 m span Malahide viaduct which carries the Dublin-Belfast
line just North of Dublin is one of the oldest railway viaducts in
the world. In August 2009, following reports of unusual flow
patterns at one of the piers, a visual inspection was performed and
no unusual distress to the structure was noted. However, within
days of this inspection the pier collapsed as a local passenger
train crossed the viaduct and the Belfast-Dublin express service
approached. The collapse, which was caused by scour of the
foundations (which was not visible to the inspector) caused the
line to be closed for seven months and resulted in a repair bill in
the region of 4 million.
Visual inspection is one of the most widely used techniques when
monitoring the current state of railway infrastructure. The
benefits of such an approach are obvious in that trained inspectors
and engineers develop an intimate knowledge of the visual condition
of existing infrastructure and in some cases (e.g. where drainage
channels have become blocked) can organise fast remedial works. A
further advantage is its cost effectiveness, as the inspectors are
typically employees of the network operator. On the other hand,
disadvantages of visual inspections include:
(i) safety concerns visual inspections involve staff walking on
railway lines that are usually live,
(ii) Lack of continuity when experienced staff retire, their
knowledge is lost. This was identified as a key significant factor
during the public enquiry into the Malahide Viaduct failure in
Ireland.
(iii) A visual inspection of a slope, tunnel or bridge will not
reveal whether some deep-seated mechanism such as a weak soil
stratum, reinforcement corrosion in concrete, or scour beneath a
foundation in a river is likely to result in imminent catastrophic
failure.
For the above reasons, it is vital that reliable methods of
providing real-time information on critical sections of
infrastructure are developed.
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In recent years, concentrated research efforts have led to
advances in embedded sensor technology. The Smart Rail project
proposes to:
(i) Use modern ICT networks to collect data from embedded sensor
networks and use said data to populate statisticals for structural
health monitoring models.
(ii) Recognise that rainfall induced landslides result from an
infiltration of water into slopes, causing the water content to
increase and the soils strength and stiffness to reduce. The use of
remotely monitored sensors to measure water content variations
would provide critical data to network operators and act as an
early warning system for slope failures. A full-scale experiment is
planned on the Irish Rail network, where an embankment carrying a
section of rail line will be instrumented and subjected to
artificial rainfall to induce a slope failure.
(iii) Investigate Techniques to measure a bridges response to
scour. Modern instrumentation can provide both direct and in-direct
measurements of scour. The use of low-cost instrumentation which
can be deployed on a network wide basis and provide for real-time,
indirect measurement of scouring will be considered.
(iv) Develop a bridge weigh-in-motion system for railway bridges
which will be capable of separating the dynamic responses of the
structure from the train vibration, thus having the ability to
detect damage in the bridge.
(v) Use Corrosion Resistant Sensors (CRS). CRS have been
developed for monitoring reinforcement corrosion in road bridges.
CRS sensors will be used for the first time on rail bridges within
the Smart Rail project.
This report focuses on the choice of instrumentation, and where
appropriate, the initial laboratory calibration of the
instrumentation chosen for the demonstration projects on bridge
scour and slope stability as set out above.
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2 Bridge Scour
2.1 Introduction The analysis and monitoring of bridge scour has
gained considerable interest in recent years. Adverse hydraulic
action, including scour, has been deemed responsible for over 53%
of bridge failures in the United States between 1989 and 2000
(Wardhana & Hadipriono, 2003). Due to the current economic
climate, the conservation and maintenance of existing
infrastructure in order to prolong its lifespan has become
increasingly important. There are three primary ways of combating
the effects of scour. These are the use of structural, hydraulic
and monitoring countermeasures. Monitoring is usually the least
expensive of the three options (Briaud et al., 2011). Within this
branch of countermeasures, there are several options available:
Visual Monitoring, Portable Instrumentation and Fixed
Instrumentation. There is a myriad of existing instrumentation
available that falls under the headings of portable and fixed
instrumentation. These aim to monitor the progress of scour during
floods, with varying levels of success. In this section, the
available instrumentation is compared in terms of its successful
deployment in detecting and monitoring scour.
2.2 Instrumentation used to monitor bridge scour Many of the
current types of instrumentation in use require underwater
installations. These can be both costly and dangerous. Several
instrument types exist and they are grouped according to the
methods they use to monitor the occurrence of scour around bridge
piers and abutments. The most novel approaches involve using the
bridge superstructure to monitor changes induced by adverse
scouring of the foundations. All of these methods are summarised
below.
2.3 Single use Buried Devices / Float-Out Devices These devices
are installed into the ground, near the pier or abutment of
interest. They can be buried at multiple depths. Signals are sent
to data acquisition systems informing the user as to their status,
be it in position or floated out. Once the device floats out of the
ground, it indicates that the depth of scour has reached this level
and the device must be re-installed once more.
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Figure 1 Positioning Float-Out Devices (Monitoring Scour
Critical Bridges, A Synthesis of Highway Practice, n.d.)
2.3.1 Tethered Buried Switch This device is buried into the soil
at the location of interest for scour measurement. It is a type of
float-out device that is buried vertically into the streambed. It
can be hard-wired to a data acquisition system. When the rod
changes from a vertical orientation to a horizontal one, (as would
occur during the float-out stage) an electrical switch triggers.
This type of instrument sends out three discrete values to the data
acquisition system 1, 2 and 3. If the rod is vertical, it emits a
signal of 1 at a rate corresponding to the chosen sampling rate the
user specifies. A value of 2 corresponds to scour levels reaching
the depth of embedment and floating out has occurred. A value of 3
indicates that the sensor is damaged and needs to be repaired.
Advantages Disadvantages
The system is a reliable indicator of scour reaching a certain
level at a given location.
The system also tells you if a fault has occurred by
transmitting a default value of 3.
It has a single use and requires re-installation once it has
floated out.
It is susceptible to damage by debris since it is hard-wired
directly to the data logger device.
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Figure 2 Tethered Buried Switch (TBS) (Briaud et al., 2011)
2.3.2 Float-Out Device A float-out device is a cylindrical
device with typical dimensions of 11.43cm in diameter and 300cm in
length. These devices may be installed in the streambed at various
locations of interest near abutments and bridge piers. They are
installed in a vertical orientation and may be installed at various
depths. They become activated when scour levels reach the upper
level of the sensor and the senor floats out of position. An
on-board trigger mechanism sends a signal to a data acquisition
system that then alerts the user when the device has floated out of
the installed position. This is indicated by its orientation
changing from vertical to horizontal.
Advantages Disadvantages
They provide an easy method of detecting if scour has reached
the datum of the sensor, thus is reliable in this regard.
They are a self-contained unit and thus are mechanically
simple.
The system is costly, both to purchase and to install.
The sensor must be reset after each float-out event, making it
impractical for remote sensing requirements.
It only works if the scour hole reaches the level of the sensor
and will only work at the location where the sensor is placed,
which may not be the exact location of maximum scour
occurrence.
