- Email: [email protected]
Sensitivity Studies on Scour Detection Using Vibration-based Systems
Available online at www.sciencedirect.com
ScienceDirect Transportation Research Procedia 14 (2016) 3982 – 3989
6th Transport Research Arena April 18-21, 2016
Sensitivity studies on scour detection using vibration-based systems Luke J. Prendergast a,b, *, Kenneth Gavin a,b, Cormac Reale a,b a
School of Civil, Structural and Environmental Engineering, University College Dublin, Dublin 4, Ireland b Earth Institute, University College Dublin, Dublin 4, Ireland
Abstract The high profile failure of the Malahide viaduct in Dublin in late 2009 was attributed to erosion of the supporting soils around the bridge piers, commonly referred to as foundation scour. This is a widespread geotechnical-structural problem, where foundation scour has been identified as the number one cause of bridge failure in the United States. Monitoring scour is of paramount importance to ensure the continued safe operation of the ageing bridge asset network. Most monitoring regimes rely on expensive underwater instrumentation that is often subject to damage during times of flooding, when scour risk is at its highest. Scour causes a rapid reduction in foundation stiffness and can lead to complete failure of one or more sub-structural components of a bridge. In this paper, a novel scour monitoring approach based on dynamic measurement techniques is described. The investigation is based on using accelerometers mounted on the structure of interest to detect losses in foundation stiffness due to scour, which manifest itself as a change in vibration characteristics. Experimental and numerical analyses were performed to validate the potential of this new monitoring framework. A significant advantage of this monitoring method over traditional approaches is that the structure itself is used to monitor the damage. Therefore, if failure is likely, it is assumed that the dynamic characteristics will indicate such and remediation works may be implemented. © The Authors. by by Elsevier B.V.B.V.. This is an open access article under the CC BY-NC-ND license © 2016 2016The Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Road and Bridge Research Institute (IBDiM). Peer-review under responsibility of Road and Bridge Research Institute (IBDiM) Keywords: Scour; vibration; SHM; frequency; acceleration
* Corresponding author. Tel.: +353-1-7163284 E-mail address: [email protected]
2352-1465 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Road and Bridge Research Institute (IBDiM) doi:10.1016/j.trpro.2016.05.495
Luke J. Prendergast et al. / Transportation Research Procedia 14 (2016) 3982 – 3989
1. Introduction 1.1. Bridge scour Scour of foundations is the number one cause of bridge collapse in bridges located over waterways. Several studies undertaken into failed bridges in the United States have indicated that scour (and other flood-related issues) have resulted in their collapse (Melville and Coleman 2000; Briaud et al. 2001 and 2005). One such study concluded that during the last 30 years 600 bridges have failed due to scour problems (Shirole & Holt 1991; Briaud et al. 1999) causing major operating disruption and financial losses (De Falco & Mele 2002). In the United States the average cost for flood damage repair of highways is estimated at $50 million per year (Lagasse et al. 1995). When corrected for inflation, this figure would be even higher today. During a single flood event in the upper Mississippi and lower Missouri river basins which occurred in 1993, at least 22 of the 28 bridges that failed were due to scour. The associated repair costs were more than $8,000,000 (Kamojjala et al. 1994). An example of a failed bridge due to scour problems is the Malahide Viaduct, which collapsed due to tidal scour in Dublin, 2009 (see Fig. 1). 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 forms of scour; namely general scour, contraction scour and local scour. General scour occurs naturally in river channels and includes the aggradation and degradation of the river bed that may occur as a result of changes in the hydraulic parameters governing the channel form such as changes in the flow rate or changes in the quantity of sediment in the channel (Forde et al. 1999). General scour is a natural erosion and deposition process. It relates to the natural evolution of the waterway and is associated with the progression of scour and filling, in the absence of obstacles (Federico et al. 2003). Contraction scour occurs as a result of the reduction in the channel’s cross-sectional area that arises due to the construction of structures such as bridge piers and abutments. Bridges tend to reduce the free cross-sectional area of a channel by the nature of pier and abutment construction within the channel. Contraction scour manifests itself as an increase in flow velocity and resulting bed shear stresses, caused by a reduction in the channel’s cross-sectional area at the location of a bridge. The increased shear stresses can overcome the channel bed’s threshold shear stress and mobilize the sediments (Briaud et al. 1999). Finally, Local scour occurs around individual bridge piers and abutments. It is caused by downward flow being induced