smartrail guideline on scour monitoring (3)

SMARTRAIL User Guidelines: BRIDGE SCOUR MONITORING A GUIDELINE

SMARTRAIL Guidelines BRIDGE SCOUR MONITORING issued 20.09.2014 1 (20)

SMARTRAIL WP1

User Guidelines

BRIDGE SCOUR MONITORING

- A GUIDELINE

This project has received funding from the European Unions Seventh Framework

Programme for research, technological development and demonstration under grant

agreement no FP7- 285683.



Table of Contents

1 Introduction ................................................................................................................... 3

2 Bridge Scour .................................................................................................................. 3

3 Bridge Scour Monitoring ............................................................................................... 4

Fixed or discrete monitoring instrumentation ...................................................................... 7

3.1.1 Single use Buried Devices ............................................................................... 7 3.1.2 Pulse / Radar Devices: ..................................................................................... 9 3.1.3 Piezo-electric Film Sensor Devices .................................................................10 3.1.4 Buried / Driven Rods: ......................................................................................10

Structural Health Monitoring ..............................................................................................12

4 Recommended practice .............................................................................................. 17

5 References ................................................................................................................... 19



1 Introduction These guidelines provide a description of the work necessary to deliver the benefits as described focused on the use of remote monitoring of bridge scour in the SMARTRAIL Work Package WP1. The project website www.fehrl.smartrail.org contains full details of the results of the other aspects of work completed in this Work Package and further information concerning the project.

2 Bridge Scour Bridge scour is the term given to the excavation and removal of material from the bed and banks of rivers as a result of the erosive action of flowing water (Hamill, 1999). There are three main forms of the scour process, namely: general scour, contraction scour and local scour. General scour occurs naturally in river channels and arises as a result of the aggradation and degradation of the riverbed due to natural river flow processes. Contraction scour occurs as a result of obstructions in the river channel obstructing the flow of water such as the presence of a bridge. The decrease in flow area leads to an increase in flow velocity and associated bed shear stresses inducing scour in the vicinity of the obstruction. Local scour occurs around bridge components such as the piers and abutments (Heidarpour, Afzalimehr, & Izadinia, 2010). Due to pressure differences, downward flow is induced at the upstream end of piers, which leads to localised erosion around the structure (Hamill, 1999). Scour reduces the foundation stiffness and can lead to resultant sudden structural failure. The three scour mechanisms combine together to create an overall depth of scour around sub-structural bridge components, which can be detrimental to the stability and safe operation of these structures. A schematic of the scour process is shown in Fig. 1. Scouring of bridge foundations is the number one cause of bridge failure in the United States (Briaud et al., 2001; Briaud, Chen, Li, Nurtjahyo, & Wang, 2005; Melville & Coleman, 2000). One US study of over five hundred bridge failures which occurred between 1989 and 2000 deemed flooding and scour to be the cause of 53% of failures of all failures (Wardhana & Hadipriono, 2003). Another review claims that over the past thirty years, 600 US bridges failed due to scour (Briaud, Ting, & Chen, 1999; Shirole & Holt, 1991). In addition to the risk to human life caused by bridge scour, these failures cause major disruption and also economic losses (De Falco & Mele, 2002). Lagasse et al. (1995) estimate that the average cost for flood damage repair of bridges in the United States is approximately $50 million per annum, a cost that is certainly higher in todays economic climate. Scour is relatively difficult to predict and poses serious risks to the stability of vulnerable structures. It typically results in a loss in foundation support, soil-structure stiffness and can compromise structural safety. Visual inspections are expensive and time consuming and tend to have limited effectiveness. There is a reliance on over the deck visual assessment, and whilst divers can be used to examine the condition of the foundation, this is dangerous in times of flooding, when the risk of scour is highest and the effectiveness of even a direct visual assessment may be reduced by the presence of debris in the water. Scour holes also tend to re-fill as floodwaters subside, making visual inspections undertaken after a flood event somewhat ineffective. These may fail to detect the loss in stiffness resulting from scour as the backfilled material may be loose and therefore have significantly reduced strength and stiffness properties. Many mechanical and electrical instruments have been developed that aim to remotely detect the presence of scour. A comprehensive overview of the instrumentation available is given in section 3.