They have a limited battery life which is in the region of seven
years, thus they require re-installation after this time has
elapsed.
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Figure 3 Typical Float-Out Device (Monitoring Scour Critical
Bridges, A Synthesis of Highway Practice, n.d.)
2.4 Pulse / Radar Devices: These devices utilize radar signals
or electromagnetic pulses to determine changes in material
properties that occur when a signal is sent through a changing
medium as would occur at a water-sediment interface. These signals
henceforth determine the depth of a scour hole, at a given
time.
2.4.1 Time-Domain Reflectometry This method was originally
developed by electrical engineers interested in detecting
discontinuities in power and communication transmission lines. It
works on the principle of measuring changes in the dielectric
permittivity constants of various materials. Measuring probes are
installed into the streambed at the location of scour interest. A
fast rising step impulse is sent down a tube, buried into the
ground. When the wave reaches an area where the dielectric
permittivity changes, a portion of the energy is reflected to the
receiver. Dielectric permittivity properties are different for air,
water and sediment/ Hence, a geophysical profile may be established
that will show the progressive depths of scour at the particular
location of interest (Hussein, 2012).
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Advantages Disadvantages
Relatively full images may be obtained that show the air/water
interface and the water/sediment interface.
A good geophysical profile is thus established showing clearly
the existence and depth of scour.
It requires that long probes be installed at bridge piers, which
is expensive and time consuming as well as requiring underwater
engineering works.
Figure 4 Time-Domain Reflectometer (Briaud et al., 2011)
2.4.2 Ground Penetrating Radar (GPR) A GPR transmitter is
floated out in a river to the location of interest for obtaining
the depth of a scour hole. An electromagnetic pulse is then sent
through the water and the waves are partially reflected as they
pass through the different media. The waves are of a very high
frequency (in the range of MHz). It works on a very similar
principle to the previous Time Domain Reflectometry (TDR) approach,
whereby changes in the dielectric properties are identified as the
waves reflect at different stages. The reflected signal is recorded
into the receiver and an overall geophysical map may be generated,
showing clearly the submerged scour hole and its depth.
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Advantages Disadvantages
The method is easy to implement and can be relatively
successful.
The method can produce an accurate model of the channel bottom
(to depths of the order of 10m) and subterranean lithological
features with thicknesses in the region of 0.3m. A 200 MHz
intermediate frequency can undertake this.
The method is non-invasive and can be moved rapidly across the
channel surface to obtain the images required for analysis.
The device does not need to be physically coupled to the water
surface and can be operated remotely.
Profiles can be extended across emerged sandbars and onto the
shore.
Requires manual use and must be floated into position.
It is dangerous to undertake these activities during a flood
scenario.
The equipment is relatively expensive.
The device only gives scour information at the time the method
is employed and is not suitable for the purpose of continuous
monitoring.
Figure 5 Typical GPR Profile (Anderson, 2007)
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2.5 Piezo-electric Film Sensor Devices These types of sensors
utilize strain deformations to generate an electrical signal, which
can alert the person monitoring that scour levels have reached a
certain level. An array of sensors can be placed onto plates that
are buried. When buried, no bending deformation occurs. When
exposed to flow, deformation occurs and a signal is sent to a
data-logger to alert that scour levels have reached the particular
level of the sensor.
2.5.1 Fibre Optic Sensors using Fibre Bragg Grating (FBG)
methods
These fibre optic sensors are composed of Fibre Bragg Grating
(FBG) elements that can monitor bridge scour in real-time. Optical
fibres are useful in that they are reliable against corrosion, long
term degradation and general environmental damage. Several Fibre
Bragg Grating sensors can be arranged linearly along an optical
fibre. This can then be mounted onto a cantilever plate, and
installed at different levels of a steel pipe fixed to a pier or
abutment. The system works on the principle of picking up strain
deformation that will occur in the plate if it becomes exposed to
the impulse force from flowing water. Only plate elements exposed
to the flow will bend, hence an accurate measurement of scour
levels can be derived from this.
Advantages Disadvantages
Method is reliable and relatively cheap to implement.
Method can be tailored to particular accuracy levels required by
augmenting the number of sensors placed onto the optical fibre.
The resolution is only as good as its number of sensors.
It may be highly sensitive to vibrations of the support pipe due
to the flowing water or traffic excitation.
For this reason, reviewers have declared little difference being
obtained in some cases between buried and exposed sensors. (May,
Ackers, & Kirby, 2002).
2.6 Buried / Driven Rods: These instruments work on the
principle of a manual or automated gravity based physical probe
that rests on the streambed and moves downward with increasing
progression of a scour hole during a flood scenario. The system
utilises some form of remote sensing mechanism to detect the level
change of the gravity sensor. The sensor must be sufficiently large
to prevent penetration into the bed while in a static stage prior
to the occurrence of scour as this will affect the accuracy of the
perceived results.
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2.6.1 Magnetic Sliding Collar This is a magnetic collar placed
around a structurally rigid pipe that is driven or augered into the
streambed at a particular location near a bridge pier. The magnetic
nature of the collar allows it to trigger sensors in the rod. As
the streambed erodes, the collar slides down along the rod. The
data may be either manually or automatically read. In the case of
automatic reading, flexible cables are attached to a datalogger and
convey magnetic switch closures corresponding to collar movement.
The manually read case requires the use of a hollow metal tube to
connect the sensor to the bridge deck.
Advantages Disadvantages
Relatively cheap and gives a good indication of maximum scour
depth attained during a flood.
Manually read scenario requires infrastructure in the form of
metal tubing that is very susceptible to damage from ice or
debris.
We can only detect scour depths specifically at the location of
the device.
It requires a pile driver to install the device into the
ground.
Once the flood waters subside, the collar will remain at the
lowest elevation reached. Hence, the device must be reset after
each individual flood event. This is both costly and labour
intensive.
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Figure 6 Magnetic Sliding Collar (Monitoring Scour Critical
Bridges, A Synthesis of Highway Practice, n.d.)
2.6.2 Scubamouse The scubamouse device consists of a steel pipe
that is buried or driven into the streambed in front of a bridge
pier. The steel pipe has a horseshoe-shaped collar around the
outside, which rests initially on the un-scoured streambed. As
scour progresses during a flood, the collar remains at rest on the
riverbed, which lowers in elevation. As water stages reduce when
the flood begins to subside, the scour hole begins to fill with
sediment, thus burying the collar. The collar remains at rest at a
depth corresponding to the maximum depth of scour reached during
the flood. A radioactive detection mechanism is slid down the
inside of the steel buried pipe in order to detect the resting
location of the collar. A signal may then be sent back to a data
logger device. This device operates very similarly to the magnetic
sliding collar described previously.
Advantages Disadvantages
This method is inexpensive and easy to deploy.
It works on the very simple principle of a weight resting on the
riverbed.
A significant disadvantage is that the device has a single
use.
This means that it needs to be reset after each individual flood
event.