at the upstream end of bridge piers which leads to very localized erosion in the direct vicinity of the structure (Hamill 1999), see Fig. 2(a). Horseshoe vortices can develop due to the separation of the flow at the edge of the scour hole upstream of the pier and this can result in pushing the down-flow inside the scour hole closer to the pier, which exacerbates the process. Horseshoe vortices are a result of initial scouring and not the primary cause of scour. Furthermore, separation of the flow at the sides of the pier result in wake vortices on the downstream end of the pier (Heidarpour et al. 2010). Local scour depends on the balance between streambed erosion and sediment deposition. Clear-water scour is the term given to describe the situation when no sediments are delivered by the river into the scour hole whereas live-bed scour describes the situation where an interaction exists between sediment transport and the scour process (Brandimarte et al. 2006). The presence of live-bed conditions leads to smaller ultimate scour depths than in clear-water conditions. Further information on the scour process is available in Prendergast & Gavin (2014). Scour results in a loss in foundation stiffness and can cause sudden failure, see Fig. 2(b).
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Fig. 1. Malahide Viaduct collapsed August 2009 (Prendergast & Gavin 2014).
Bridge Pier
Removal of foundation soil by water action
Downflow
Wake Vortices
Scour Hole Horseshoe Vortices
Loss in foundation stiffness
Potential (sudden) bridge collapse
(a)
(b) Fig. 2 (a) Local scour schematic; (b) Scour process.
1.2. Scour monitoring Traditionally, scour monitoring was undertaken by way of diving inspections whereby divers would manually inspect the condition of critical bridge foundations using crude depth-measuring instrumentation, see Fig. 3(a). This practice is dangerous and outdated. In general diving inspections cannot be undertaken during heavy flood flow, when scour risk of occurrence is highest and also the fact that scour holes tend to refill upon the subsidence of flood waters means that the scour depth measured may often not be that of the maximum depth attained during the flood. There are a number of systems available that aim to remotely monitor the presence of scour around bridge foundations. These systems range from sound monitoring devices and electrical conductivity devices to physical probes resting on the riverbed. In general these devices aim to detect changes in the depth of a scour hole around a foundation and can either be operated continuously or discretely as part of standard maintenance procedures. An
Luke J. Prendergast et al. / Transportation Research Procedia 14 (2016) 3982 – 3989
example of a number of these devices is shown schematically in Fig. 3(b). This figure is reproduced from Prendergast & Gavin (2014) and shows a Magnetic Sliding Collar (MSC), Float-Out Device (Briaud et al. 2011), Time-Domain Reflectometry (TDR) (Yankielun & Zabilansky 1999; Yu 2009), Sonic Fathometer and Ground Penetrating Radar (GPR) (Anderson et al. 2007) systems. (a
(b
Fig. 3 (a) Schematic of diving inspections for scour measurement; (b) Schematic of scour-measuring instrumentation (Prendergast&Gavin 2014).
2. Vibration-based scour detection 2.1. Background By virtue of the process of eroding away soil from around foundation elements, scour causes a reduction in the stiffness of a foundation. Traditional Structural Health Monitoring (SHM) utilizes losses in stiffness of a structural element to detect damage as stiffness loss typically manifests itself as a change in modal characteristics of a structure. The same is true for the detection of scour. Losses in soil contact with a foundation can manifest itself as a change in modal properties (natural frequencies, damping ratios and mode shapes etc.). The key question with relation to the detection of scour is how sensitive these parameters are to the scour process. Several authors have investigated vibration-based scour detection methods, see Briaud et al. (2011); Elsaid & Seracino (2014); Foti & Sabia (2011); Prendergast et al. (2013); Prendergast et al. (2015); Klinga & Alipour (2015); Ju (2013); Chen et al. (2014). The detection and monitoring of scour is arguably more complex than crack damage detection in bridge beams for example as there are a significant number of parameters at play in the problem. Foundation geometry, bridge superstructure properties, soil type and stiffness nonlinearities are all issues which affect the sensitivity of scour detection using vibrations. In particular, the nonlinear and inelastic nature of foundation soils at intermediate operational strains can alter the dynamic response of a foundation system (and by extension, a superstructure). This is shown by the change in secant stiffness of a shallow pad foundation element due to scour undermining the foundation (Prendergast & Gavin 2014). The loss of soil contact leads to an increased stress (and strain) on the remaining soil, and since soil stiffness is strain-dependent, a loss of stiffness can occur. This adds a further degree of complexity to the global issue of trying to identify foundation scour using vibration-based damage detection methodologies.