http://www.fehrl.smartrail.org/



Wake Vortices

Bridge Pier

Scour Hole

Downflow

Horseshoe Vortices

Figure 1 Scour prcess schematic (Prendergast & Gavin, 2014)

3 Bridge Scour Monitoring An effective method of combatting scour is to monitor its evolution over time and implement remediation works as they are required (Briaud et al., 2011). The most widespread monitoring scheme in place as part of any national bridge asset management framework is to undertake visual inspections. Visual inspections are commonplace in engineering and are used to detect structural anomalies such as cracking and other damage (Sohn et al., 2004). With regard to scour, visual inspections involve the use of divers to inspect the condition of foundation elements and to measure the depth of scour using basic instrumentation (Avent & Alawady, 2005). Two particular disadvantages associated with this inspection method include the fact that inspections cannot be carried out during times of flooding, when the risk of scour is highest, and the maximum depth of scour may not be recorded as scour holes tend to fill in as flood waters subside (Foti & Sabia, 2011; Lin, Lai, Chang, Chang, & Lee, 2010). The fact that scour holes tend to refill can be dangerous and misleading as the true extent of the scour problem may be missed in the inspection. A more effective alternative is to use fixed or discrete scour depth recording instrumentation, See Table 1. A number of instruments have been developed that can monitor the depth of scour around bridge piers and abutments. Some of these sensing instruments are discussed in the following subsections. Another alternative is to use damage detection methods that are currently under development in the area of Structural Health Monitoring (SHM). These methods are also discussed in the following subsections.



SMARTRAIL Work Package (WP) 1 BRIDGE SCOUR MONITORING - A GUIDELINE

Title: Assessment Techniques for Bridge Scour

Failure Mode:

Detection Mechanism:

1 Pier Tilting (Due to differential settlement) Inclinometer

2 Pier Settlement

Strain Gauge placed at deck level will alert to stresses from differential

settlement of different piers / supports

3 Pile Group Tilting Inclinometer

4 Deck sliding off supports due to hydraulic loading Stage Height Measurement Device such as a Stream Gauge

will alert before river height reaches deck level

5 Scouring of foundation leading to lack of lateral pile Accelerometers detect differences in acceleration signals / frequencies

Stability

6 Deck falling off abutment due to adverse tilt of support Inclinometer, Strain Gauge

7 Deck buckling upwards due to adverse tilt of support Inclinometer, Strain Gauge

8 Scour holes develop and become filled in once flood Ground Penetrating Radar

subsides (compromising of support from soil)



9 Scour hole developing over time Sounding Rod (Rests on bed and moves down as hole appears, base

rests on riverbed at lowest elevation of scour hole)

Measure displacement of top of rod to determine scour hole depth

10 Scour hole developing over time

Sonic Fathometer (Like GPR above) - Reflects Acoustic Wave to detect

interface between water and riverbed

11 Scour hole developing over time "Scubamouse" - Used in NZ - A radioactive collar is placed around a

vertical tube and rests on the riverbed. Once scour occurs, the

displacement of the collar can be measured from a probe placed down

the tube. (Collar remains on riverbed and sinks as hole develops)


"Wallingford Tell-Tail Device" - Omni-directional motion sensor

placed at various depths and connected to a data-logger. Measures

motion on the soil thus detecting scour hole development

13 Scour hole developing over time Buried Rods such as:

(a) Piezoelectric film sensors - Exposure by scour detected by change in

electrical signal caused by vibration

(b) Mercury Tip Switch - Flip down and break circuit when exposed by

Scour

(c) Magnetic Sliding Collar - Movement detected by magnets in tube


Time-Domain Reflectometery (TDR):

Using buried sensors that detect changes in dielectric properties of

surrounding materials in order to detect movements in the soil-water

Interface


SMARTRAIL Guidelines BRIDGE SCOUR MONITORING issued 20.08.2014

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Fixed or discrete monitoring instrumentation

3.1.1 Single use Buried Devices

These devices are installed in the river bed, near the pier or abutment of interest. They can be buried at multiple depths. They cummincate with a data acquisition system 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 scour hole has reached this level. The difficulty with these devices is that they typically have a single use and once the scour depth reaches their installation depth, they must be re-installed. They also have a fixed battery life and only give an indication of scour depth at a single point, i.e. scour above the device and further scour below is not known.