This makes it very impractical for bridges that are susceptible
to frequent flooding.
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2.6.3 Wallingford Tell Tail Device This type of device has been
installed on many older high risk structures in the UK at the
location where the maximum scour is expected. It consists of a set
of omni-directional motion sensors, mounted on tails connected to a
rod and buried in the streambed at a range of depths. It can be
connected to a data logger via a cable. The motion sensors detect
bed movements that are indicative of scour having reached the depth
of embedment of the sensor.
Advantages Disadvantages
This device is relatively reliable and has a low power
consumption.
One significant disadvantage is that the device must be reset
when the level of the sensor is reached.
2.6.4 Mercury Tip Switch A number of mercury tip switches can be
arranged along a support pipe that is driven or augured into the
ground near the front of a bridge pier or abutment. The devices
work on the premise that as the rod or pipe is driven into the
streambed, the tip switches become folded up against the rod, which
closes the circuit. The presence of the streambed material is what
is responsible for ensuring that the switch remains open. As the
streambed erodes away due to scour, the material around the switch
will no longer hold the switch open and it will flip into the
closed position, thereby breaking the circuit.
Advantages Disadvantages
Due to the simple switching mechanism, the parts can be
purchased in any electronics shop.
Also, due to the simplicity of the technology, it is easy to
develop a rugged sensor array that can endure long-term exposure to
the elements.
The accuracy of the system can also be tailored to the needs
required. By spacing the switch array closer together, a more
accurate scour monitoring system can be developed.
One disadvantage of this type of sensor is that the use of
mercury in the tip could be perceived as an environmental
hazard.
Even though the housing is extremely durable, environmental
damage from debris or ice may release the mercury within.
Also, once the depth of scour is reached, this sensor type will
not show any further scour activity such as scour hole in-fill or
re-scour.
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2.7 Superstructure Monitoring This is a relatively new area in
the region of scour monitoring. It proposes using the
superstructure to detect changes in the support conditions caused
by excessive scouring of pier and abutment foundations. There are a
number of ways to detect this damage. The following instruments aim
to use structural characteristics as a damage indicator.
2.7.1 Tiltmeter A Tiltmeter (or Inclinometer) is a device used
to measure the rotation of a structural element caused by
compromised support conditions induced by progressive scouring of
foundations. They can be used in two primary ways. If progressive
scour causes adverse settlement of a pier support, an inclinometer
placed on the bridge deck near the pier interface should show
rotation of the deck. The other method is to place these devices in
a line along a rigid pier. If differential settlement occurs due to
differential undermining of a pier support, the inclinometer should
detect this as a rotation of the pier. They can be combined
together in a housing at orthogonal orientations to create a dual
axis Tiltmeter. This will provide information on movements in two
planes of rotation. This can be desirable and useful due to the
three-dimensional problem scour poses. A positive output denotes
clockwise rotation.
Advantages Disadvantages
These devices can be used in remote sensing applications.
They are robust and reliable.
Another major advantage is that these sensors do not require
high sampling rates, thus they conserve energy.
The output is simple to read as it is effectively the degrees of
rotation vs. time.
No major analysis is required in order to obtain meaningful
data.
Rotation values may be an indication that complete compromised
support conditions have been met and it may be effectively too
late.
These may not show the progression of scour, but merely show it
when it has reached significant levels.
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Figure 7 Tiltmeter (Monitoring Scour Critical Bridges, A
Synthesis of Highway Practice, n.d.)
2.7.2 Accelerometer An accelerometer is a motion sensing device
that can be used to obtain the change in acceleration profile of a
super-structural element subject to excitation (ambient or forced).
It operates by taking data points at a sampling rate that is high
in comparison with the structural vibration that is expected
depending on the scale of the structure. This acceleration profile
can be used to obtain dynamic characteristics such as natural
frequency and damping ratio. Any changes in the structural support
scheme caused by scour can be detected using these sensors, which
are placed on piers.
Advantages Disadvantages
This has the potential to be a robust method, if used
correctly.
It can be used as a remote sensing system.
It does not require an underwater inspection or expensive
underwater installations, hence it is easy to install when compared
with other types of instrumentation.
It can be used to show the progression of a scour hole as is
indicated by changes in the acceleration profile as the hole
develops.
There is an issue with high power consumption (Briaud et al.,
2011). There is also the possibility that one may not obtain
adequate excitation from ambient traffic or rail loading to obtain
a high signal to noise ratio.
Rigid pier structures may not vibrate adequately to pick up the
signals.
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2.8 Sound Wave Monitoring These devices utilise sound wave
reflection from material of different densities and other
properties to establish the location of the sediment-water
interface and hence, the depth of a scour hole.
2.8.1 Sonic Fathometer This device can be mounted onto a bridge
pier or abutment, immediately below the level of the water stage. A
sonic pulse is emitted from a pulse generator, which travels
through the given medium until it comes to the sediment-water
interface. At this location, partial reflection occurs and the
reflected wave passes back to a receiver. By applying known
material properties to the data obtained, meaningful information
regarding the location and condition of the streambed may be
assessed. The scour hole, if present, will be measurable with this
method. It works on a very similar principle to that of the pulse /
radar devices described previously but differs by using sound waves
in lieu of electromagnetic or radar pulses.
Advantages Disadvantages
The devices are cheap and easy to deploy.
They also prove to be quite accurate over small distances.
Fixed sonar monitoring can provide continuous data for the soil
erosion and the nature of the streambed.
If high levels of air entrainment exist due to high flow
turbulence, or if a particularly high concentration of moving
sediment just above the static sediment interface exists, the
device will not work accurately.
The device is only accurate within certain depth tolerances.
Too shallow an installation will lead to useless data being
obtained.
The devices are only useful within a narrow area.
The state of the streambed outside of this bracket will not be
known, thus effective placement on the device is imperative.
Since the device is placed below the waterline, any debris
present can potentially damage the device rendering it useless.
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2.8.2 Reflection Seismic Profilers This type of device typically
employs a coupled acoustic source transducer / receiver transducer
placed immediately beneath the surface of the water. The acoustic
source transducer produces short period pulsed acoustic signals (in
the range of kHz) at regular time intervals or distance intervals
as it is towed across the water surface. The high frequency seismic
pulsed signal propagates through the water column into the
subterranean sediments below. At this interface, some of the
acoustic signal is reflected back to the receiver. This receiver
measures and can digitally record the magnitude of the reflected
signal in terms of its energy and two-way travel time. The
magnitude of the reflected signal vs. its travel time for the
different signalled locations can be displayed on a time trace.
This allows for an effectively continuous depth profile to be
obtained across the river cross-section, by combining the signals
from multiple locations. Estimated seismic interval velocities can
be used to transform the time-depth profile into a depth profile.
Water velocities are a function of suspended sediment concentration
and can vary appreciably (Anderson, 2007).
Advantages Disadvantages
This tool can provide an accurate depth-structure model of the
channel bottom to depths of the order of tens of meters.