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V
V
Bridge Pier
Bridge Pier
Scour Hole
No Scour
E1
E2
Stress
E1 = Stiffness before scour E2= Stiffness after scour E2 < E1
Strain
Fig.4. Stiffness loss due to scour (Prendergast & Gavin 2014).
2.2. Modelling of foundation scour In this paper, a numerical model of a two-span integral bridge is created to highlight the sensitivity of structural vibrations to scour of the foundation. A schematic of the bridge is shown in Fig. 5. The bridge model is loaded by a point load (representing the passage of a vehicle along the bridge deck). The dynamic response of the bridge due to this point load traversing is calculated in the model and outputted as bridge dynamic displacement, velocity and acceleration from the top of the pier.
Fig. 5. Two-span integral bridge schematic with moving point load.
The bridge properties correspond to a typical two-span integral bridge, see Prendergast et al. (2015) for case study properties adopted. The soil is modelled using a Winkler spring philosophy whereby the soil stiffness is discretised into individual springs for ease of modelling (Winkler 1867; Dutta & Roy 2002; Prendergast et al. 2013). The method for deriving soil stiffness is available in L J Prendergast et al. (2015).
Luke J. Prendergast et al. / Transportation Research Procedia 14 (2016) 3982 – 3989
2.3. Results of analysis To highlight the sensitivity of the dynamic response of a typical integral bridge to pier scour, a brief analysis is conducted herein. The load (50 kN) traverses the bridge at 50 km/hr (13.88 m/s) and the lateral dynamic displacement, velocity and acceleration is calculated at the pier top (see Fig. 5 for sensor location in schematic). 10 seconds of free vibration (damped at 2%) is included in the analysis. This is shown in Fig. 6.
Fig. 6. (a) Pier top lateral displacement; (b) Pier top lateral velocity; (c) Pier top lateral acceleration.
The results from Fig. 6. show a typical lateral bridge response due to the passage of a load along the bridge deck. For completeness, the displacement, velocity and acceleration signals generated are shown. Both the forced vibration component (when the load is on the bridge) and the free vibration component (when the load has departed) are highlighted. In reality, the dynamic response will be measured by an accelerometer; therefore it is of interest to assess the effect of scour of the pier on the lateral pier top acceleration response. This is shown in Fig. 7. For this analysis, the load traverses the bridge deck at 50 km/hr for zero scour and for 10 m of scour around the central pier foundation and the change in the acceleration response is shown.
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-3
12
x 10
zero scour 10m scour
-3
1
10
x 10
8 0
acc (m/s 2)
6 4
-1
2
6
8
7
0 -2 -4 -6 -8
0
2
4
8
6
10
12
14
time (s) Fig. 7. Zero scour and 10m scour lateral pier top acceleration responses due to passage of a vehicle over the bridge.
