Ventajas e Inconvenientes sistemas de monitorización puentes

(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


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

SMARTRAIL- Smart maintenance analysis and remediation of transport infrastructure


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.



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 ..................................................................................................... 49

4.6 Discussion ................................................................................................ 52



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 ................................................................................................ 60

5.5 Summary ................................................................................................. 61

6 Conclusions ............................................................................................................ 62

7 References ............................................................................................................. 63



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.



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.



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.



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.



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.



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.


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.



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).




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.




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)



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.


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.



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.


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.



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.


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.



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.


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.


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.



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.


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.



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.


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.



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.


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.



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).


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



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).


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.




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.



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.



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.



Figure 8 Transfer of data from Site to Computer (http://www.tencate.com)


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.



(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/)



Figure 11 Typical Site after installation (http://www.geobeads.com/)


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.



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)


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.



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


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.



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)




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)




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




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.


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



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.


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).




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/)



Figure 20 Piezometer (http://www.geo-observations.com/


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.




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.



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.



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



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.



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



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/



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



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



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



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



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



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



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.



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

Ventajas e Inconvenientes sistemas de monitorización puentes

Documents

Transcript of Ventajas e Inconvenientes sistemas de monitorización puentes