Post-acquisition processing of the data can be applied.
Depending upon the source frequency, the tool can provide very
accurate imagery of the channel sub-features, including in-filled
scour holes.
The source and receiver need to be submerged.
The tool cannot be used, therefore, to gain profiles across
sandbars or other structures above the waterline.
Contamination of data by noise is plausible due to the
multi-faceted nature of the bed and cross-over signalling, as well
as shoreline and bridge pier echoing.
It requires manual operation and information on scour holes is
only obtained at the particular instant when the method is
applied.
2.8.3 Echo Sounders These devices are similar to reflection
seismic profilers in that they also employ a coupled acoustic
source transducer / receiver transducer placed immediately below
the surface of the water. The devices differ from seismic profilers
in that they emit a higher frequency acoustic source pulse (in the
100 kHz range), some of which is reflected at the channel bottom
and returned to the receiver. Due to the rapid attenuation of the
high frequency pulsed acoustic energy, relatively little signal is
transmitted into or
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reflected from within the sub-bottom sediment. Traces from
adjacent source / receiver locations can be plotted side by side to
generate a coherent time-depth profile. By applying estimated
seismic interval velocities, these plots can be converted into
depth profiles (Anderson, 2007).
Advantages Disadvantages
This tool can provide an accurate depth profile of the river
cross-section, if the acoustic velocities are known.
Post-acquisition processing can be applied.
Both the source and receiver must be submerged.
Therefore, profiles cannot extend over sand bars or other
over-water structures.
Noise contamination by pier / shoreline reflection can occur,
which can skew the data received.
Since the method uses high frequency waves that do not penetrate
into the sub-bottom strata, the device will not show the presence
of in-filled scour holes.
2.9 Electrical Conductivity Devices These devices measure the
ability of a solution to conduct an electric current between two
electrodes. If the material between the probes changes the ability
to draw a current also changes. This can act as a scour
indicator.
2.9.1 Electrical Conductivity Probes These devices measure the
ability of a solution to conduct an electric current between two
electrodes. In solution, currents flow by ion transport. Therefore,
an increase in ion concentration will result in higher conductivity
values. Conductivity probes actually measure conductance, which is
the reciprocal of resistance. Conductance is measured using the SI
parameter Siemens. The use of this method in the context of scour
measurement is based on the idea that the conductance of the river
bed is different to that of flowing water. The nature of suspended
sediments, dissolved ions and chemical characteristics of water
determine its conductivity value. Parent materials and the
composition of the water in the sediments determine the electrical
conductivity of the riverbed. Using this technique, multiple
sensors are placed on a probe that is driven vertically into the
riverbed and left,at the desired location of interest for periodic
monitoring. One sensor should be left above the sediment interface
as a control, while the rest should be submerged below the
riverbed. If scour occurs, thereby revealing the buried sensors,
then these should measure the conductivity of the flowing water
instead of that of the sediment, thus observing the presence of
scour.
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Advantages Disadvantages
This method allows for long-term monitoring and is relatively
robust.
It works well, provided that the conductivity properties of the
sediment and the water vary significantly. It is possible to tailor
this tool to the required accuracy by increasing the density of the
sensor array placed along the buried probe.
The tool only monitors the development of scour directly at the
location where it is buried.
It cannot be used to identify scour in-filling. Scour features
can be significantly underestimated, particularly if the sensor is
not located at the location of maximum depth of the scour hole.
The tool cannot be used to image scour features that are below
the subchannel bottom sediments.
2.10 Discussion on Scour Monitoring Equipment There are a range
of devices available to monitor scour. Some devices will measure
progression of scour holes as they develop during times of great
flooding. Other devices will only give a static value of scour at
the given time the monitoring took place. The reliability of many
of these devices is questionable. Those devices that depend on the
mechanical movement of certain parts are less reliable in that
mechanical failure is much more likely given the hostile nature of
the environment of the underwater sensor.
From the perspective of monitoring scour-critical bridges, any
of the devices should prove adequate. Those requiring
re-installation may prove troublesome due to the time and economic
cost involved. The buried float-out devices are particularly
relevant here. Once the scour hole has reached their level, they
will simply float out and are essentially no longer operational at
that point. The pulse / radar devices such as Ground Penetrating
Radar (GPR) and the devices using sound waves such as Sonic
Fathometers and Echo Sounders are only useful in giving scour
information at a particular time (usually after a flood event).
They are particularly unsuitable for the analysing of scour hole
progression as maximum scour depths are attainable within a number
of hours during a flood event in sediment streambeds composed of
sand. Limited information, in this regard, is obtained by using
these devices. As a preliminary scour assessment, they are quite
appropriate. This refers to portable monitoring versions of this
equipment only. Fixed Sonic Fathometers, on the other hand, can be
used to analyse the progression of scour.
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The concept of scour holes re-filling upon flood subsidence is
particularly important in that the carrying capacity and stiffness
of the in-fill material may be significantly less than the original
sediment in the streambed. Most of the methods for scour monitoring
are incapable of measuring these effects. GPR provides a good
analysis but is limited as described previously. Most of the
mechanical based apparatuses such as the Magnetic Sliding Collar,
Scubamouse and Mercury Tip Switches are particularly ineffective in
this regard and also require re-installation once scour levels
reach depths below their operational elevations. The methods
utilising Piezoelectric Film Sensors and Electrical Conductivity
Probes offer promise in that they have no mechanical parts prone to
failure. Alas, they are unsuitable to monitoring scour holes
re-filling upon flood subsidence.
Recent developments in scour monitoring instrumentation look at
using the superstructure to monitor the presence and development of
scour. The structure will respond to ambient loading differently if
the foundations become compromised or undermined. Accelerometers
can be used to measure natural frequency and subsequently damping
ratios of bridge piers subject to train loading. The novel aspect
here is that underwater instrumentation is not required. This is a
consideration that can reduce the cost of monitoring significantly.
Recent developments in this area have shown promising results with
the use of accelerometers. Tiltmeters can also be used to observe
the occurrence of differential settlement of piers due to
undermining. In terms of scour monitoring, they only become
effective when the situation has reached a critical level and thus
may be seen to be too late from the perspective of a bridge manager
(Briaud et al., 2011).
2.11 Instrumentation of choice for Scour Due to the novel nature
of the dynamic approach to bridge scour monitoring and assessment,
the use of accelerometers as a method of assessing the progress of
scour holes during floods will be investigated in the SMARTRAIL
project. The frequency response of bridge piers will change
depending on the level of supporting soil surrounding the
foundations. As scour progresses, the pier support condition will
vary from somewhere between a fixed and free support closer to a
free end support. This lengthening of the exposed pier should have
a corresponding decrease in natural frequency. Damping ratios may
also be analysed. Ongoing research is being undertaken with regard
to use of this equipment in this regard.