Fig. 7. shows the lateral pier top acceleration for zero and 10m scour affected the bridge pier. As is evident, scour has the effect of increasing the period of the vibration for the scoured signal (see the insert on the figure). This is sensible as an increased effective length leads to a reduced flexural stiffness therefore an increased period is expected. The scoured signal also has a lower amplitude in the free vibration than the zero scour signal. Scour therefore has a noticeable effect on the dynamic response of an integral bridge system. 3. Conclusions Scour is the number one cause of bridge failure in bridges located over waterways. It leads to a rapid loss in foundation stiffness and can result in sudden catastrophic collapse. Traditionally, diving inspections were used to detect and monitor scour around critical sub-structure components of bridges. More recently, a range of automated and manually operated instrumentation has become available that aims to detect the change in scour depths around bridge foundations either continuously or discretely. These instruments have associated advantages and disadvantages. This paper focusses on the topic of applying vibration-based Structural Health Monitoring techniques to the detection and monitoring of scour. Preliminary results show that the lateral pier top acceleration response is highly affected by the presence of scour around the pier and that the method may be applicable as a lowmaintenance scour monitoring alternative to the more intensive and expensive instrumentation available on the market. Further research is required to assess the potential of the approach on actual structures as the results in this paper were derived from numerical simulations.
Acknowledgements The authors would like to acknowledge the support of the Earth and Natural Sciences (ENS) Doctoral Studies Programme, funded by the Higher Education Authority (HEA) through the Programme for Research at Third Level Institutions, Cycle 5 (PRTLI-5), co-funded by the European Regional Development Fund (ERDF), the European Union Framework 7 project SMART RAIL (Project No. 285683) and the University College Dublin (UCD) Earth Institute.
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References Anderson, N.L., Ismael, A.M. & Thitimakorn, T., 2007. Ground-3HQHWUDWLQJ 5DGDUௗ $ 7RRO IRU 0RQLWRULQJ %ULGJH 6FRXU Environmental & Engineering Geoscience, XIII(1), pp.1–10. Brandimarte, L. et al., 2006. Stochastic Flow Analysis for Predicting River Scour of Cohesive Soils. Journal of Hydraulic Engineering, 132(5), p.493. Available at: http://link.aip.org/link/JHEND8/v132/i5/p493/s1&Agg=doi. Briaud, J.L. et al., 2001. Erosion Function Apparatus for Scour Rate Predictions. Journal of Geotechnical and Geoenvironmental Engineering, pp.105–113. Briaud, J.L. et al., 2011. Realtime monitoring of bridge scour using remote monitoring technology, Austin, TX. Available at: http://tti.tamu.edu/documents/0-6060-1.pdf. Briaud, J.L. et al., 2005. SRICOS-EFA Method for Contraction Scour in Fine-Grained Soils. Journal of Geotechnical and Geoenvironmental Engineering, 131(10), pp.1283–1295. 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), pp.237–246. Chen, C.-C. et al., 2014. Scour evaluation for foundation of a cable-stayed bridge based on ambient vibration measurements of superstructure. NDT & E International, 66, pp.16–27. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0963869514000589 [Accessed February 23, 2015]. Dutta, S.C. & Roy, R., 2002. A critical review on idealization and modeling for interaction among soil–foundation–structure system. Computers & Structures, 80(20-21), pp.1579–1594. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0045794902001153. Elsaid, A. & Seracino, R., 2014. Rapid assessment of foundation scour using the dynamic features of bridge superstructure. Construction and Building Materials, 50, pp.42–49. Available at: http://linkinghub.elsevier.com/retrieve/pii/S095006181300809X [Accessed October 29, 2013]. De Falco, F. & Mele, R., 2002. The monitoring of bridges for scour by sonar and sedimetri. NDT&E International, 35, pp.117–123. Federico, F., Silvagni, G. & Volpi, F., 2003. Scour Vulnerability of River Bridge Piers. Journal of Geotechnical and Geoenvironmental Engineering, 129(10), pp.890–900. Forde, M.C. et al., 1999. Radar measurement of bridge scour. NDT&E International, 32, pp.481–492. Foti, S. & Sabia, D., 2011. Influence of Foundation Scour on the Dynamic Response of an Existing Bridge. Journal Of Bridge Engineering, 16(2), pp.295–304. Hamill, L., 1999. Bridge Hydraulics, 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), pp.411–422. Available at: http://linkinghub.elsevier.com/retrieve/pii/S1001627911600085 [Accessed January 23, 2012]. Ju, S.H., 2013. Determination of scoured bridge natural frequencies with soil–structure interaction. Soil Dynamics and Earthquake Engineering, 55, pp.247–254. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0267726113002017 [Accessed October 29, 2013]. Kamojjala, S. et al., 1994. Analysis of 1993 Upper Mississippi flood highway infrastructure damage. In Proceedings of the 1st International Conference of Water Resource Engineering. New York, NY: ASCE, pp. 1061–1065. Klinga, J. V. & Alipour, A., 2015. Assessment of structural integrity of bridges under extreme scour conditions. Engineering Structures, 82, pp.55–71. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0141029614004398 [Accessed November 14, 2014]. Lagasse, P.F. et al., 1995. Stream stability at highway structures, Washington, DC. Melville, B.W. & Coleman, S.E., 2000. Bridge scour, Highlands Ranch, CO: Water Resources Publications. Prendergast, L.J. et al., 2013. An investigation of the changes in the natural frequency of a pile affected by scour. Journal of Sound and Vibration, 332(25), pp.6685–6702. Prendergast, L.J. & Gavin, K., 2014. A review of bridge scour monitoring techniques. Journal of Rock Mechanics and Geotechnical Engineering, 6(2), pp.138–149. Prendergast, L.J., Gavin, K. & Doherty, P., 2015. An investigation into the effect of scour on the natural frequency of an offshore wind turbine. Ocean Engineering, 101, pp.1–11. Available at: http://dx.doi.org/10.1016/j.oceaneng.2015.04.017. Prendergast, L.J., Hester, D. & Gavin, K., 2015. Development of a Vehicle-Bridge-Soil Dynamic Interaction Model for Scour Damage Modelling. Shock and Vibration, In Press. Shirole, A.M. & Holt, R.C., 1991. Planning for a comprehensive bridge safety assurance program. In Transport Research Record. Washington, DC: Transport Research Board, pp. 137–142. Winkler, E., 1867. Theory of elasticity and strength, Dominicus Prague. Yankielun, N. & Zabilansky, L., 1999. Laboratory Investigation of Time-Domain Reflectometry System for Monitoring Bridge Scour. Journal of Hydraulic Engineering, 125(12), pp.1279–1284. Available at: http://colleges.ksu.edu.sa/Papers/papers/TDRforBridge.pdf [Accessed January 26, 2012]. Yu, X., 2009. Time Domain Reflectometry Automatic Bridge Scour Measurement System: Principles and Potentials. Structural Health Monitoring, 8(6), pp.463–476. Available at: http://shm.sagepub.com/cgi/doi/10.1177/1475921709340965 [Accessed September 22, 2013].
ScienceDirect Transportation Research Procedia 14 (2016) 3982 – 3989
6th Transport Research Arena April 18-21, 2016
Sensitivity studies on scour detection using vibration-based systems Luke J. Prendergast a,b, *, Kenneth Gavin a,b, Cormac Reale a,b a
School of Civil, Structural and Environmental Engineering, University College Dublin, Dublin 4, Ireland b Earth Institute, University College Dublin, Dublin 4, Ireland
Abstract The high profile failure of the Malahide viaduct in Dublin in late 2009 was attributed to erosion of the supporting soils around the bridge piers, commonly referred to as foundation scour. This is a widespread geotechnical-structural problem, where foundation scour has been identified as the number one cause of bridge failure in the United States. Monitoring scour is of paramount importance to ensure the continued safe operation of the ageing bridge asset network. Most monitoring regimes rely on expensive underwater instrumentation that is often subject to damage during times of flooding, when scour risk is at its highest. Scour causes a rapid reduction in foundation stiffness and can lead to complete failure of one or more sub-structural components of a bridge. In this paper, a novel scour monitoring approach based on dynamic measurement techniques is described. The investigation is based on using accelerometers mounted on the structure of interest to detect losses in foundation stiffness due to scour, which manifest itself as a change in vibration characteristics. Experimental and numerical analyses were performed to validate the potential of this new monitoring framework. A significant advantage of this monitoring method over traditional approaches is that the structure itself is used to monitor the damage. Therefore, if failure is likely, it is assumed that the dynamic characteristics will indicate such and remediation works may be implemented. © The Authors. by by Elsevier B.V.B.V.. This is an open access article under the CC BY-NC-ND license © 2016 2016The Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Road and Bridge Research Institute (IBDiM). Peer-review under responsibility of Road and Bridge Research Institute (IBDiM) Keywords: Scour; vibration; SHM; frequency; acceleration