Figure 2 Positioning Float-Out Devices (NCHRP, 2009)

3.1.1.1 Instrument: Tethered Buried Switch

This device is buried in the river bed at the location of interest for scour measurement (see Fig. 2). 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. An image of a typical sensor is shown in Fig. 3.



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Figure 3 Tethered Buried Switch (TBS) (Briaud et al., 2011)

3.1.1.2 Instrument: Float-Out Device

A float-out device is a cylindrical device (see Fig. 4) that may be installed in the streambed at various locations of interest near abutments and bridge piers. They are installed using a vertical orientation. They may be installed at various depths. They become activated when scour levels reach the upper level of the sensor and the sensor floats out of position. An on-board trigger mechanism sends a signal to a data acquisition system that alerts the user that the device has floated out of the installed position. This is indicated by the orientation changing from vertical to horizontal. These are reliable instruments in that they provide an easy method to detect if scour has reached the sensor datum. Their use is recommended in areas where exceeding a certain scour depth is very detrimental to stability, i.e. at the formation level of a shallow foundation.

Figure 4 Typical Float-Out Device (NCHRP, 2009)



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3.1.2 Pulse / Radar Devices:

Pulse / radar 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.

3.1.2.1 Instrument: 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 in the streambed at the location of 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 (Elsaid, 2012). It is a good method in that relatively clear geophysical images can be obtained that show the water-sediment interface and hence, the depth of scour. However, the requirement that long probes be installed into the riverbed can make it a somewhat expensive method for scour monitoring (See Fig. 5).

Figure 5 Time-Domain Reflectometer (Briaud et al., 2011)

3.1.2.2 Instrument: Ground Penetrating Radar (GPR)

A GPR transmitter is floated out in a river to the location. 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). The method works on a very similar principle to 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 by the receiver and an overall geophysical map may be generated, showing clearly the submerged scour



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hole and its depth. An example of a geophysical map is shown in Fig. 6 where the scour depth is cleary detectable. The disadvantages of the method is that it requires manual operation, is not ideally suited to continuous monitoring and it cannot be deployed in a flood scenario as the equipment would be washed away. However, if used as part of a discrete bridge monitoring scheme, it is quite adept at detecting scour.

Figure 6 Typical GPR Profile (Anderson, Ismael, & Thitimakorn, 2007)

3.1.3 Piezo-electric Film Sensor Devices

Piezo-electric film 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 datalogger to alert that scour levels have reached the particular level of the sensor.

3.1.3.1 Instrument: 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 of flowing water. Only plate elements exposed to the flow will bend hence an accurate measurement of scour levels can be derived from this. The resolution of scour depth monitoring is only as good as the number and spacing of sensors along the array.

3.1.4 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



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stage, prior to the occurrence of scour as this will affect the accuracy of the perceived results.

3.1.4.1 Instrument: Magnetic Sliding Collar

A Magnetic Sliding Collar incorporates 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 or abutment. The magnetic nature of the collar allows it to trigger sensors in the rod at the location of the sensor. As the streambed erodes, the collar slides down along the rod allowing magnetic triggers to detect that it has changed its elevation. The data from the device may be manually or automatically read. In the automatic case, 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. This device is useful and pretty accurate at detecting scour. It cannot, however, measure infill as the sensor typically remains at the maximum scour depth. It gives a good indication of the previous maximum scour depth measured at the location of the sensor. The device is only good at measuring scour at its installation location and may miss the global effect of scour if placed in an area where the scour is not so severe (see Fig. 7).

Figure 7 Magnetic Sliding Collar (NCHRP, 2009)

3.1.4.2 Instrument: Scubamouse

The scubamouse is a device that was created in New Zealand. It 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



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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 datalogger device. This device operates on similar principles to the magnetic sliding collar.

3.1.4.3 Instrument: 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 omnidirectional 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 datalogger via a cable. The motion sensors detect bed movements that are indicative of scour having reached the depth of embedment of the sensor. The device must be reset when scour reaches its depth of installation, which can make it labour intensive which has a significant cost implication.

3.1.4.4 Instrument: 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 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, breaking the circuit.