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3 Slope Monitoring
3.1 Introduction One of the effects of climate change is
increased rainfall, a factor which is having a detrimental effect
on the integrity of the slopes. It is imperative to the safe
operation of the railway that we can monitor and analyse slope
safety in real time. Soil matric suction is a critical strength
component in embankment stability. Rainfall infiltration has been
shown to reduce soil suction (Gavin & Xue, 2009; Ridley,
McGinnity, & Vaughan, 2004; Xue & Gavin, 2007) thereby
decreasing the safety of the slope. Several techniques have been
developed to monitor negative pore water pressure (soil suction).
These are set out in detail below. Soil moisture content is also a
critical parameter as it has a direct correlation to soil suction
and there is a lot of existing data which enables users to predict
soil suctions using a soil-water characteristic curve for a
particular soil. Soil moisture has been measured for many years (A
Tarantino, Ridley, & Toll, 2008) and numerous companies provide
a highly accurate means of doing so. Further details are outlined
below.
Several methods of monitoring slope deformations are also
outlined in this report. However, many of these methods are
considerably more expensive than monitoring pore pressures and soil
moisture content and typically give less warning time.
3.2 Slope Monitoring Instrumentation
3.2.1 Tencate GeoDetect Tencate is a French company which has
produced a geotextile called GeoDetect ,which is outfitted with
fibre optic cables. These cables act as sensors and can be
monitored for changes in strain and temperature. It can be used to
provide an early warning system, as a structural health monitoring
sensor or simply as soil reinforcement. When installed correctly,
strains as low as 0.02% can be monitored within the soil. The
system is connected to an optical interrogator which sends pulses
of light through the fibre optic cables embedded within the
geotextile. If there is a strain change within the cable the light
will be refracted at this point. This refraction is then picked up
by the optical interrogation unit. This information is then relayed
back to a computer where it can be interpreted in real time.
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Figure 8 Transfer of data from Site to Computer
(http://www.tencate.com)
Advantages Disadvantages
An extremely accurate way to monitor ground movements.
Extremely easy to install in new embankments geotextile.
Easily applicable to real time monitoring.
Proven track record in monitoring railway settlements having
previously been used to great success by SNCF.
The optical interrogator needed to interpret the changes in
light pulses is currently quite expensive and makes the technology
quite prohibitive on a large scale.
While easily to install on new embankments, medium scale
earthworks would be required to install on an existing
embankment.
Equipment is buried. Therefore, maintenance could prove to be
potentially problematic.
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(a) (b) Figure 9 Close up of Geotextiles showing Fibre optic
cables (b)
(http://www.tencate.com)
3.2.2 Geobeads Alert Solutions Geobeads is a multi parameter
sensor manufactured by Alert Solutions in the Netherlands. Geobeads
is quite innovative as it consists of an array of up to 100 nodes
embedded in one cable. The cable itself serves as a power supply
whilst simultaneously enabling data transmission. It can be up to
1000m in length. Each node can contain multiple sensors to monitor
any combination of inclination, positive soil pore water pressure
and temperature. Their pore water pressure sensors are also able to
detect negative pore water pressures (soil suction).
Figure 10 Close up of a Geobeads
sensor(http://www.geobeads.com/)
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Figure 11 Typical Site after installation
(http://www.geobeads.com/)
Advantages Disadvantages
Easily scalable due to its nodal nature, it is straightforward
to expand the operation.
Easier to install than most systems as all sensors are combined
within one small cable, therefore allowing for fast installation of
multiple nodes at once.
Continuous and automatic measuring, frequencies of measurement
and alarms can be remotely set.
Remote sensing can be viewed online 24 hours a day. There is no
need to visit test sites unless a problem arises.
As data is delivered online it can be accessed from any computer
with an internet connection.
Multiple nodes can be attached to one network controller (up to
100). Has previously been used by railways SNCF and in embankments
in the Ijk Dijk project.
Incompatible with other data loggers. Depending on installation
method, sensors may be sacrificial.
While sensors are relatively affordable, network controllers and
project fees (monthly fees necessary to use online remote access)
are costly.
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2.1.3 Laser Scanner
Terrestrial laser scanning can be applied to monitor surface
deformations of slopes. This technique involves taking 3D scans of
the slope in question at different time intervals. A comparison can
then be made between scans and any volume change can be picked up
on. Hansje Brinker applied laser scanning on the Ijkdijk project
and were able to detect a statistically significant deformation 26
hours before the dijk failed. LIDAR is fast, efficient, and
portable. Typically it is accurate to 2mm in 100m.
Figure 12 Typical Laser scanner produced by Trimble
(http://www.trimble.com)
Advantages Disadvantages
Portable and easy to use.
Fast accurate results.
Stand alone equipment does not rely on data loggers etc.
Highly versatile piece of equipment which has great
reusability.
Expensive.
A series of scans is needed for any deformations to be
noticed.
Equipment is not suited to permanent installations.
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2.1.4 Extensometers
Extensometers can be used to monitor heave and settlement within
embankments. They can also provide data on the depth at which
settlement has occurred. A number of different types exist. One of
the most common is magnet extensometers which consist of a number
of magnets coupled with the surrounding soil. A probe is then
passed through a nearby access pipe which records the depths of the
magnets by interpreting the strength of their magnetic field. If
the access pipe is stable these depths can be referenced to a datum
magnet at the base of the pipe otherwise the top of the access pipe
must be surveyed prior to measurements being taken.
Figure 13 Magnet Extensometer by Slope Indicator
Advantages Disadvantages
Accurate.
Easy to use.
No post processing.
Requires manual readings.
Installation can cause destruction to surroundings.
Very localised monitoring.
Can be used to monitor vertical or horizontal deformation but
not both.
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2.15 Instrument: Inclinometers Inclinometers are used to detect
lateral movement and shear planes in slopes. An inclinometer casing
is first installed in the slope. This casing has precast orthogonal
grooves in its interior walls. The casing is installed with one of
the grooves facing in the direction of principal deformation. An
inclinometer sensor with orthogonal tilt sensors is then inserted
into the casing. The sensor has wheels which slot into the grooves
in the casing enabling it to move along the length of the casing.
The tilt sensors then monitor the angle of inclination of the
casing at regular intervals and generate a profile. Subsequent
measurements are then compared with this initial profile to monitor
the rate of displacement.
Figure 14 Inclinometer Casing showing precast grooves
(www.slopeindicator.com)
Figure 15 Inclinometer Probe (www.slopeindicator.com)
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Advantages Disadvantages
Accurate.
Established method.
Good for long term monitoring.
One inclinometer sensor can be used in multiple casings.
Initially expensive.
Destructive installation process.
May add rigidity to soft soils.
Manual operation is required.
3.3 Positive Pore Water Pressures
3.3.1 Instrument: Piezometers: Casagrande Standpipe A standpipe
or Casagrande piezometer constitutes a porous filter tip which is
connected to a riser pipe. The porous tip is sealed in the soil at
a certain depth using a bentonite grout. Water is then free to
enter or exit the riser pipe through the porous tip. Therefore, as
pore water pressure increases or decreases the water level within
the riser pipe rises and falls respectively. Then by monitoring the
change in water depth the flux in pore water pressure can be
observed.
Figure 16 Casagrande Standpipe
(http://www.rstinstruments.com)
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Advantages Disadvantages
No electronic components.
Easy to measure.
Manual readings.
Requires access to the top of the pipe.
Response time is directly dependent on the permeability of the
soil.
3.3.2 Instrument: Vibrating Wire Piezometer
Vibrating wire piezometers are used in conjunction with a data
logger. They consist of a diaphragm based pressure transducer and a
signal output cable. They are available for a wide range of
pressures and can be used in all soil types. They can be installed
completely encased within a bentonite cement grout, or, they can be
installed in sand in a take zone with a bentonite seal. They are
based on the vibrating wire theory whereby tension in a wire is
proportional to its natural frequency squared. The tension on the
wire is controlled by the external pressure acting on the
diaphragm. The wire is then excited causing it to vibrate at its
natural frequency. This frequency is then recorded and calibrated
against pressure to produce pore water pressure readings.
Figure 17 A selection of Vibrating Wire Piezometers from
Geokon
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Advantages Disadvantages
Quick response times. Suitable for automated logging. Multiple
piezometers can be grouted in the same borehole.
Requires a data logger.
Requires calibration.
The calibrated component is buried.
3.4 Negative Pore Water Pressures
3.4.1 Instrument: Tensiometer
Tensiometers consist of a porous ceramic cup attached to a
tubular body of varied length. Contained within the tubular body is
a diaphragmatic pressure transducer. The tube and body are filled
with de-aired water. Tensiometers can be inserted directly into the
ground without the need for grout provided that the tip of the
borehole augured is slightly smaller in diameter than that of the
tensiometer. This ensures a snug fit and ensures good contact
between the porous disk and the surrounding soil.
Water is able to flow through the ceramic disk when saturated
while the flow of air is prevented. This means that when
equilibrium is reached between the soil and the tensiometer, the
pore-pressure in the soil will be the same as the pressure in the
water within the tensiometer. Modern tensiometers can measure
negative pore water pressures of approximately 90kPa. After this,
the water within the tensiometer begins to cavitate, making further
measurements unreliable. There are several different types of
tensiometers on the market including; regular tensiometers,
Jet-fill tensiometers, miniature tensiometers, and self-refilling
tensiometers.
Advantages Disadvantages
Accurate to -90kPa.
Affordable.
Can refill without removing.
High maintenance.
Can be damaged in dry or frozen soils.
3.4.2 Instrument: Jet-Fill Tensiometers
A jet fill tensiometer has a water reservoir on top of the
tensiometer which helps remove air bubbles from the body of the
tensiometer. This is done by pressing a button
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which releases water into the body of the tensiometer from the
reservoir, displacing air as it does so. These air bubbles then
move upwards accumulating at the top of the reservoir.
Advantages Disadvantages
Easy to remove air from Expensive
3.4.3 Instrument: Automatic Fillling Tensiometers Automatic
filling tensiometers are highly specialised tensiometers suited to
dry ground. Normally when dry soil removes water from the porous
cup, tensiometers need to be refilled again which requires a site
visit. However, these tensiometers will refill themselves at the
next rainfall event and will automatically de-aerate themselves.
Furthermore, tensiometers are usually irreparably damaged by frost.
However, these sensors will detect freezing conditions and purge
the system of all water until after the event.
Figure 18 A Selection of different tensiometers from left to
right standard tensiometer (http://www.decagon.com/),
jet fill tensiometer(http://www.soilmoisture.com), Ts1 smart
tensiometer(http://www.decagon.com/) and
miniature
tensiometers(http://www.earthsystemssolutions.com).
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Advantages Disadvantages
Accuracy.
Automatic self-refilling and de-aerating .
Automatic emptying before frost.
Continuous fill level controlling.
Low maintenance.
Expensive.
3.4.4 Geotechnical Observations Flushable Piezometers
Geotechnical Observations flushable piezometers allow for the
measurement of positive and negative pore water pressures. They are
suitable for use in any earthen structure. They are of a similar
construct to tensiometers in that they have a water reservoir
within a porous cup and the stress exhibited in this water is
measured by an attached pressure transducer. However, the flushable
piezometers are connected to a pumping system which can circulate
de-aerated water around the system, flushing any air which has
built up in the system.
Figure 19 Cross-section of Flushable Piezometer
(http://www.geo-observations.com/)
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Figure 20 Piezometer (http://www.geo-observations.com/
Advantages Disadvantages
Accuracy.
Measures positive and negative pore water pressure.
Each sensor has an independent stand alone data logger
inbuilt.
The system removes air.
Only available to rent.
Requires trained personnel for instalation.
3.5 Soil Moisture
3.5.1 Instrument: Water Content Reflectometers Water content
reflectometers measure the volumetric water content of porous media
such as soil. They consist of a pair of stainless steel rods
connected to a circuit board. The water content is obtained from
the probes sensitivity to the dielectric constant of the soil in
which it is embedded. They can measure volumetric soil moisture
from 0% to full saturation. Probes can be fully buried in the soil
or they can be inserted from the surface for near surface
measurements.
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Advantages Disadvantages
High accuracy and precision.
Fast response time.
Perfect for long term monitoring.
Compatible with a wide range of data loggers and
multiplexers.
Inexpensive.
Awkward to bury.
Figure 21 Volumetric Soil Moisture probe from Campbell
Scientific
3.6 Summary Whilst major advances in monitoring slope movements
using laser scanning techniques have been made, such systems tend
to be expensive and reactive. In the SMARTRAIL project, the use of
embedded sensors to measure suction and water contents will be
investigated. Such sensors measure the physical response of soil to
rainfall infiltration and have the potential to act as an early
warning system for stability problems.
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4 Laboratory Study
4.1 Overview In order to assess the efficacy and to calibrate
some of the chosen instruments, laboratory studies were undertaken
to examine whether the dynamic response of a simple structure
varied in response to the occurrence of scour in a sand stratum. In
addition, a laboratory study of the effect of rainfall on the
suctions measured in glacial till used to construct Irish railway
embankments was undertaken.
4.2 Use of Accelerometers to investigate bridge scour
4.2.1 Background Larger and more frequent flood flows expose
foundation soils to stronger erosive forces, thus increasing the
likelihood that scour of piers (and abutments) will compromise the
structural integrity of some bridges. The development of low-cost,
low maintenance, non-destructive methods of bridge scour analysis
is therefore becoming increasingly more important in light of the
current economic climate. The use of embedded sensors that measure
the vibration responses of a structure may offer the potential to
track changes in the foundation soil stiffness matrix caused by
scour, and may also inform engineers when implementing appropriate
protection schemes. This paper presents a laboratory investigation
in which the dynamic response of a scaled pier installed in a bed
of sand and instrumented with an accelerometer is recorded for a
constant and repeatable excitation. Sand stiffness properties were
manually altered by increasing the scour depth in progressive
experiments. For each experiment, a vibration response was recorded
and this was converted to a frequency response using a fast Fourier
transform (FFT). Differences between the dynamic signatures of the
piers for the different scour conditions investigated were analysed
in order to explore whether this type of non-destructive testing
could provide a viable method of detecting scour before the
structural integrity of the bridge reaches a critical stage.