Structural Health Monitoring

Structural Health Monitoring (SHM) techniques have been developing rapidly in recent years. Advances in this technology have led to the use of the structure itself to detect damage by observing changes in the structures condition. The method explored for scour detection as part of this project is to use the frequency response of the structure to detect the presence and in some cases the extent of the scour. The frequency response of the structure is expected to change as scour removes soil from around the foundation elements. Fig. 8 shows a schematic of the pre- and post-scour process on a bridge.



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Road Surface

Water Surface

Riverbed

Embankment

Vehicle

Previously

validated

pile model

(a)

Road Surface

Water Surface

Scour Hole

Embankment

Vehicle

(b)

Figure 8 Frequency change due to scour schematic

Accelerometers placed on the bridge structure can be used to detect the presence and extent of scour by measuring vibrations and obtaining the frequency content of the signals. An experiment undertaken on a pile, the dimensions of which were typical of those used to support road and rail bridges, highlights the change in frequency due to scour. A schematic of this test is shown in Fig. 9.



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Blessington Sand

Initial Level

2260

6500

Scour Level -1

Scour Level -2

Scour Level -3

Scour Level -4

Scour Level -5

Scour Level -6

Scour Level -7

Scour Level -8

Scour Level -9

Scour Level -10

Scour Level -11

Scour Level -12

Base Level

500

Pile

500

500

1000

A A

Section A-A

R170

R157Accelerometer 4

Accelerometer 3

Accelerometer 2

Accelerometer 1

8760

Figure 9 Experimental test (Prendergast, Hester, Gavin, & OSullivan, 2013)

A vibration test was undertaken on the pile in order to establish the sensitivity of the frequency response of the structure to a change in the level of soil surrounding the structure. The results of this test are shown in Fig. 10. From the figure, it is evident that significant changes in frequency can be obtained due to scour. Therefore, this method is suggested as a way to monitor the presence of scour developing around critical foundation elements of bridge structures. Fig. 10 also shows the frequency change of a cantilever with the same free length as the scoured pile to highlight that the results are as expected (i.e. they should be less than a cantilever at each depth because this has an infinitely stiff foundation).



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0 5 10 15 20 25 30 35 40-7

-6

-5

-4

-3

-2

-1

0

Frequency (Hz)

Sco

ur

Dep

th (

m)

Experimental Frequency

Fixed Cantilever Frequency

Figure 10 Frequency change with scour (Prendergast et al., 2013)

The method of using changes in frequency to detect scour was further tested by developing a full numerical finite-element bridge model and subjecting it to a vehicular moving load to assess if it is possible to detect frequency changes arising from a loss of soil support from around the foundation. The bridge modelled is shown in Fig. 11.



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6001800

150

00

38

00

19

00

32

50

26

25

171

00

3500

4500

60

00

25000 25000

500 1375 500

750

16

00

2500

A A

Bridge Deck

Abutment

Columns

Bridge Pier

Pier Foundation

Pier Cross-HeadBank Seat

Abutment Piles

6001800

Piles

Pier Column

Piles

Abutment Columns

Reinforced Earth

(a)

(b)

Figure 11 Bridge model

Scour was modelled as the removal of numerical springs from the model, similar to those shown in Fig. 8. The purpose of the investigation was to assess if it is possible to detect scour using the vibration response of the bridge due to the passage of a vehicle along the deck (similar to a train carriage or otherwise). The results of this numerical investigation are shown in Fig. 12, where a clear reduction in detected natural frequency may be observed with increasing depth of scour.



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0.9 1 1.1 1.2 1.3 1.4 1.5 1.6-10

-8

-6

-4

-2

0

Frequency (Hz)

Sco

ur

dep

th (

m)

Loose sand

Figure 12 Change in frequency with scour - numerical bridge model

4 Recommended practice The recommendation for scour monitoring using instrumentation (in lieu of visual inspections) is to use a combination of instruments at critical scour locations to reduce the dependency on any one system, each of which has its limitations. The Smartrail project has demonstrated clearly the potential to couple the use of accelerometers (for frequency measurement) with a depth monitoring instrument such as a magnetic sliding collar or similar so that an estimate of frequency change with scour depth can be obtained. This allows for the estimation of a datum for the use of accelerometers only on certain bridges to reduce maintenance associated with re-installing instruments as discussed in previous sections. The following flowchart highlights the recommended practice:



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5 References

Anderson, N. L., Ismael, A. M., & Thitimakorn, T. (2007). Ground-Penetrating Radar : A Tool for Monitoring Bridge Scour. Environmental & Engineering Geoscience, XIII(1), 110.