Results indicate that significantly different frequency responses
are recorded for decreasing elevations of bed material around the
model pier. This indicates that the method may provide the basis
for a simple and effective means of monitoring scour around bridge
piers.
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Scour can be defined as the excavation and removal of material
from the bed and banks of streams as a result of the erosive action
of flowing water (Hamill 1999). There are three main forms, namely;
general scour, contraction scour, and local scour. General scour
includes the aggradation and degradation scour that may result from
changes in the fundamental parameters that control channel form
such as flow rate and changes in the sediment supply to the river
system (Forde et al. 1999). Constriction scour occurs due to an
increase in flow velocity and resulting shear stresses caused by a
decrease in the river cross-sectional area due to the presence of a
bridge. Local scour arises due to increased velocities and
associated vortices as water accelerates around the corner of
abutments and piers, inducing downward flow and subsequent scour of
the riverbed (Hamill 1999). The scour hole generated can reduce the
carrying capacity of the foundation and can lead to catastrophic
structural collapse. Adverse hydraulic action, including scour, are
deemed to have accounted for over 53% of bridge failures in the
United States between 1989 and 2000 (Wandhanna and Hadpriono 2003).
This work assesses whether dynamic vibration signals can be used to
detect changes in the fundamental frequency of a pier arising from
changes in the stiffness of the foundation system from increased
local scour. The assessment utilises a laboratory arrangement in
which a vertical pier installed in a sand matrix and instrumented
with an accelerometer is subjected to a constant and repeatable
excitation for varying scour condition.
Scour poses significant risks to bridges and can be difficult to
detect, particularly for situations where the scour hole fills
after a flood has subsided. The concept of instrumenting bridges
and their foundations to detect changes in scour levels has gained
considerable interest in recent years. Many different methods have
developed over time, and these are employed to monitor scour around
piers and abutments. The use of Ground Penetrating Radar (GPR) as
outlined in Forde et al. (1999) can be particularly effective in a
freshwater environment as it can detect geophysical subterranean
changes that occur when a scour hole develops and becomes filled
in. It can prove difficult, however, to undertake these surveys
during flood conditions, as water flow rates can often be
dangerously high. Other methods such as the use of sonar detection
systems mounted on bridge piers, together with the installation of
buried Sedimetri systems close to piers, can be quite promising.
These, however, require care in accurately interpreting the results
(Falco and Mele 2003). Recently, the use of accelerometers on
bridge piers to detect changes in dynamic frequency has gained a
high level of interest as a method of long-term, non-intrusive
monitoring of bridge stability. In one example, a field test is
described where a pair of bridge piers, instrumented with wireless
accelerometers, were subjected to free vibration before and after a
simulated scour event with the aim of detecting changes in their
natural frequency (Lin et al. 2011). Another case outlines a study
of a road bridge in Turin, Italy, that was instrumented with
accelerometers to detect changes in dynamic signatures of different
piers relative to one another during the progression of scour as
well as before and after the planned retrofitting of one of the
piers (Foti and Sabia 2011). Briaud et al. (2011) describes a major
study
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aimed at developing the correlations between different scour
assessment techniques with the change in acceleration profile and
natural frequency as scour holes develop both under laboratory
schemes and on real bridges subjected to traffic loading.
4.3 Experimental Apparatus
Soil Characteristics Blessington sand (Co. Wicklow, Ireland)
which has a bulk density in the region of 2.03Mg/m3 was used in the
experiments. A sieve analysis was undertaken on the soil in order
to establish its grading (Figure 22). Grading indicates that the
sand is closely graded with 60% by weight, being less than 0.3mm.
The moisture content of the sand was calculated to be 13%, and this
value was taken as the matrix moisture content at the commencement
of each experiment.
Figure 22 Sieve Analysis
4.3.1 Steel Container Set-up The experiment was assembled in a
bolted together steel box with dimensions of 1m x 1m x 1m (Figure
23). The box housed the vertical pier installed in the bed of
Blessington sand. The significant mass of the box provided a rigid
structural framework in which to conduct the dynamic tests on the
pier. It was also sufficiently strong to support the weight of soil
to be placed in the box.
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Figure 23 Steel Box used in lab experiments
4.3.2 Upright Cantilever The upright pier structure was a hollow
steel box-section with properties as defined in the Table
below.
Table Hollow Section Properties
Property: Value:
Mass (kg): 31.182 Length (m): 1.260 X-Sectional Width (m):
0.1
X-Sectional Length (m):
0.1
Thickness (m): 0.008 X-Sectional Area (m2)
2.944 x 10-3
Moment of Inertia (m4):
4.181 x 10-6
Assumed Density (kg/m3)
7850
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Figure 24 Pier Structure used in lab experiment The pier was
placed on the bottom of the steel box in sand to a depth of 300mm.
This distance from the base should be enough to neglect the edge
effects of the support condition here, i.e. the zone of influence
should be within this length. The pier was instrumented with an
accelerometer mounted on its top (the unrestrained end of the
structure). The mass of the accelerometer is negligible compared to
the mass of the pier and its influence on the overall vibration is
therefore considered to be insignificant.
4.3.3 Accelerometer The type of accelerometer used was a BDK3
model from Sensors UK1. It is a capacitive spring-mass
accelerometer with integrated sensor electronics. The accelerometer
has a bolt-like appearance allowing for ease of installation onto
the hollow section and has properties as outlined in the Table
below:
1 Accelerometer information available at:
http://www.sensoruk.com/
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Table Accelerometer Specifications
Property: Specification:
Measuring Range:
3g (ca.30ms-2)
Resolution: < 10-3g
Frequency Range:
1 - 300Hz
Sensitivity at UB=5V:
Appr. 150mV/g
Temperature Drift of Sensitivity:
< + 6 x 10-2%/K
Temperature Drift of zero point:
< 0.1mV/K
Zero Offset: (2.5 0.1) Volt Output Impedance:
Approx. 100 Ohm
Linearity Deviation:
< 1%
Nominal Supply Voltage:
UbN = 5V
Permissible Supply Voltage:
UbZ = 2V 16V
4.3.4 Datalogger The data-logger used was the CR9000x model from
Campbell Scientific2. It is capable of sampling at a frequency of
1000 Hz, a value that is ideal for observing the acceleration
signal from a vibrating structure. This high sampling rate allows
for the reception of a relatively full waveform, which can be
analysed via a fast Fourier transform (FFT) to obtain the frequency
of the signal and hence the natural frequency of the structure. The
data was acquired using accompanying loggernet software, which
stores the data in real-time.
2 Campbell Scientific, UK. Specification available at
www.campbellsci.com/cr90000x
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4.3.5 Excitation Device In order to excite the hollow section in
an appropriate manner, it was required to establish the most likely
mode shape that will result prior to deciding at which location to
apply the force . Since it is the first natural frequency that we
will most likely obtain (other frequencies are also possible), it
is the first mode shape corresponding to this that we should aim to
achieve. For an upright cantilever, ignoring the self-weight
(gravitational) on natural frequency, the mode shape in Figure 4
corresponds to the first natural frequency (Virgin et al. 2007).
The equation shown in Figure 25 is true for a mass distributed over
the entire length of the pier.
In order to excite the hollow section appropriately, a load on a
swinging arc was applied to the top of the section as an impulse
force. The swinging arc mechanism allowed for repeatability of the
same force to maintain consistency in the experiment. The
subsequent excitation was at the first natural frequency of
vibration (Chopra 1981). The experimental configuration that
consisted of a pendulum device clamped into a supporting retort
stand and allowed to swing through a fixed arc is shown in Figure
26. By pulling back to a set point, repeatability of the impulse
force can be achieved. A small amount of cushioning material was
placed around the top of the section to prevent a high frequency
ping from distorting the data. This ensured that the majority of
the kinetic energy is transferred into the pier.
L ( ) 42875.121
mLEIf
pi=
Figure 25 Mode Shape at First Natural Frequency
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Figure 26 Photograph of experimental set-up
4.4 EXPERIMENTAL METHODOLOGY The first step was to assemble the
steel box by bolting together the sides and fixing to the base.
Using the roof crane in the Civil Engineering laboratory, a bag of
Blessington sand was lifted into the air above the box and the box
was filled to a level of approximately 100mm. Using a compaction
hammer the sand was compacted in order to create a stiffer base
upon which to found the model pier. It is important to compact in
100mm increments to ensure that adequate compaction and uniformity
of density is achieved. The sand was filled to an initial height of
300mm above the base. The model pier was placed vertically in the
centre of the box equidistant from all four. Sand was continually
added in increments of 100mm until a final fill level of 700mm had
been achieved and a free space of approximately 300mm from the top
remained.
Figure 27 Photograph of CR9000x Datalogger
The accelerometer was placed on the top of the pier, ensuring
that it was orientated correctly and fixed in place. The datalogger
(Figure 27) was connected and programmed accordingly using the
loggernet software to take readings at a frequency of 1000 Hz. The
free acceleration of the pier was measured after subjecting it to
an impulse force at the free end in order to infer initial
displacement Chopra (1981).This step was repeated a number of times
to ensure a consistency of data. To emulate the effects of scour,
it was decided to add sand to the box in 100mm increments. This is
in essence the reverse of a scour process, but it allows for
re-testing by
Pendulum
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removing the sand layers thereafter. The sand was re-compacted
after each fill event. A new acceleration signal was obtained at
each new level to display a static scheme of signals as a scour
process develops over time. The acceleration signal for these steps
should be different from those found previously.
For continuity of data purposes, a normal scour process was also
emulated upon reaching the fill capacity, whereby sand was removed
from around the pier in increments of 50mm and the acceleration
signals obtained at each level. The purpose of re-testing was to
offset the effects of placing new sand on top of existing layers
and the associated loss of homogeneity in soil conditions
associated with this. For instance, the new soil that was added may
have had a different moisture content to that of the existing sand
in the box, and the effects of this may have gone un-noticed. For
this reason, it was imperative to leave the latter testing phase
until some time had passed, where the soil could gain a more
uniform constitution. Moisture contents were assessed over a number
of days before re-testing.
Once all the data had been obtained, an FFT analysis was
undertaken in MATLAB to ascertain the natural frequency peaks at
each bed level.
Figure 28 Photogaph showing accelerometer attachment
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4.5 Results The levels at which scour emulation takes place are
divided up as base level 0, level 1, level 2 and level 3. These
correspond to the fill levels for initial scour testing and
represent sand depths along the pier separated by 100mm intervals.
At each bed level, an acceleration signal was obtained in the form
of a voltage readout vs. time from the datalogger. This was then
converted to acceleration in terms of gravity (g) using the
conversion factors specified by the manufacturer. The signal
obtained varies as the pier vibrates. A typical example is
displayed in Figure 29. The time period is normalised for the
purpose of graphical representation.
Figure 29 Typical Acceleration Signal This signal was then fed
through an FFT in MATLAB, where it was converted into a frequency
plot, the magnitude of which is displayed on the vertical axis. The
plot corresponding to the signal in Figure 29 is shown in Figure
30.
Figure 30 Typical Frequency Plot
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The actual signal obtained can be compared to the theoretical
signal for an upright cantilever with simplified lumped mass at the
top founded on an infinitely stiff base as calculated using Eqn.
1;
4ALEI3
21f
pi=
Where f is the frequency (Hz), E is the Youngs Modulus (GPa); I
is the moment of inertia (m4), is the density (kg/m3) and A is the
Area (m2) and L = Length (m). Values from this Equation show the
upper bound obtainable solution. The table below sets out the pier
properties during the fill testing phase.
Table Bed Levels Modelled
Level: Pier Length (m):
Theoretical Frequency (Hz)
Measured Response (Hz)
Level 0
0.968 56.0 29.58
Level 1
0.868 69.6 42.82
Level 2
0.768 88.9 60.22
Level 3
0.668 117.5 73.89
Figure 31 Frequency Change with Scour Level
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Once the fill testing phase has been completed, actual scour
emulation may take place by manually removing sand from around the
base in the reverse sequence of the original testing regime. The
benefit of this is that soil properties (such as moisture content)
will remain constant throughout the experiment duration (which is
short). Thus, the only factor affecting stiffness changes is the
level of sand on the pier itself. Sand is removed to level 2 and
removed in 50mm increments thereafter. The results of this are set
out below.
Table Measured and Predicted frequencies
Level: Pier Length (m):
Theoretical Frequency (Hz)
Measured Response (Hz)
Level 2 0.768 88.9 68.36
Level 2-1
0.818 78.4 59.9
Level 1 0.868 69.6 49.16
Level 1-0
0.918 62.2 41.83
Level 0 0.968 56 34.18
Figure 32 Frequency Change with Scour Level The purpose of
removing the top layer of sand from level 3 to level 2 is to offset
the fact that surface sand may exhibit different properties to
other sand at greater depths. For reasons of homogeneity, the
results from level 3 to level 2 are omitted. In-situ sand
properties should be more homogeneous at levels below these.
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Figure 33 Illustration of bed Levels considered in
experiment
4.6 Discussion As is evident, changes in the natural frequency
can be detected by changing the level of the sand around the pier
in the box test. It must be noted, however, that the conditions in
which this experiment was undertaken are highly idealised. An
actual bridge pier does not have a free end, thus placement of
accelerometers on real bridges would require a more detailed
primary