Avent, R. R., & Alawady, M. (2005). Bridge Scour and Substructure Deterioration : Case Study. Journal Of Bridge Engineering, 10(3), 247254.

Briaud, J. L., Chen, H. C., Ting, F. C. K., Cao, Y., Han, S. W., & Kwak, K. W. (2001). Erosion Function Apparatus for Scour Rate Predictions. Journal of Geotechnical and Geoenvironmental Engineering, 105113.

Briaud, J. L., Chen, H., Li, Y., Nurtjahyo, P., & Wang, J. (2005). SRICOS-EFA Method for Contraction Scour in Fine-Grained Soils. Journal of Geotechnical and Geoenvironmental Engineering, 131(10), 12831295.

Briaud, J. L., Hurlebaus, S., Chang, K., Yao, C., Sharma, H., Yu, O., Price, G. R. (2011). Realtime monitoring of bridge scour using remote monitoring technology. Security (Vol. 7, pp. 1440). Austin, TX. Retrieved from http://tti.tamu.edu/documents/0-6060-1.pdf

Briaud, J. L., Ting, F., & Chen, H. C. (1999). SRICOS: Prediction of Scour Rate in Cohesive Soils at Bridge Piers. Journal of Geotechnical and Geoenvironmental Engineering, (April), 237246.

De Falco, F., & Mele, R. (2002). The monitoring of bridges for scour by sonar and sedimetri. NDT&E International, 35, 117123.

Elsaid, A. (2012). Vibration Based Damage Detection of Scour in Coastal Bridges. North Carolina State University.

Foti, S., & Sabia, D. (2011). Influence of Foundation Scour on the Dynamic Response of an Existing Bridge. Journal Of Bridge Engineering, 16(2), 295304. doi:10.1061/(ASCE)BE.1943-5592.0000146.

Hamill, L. (1999). Bridge Hydraulics (pp. 1367). London: E.& F.N. Spon.

Heidarpour, M., Afzalimehr, H., & Izadinia, E. (2010). Reduction of local scour around bridge pier groups using collars. International Journal of Sediment Research, 25(4), 411422. doi:10.1016/S1001-6279(11)60008-5

Lagasse, P. F., Schall, J. D., Johnson, F., Richardson, E. V., & Chang, F. (1995). Stream stability at highway structures. Washington, DC.

Lin, Y. Bin, Lai, J. S., Chang, K. C., Chang, W. Y., & Lee, F. Z. (2010). Using mems sensors in the bridge scour monitoring system. Journal of the Chinese Institute of Engineers, 33(1), 2535.

Melville, B. W., & Coleman, S. E. (2000). Bridge scour. Highlands Ranch, CO: Water Resources Publications.

NCHRP. (2009). Monitoring Scour Critical Bridges - A Synthesis of Highway Practice. Traffic Safety. Washington, DC.



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Prendergast, L. J., & Gavin, K. (2014). A review of bridge scour monitoring techniques. Journal of Rock Mechanics and Geotechnical Engineering, 6(2), 138149.

Prendergast, L. J., Hester, D., Gavin, K., & OSullivan, J. J. (2013). An investigation of the changes in the natural frequency of a pile affected by scour. Journal of Sound and Vibration, 332(25), 66856702. doi:http://dx.doi.org/10.1016/j.jsv.2013.08.020i

Shirole, A. M., & Holt, R. C. (1991). Planning for a comprehensive bridge safety assurance program. In Transport Research Record (Vol. 1290, pp. 137142). Washington, DC: Transport Research Board.

Sohn, H., Farra, C. R., Hemez, F., Shunk, D., Stinemates, D., Nadler, B., & Czarmecki, J. (2004). A Review of Structural Health Monitoring Literature : 1996 2001 (pp. 1311).

Wardhana, K., & Hadipriono, F. C. (2003). Analysis of Recent Bridge Failures in the United States. Journal of Performance of Constructed Facilities, 17(3), 144151. doi:10.1061/(ASCE)0887-3828(2003)17:3(144)

smartrail guideline on scour monitoring (3)

Documents

bridge scour bridge

general scour

local scour

contraction scour

scour process

bridge components

smartrail user guidelines

structural health monitoring