* Corresponding author. Tel.: +353-1-7163284 E-mail address: [email protected]
2352-1465 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Road and Bridge Research Institute (IBDiM) doi:10.1016/j.trpro.2016.05.495
Luke J. Prendergast et al. / Transportation Research Procedia 14 (2016) 3982 – 3989
1. Introduction 1.1. Bridge scour Scour of foundations is the number one cause of bridge collapse in bridges located over waterways. Several studies undertaken into failed bridges in the United States have indicated that scour (and other flood-related issues) have resulted in their collapse (Melville and Coleman 2000; Briaud et al. 2001 and 2005). One such study concluded that during the last 30 years 600 bridges have failed due to scour problems (Shirole & Holt 1991; Briaud et al. 1999) causing major operating disruption and financial losses (De Falco & Mele 2002). In the United States the average cost for flood damage repair of highways is estimated at $50 million per year (Lagasse et al. 1995). When corrected for inflation, this figure would be even higher today. During a single flood event in the upper Mississippi and lower Missouri river basins which occurred in 1993, at least 22 of the 28 bridges that failed were due to scour. The associated repair costs were more than $8,000,000 (Kamojjala et al. 1994). An example of a failed bridge due to scour problems is the Malahide Viaduct, which collapsed due to tidal scour in Dublin, 2009 (see Fig. 1). 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 forms of scour; namely general scour, contraction scour and local scour. General scour occurs naturally in river channels and includes the aggradation and degradation of the river bed that may occur as a result of changes in the hydraulic parameters governing the channel form such as changes in the flow rate or changes in the quantity of sediment in the channel (Forde et al. 1999). General scour is a natural erosion and deposition process. It relates to the natural evolution of the waterway and is associated with the progression of scour and filling, in the absence of obstacles (Federico et al. 2003). Contraction scour occurs as a result of the reduction in the channel’s cross-sectional area that arises due to the construction of structures such as bridge piers and abutments. Bridges tend to reduce the free cross-sectional area of a channel by the nature of pier and abutment construction within the channel. Contraction scour manifests itself as an increase in flow velocity and resulting bed shear stresses, caused by a reduction in the channel’s cross-sectional area at the location of a bridge. The increased shear stresses can overcome the channel bed’s threshold shear stress and mobilize the sediments (Briaud et al. 1999). Finally, Local scour occurs around individual bridge piers and abutments. It is caused by downward flow being induced at the upstream end of bridge piers which leads to very localized erosion in the direct vicinity of the structure (Hamill 1999), see Fig. 2(a). Horseshoe vortices can develop due to the separation of the flow at the edge of the scour hole upstream of the pier and this can result in pushing the down-flow inside the scour hole closer to the pier, which exacerbates the process. Horseshoe vortices are a result of initial scouring and not the primary cause of scour. Furthermore, separation of the flow at the sides of the pier result in wake vortices on the downstream end of the pier (Heidarpour et al. 2010). Local scour depends on the balance between streambed erosion and sediment deposition. Clear-water scour is the term given to describe the situation when no sediments are delivered by the river into the scour hole whereas live-bed scour describes the situation where an interaction exists between sediment transport and the scour process (Brandimarte et al. 2006). The presence of live-bed conditions leads to smaller ultimate scour depths than in clear-water conditions. Further information on the scour process is available in Prendergast & Gavin (2014). Scour results in a loss in foundation stiffness and can cause sudden failure, see Fig. 2(b).
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Fig. 1. Malahide Viaduct collapsed August 2009 (Prendergast & Gavin 2014).
Bridge Pier
Removal of foundation soil by water action
Downflow
Wake Vortices
Scour Hole Horseshoe Vortices
Loss in foundation stiffness
Potential (sudden) bridge collapse
(a)
(b) Fig. 2 (a) Local scour schematic; (b) Scour process.
1.2. Scour monitoring Traditionally, scour monitoring was undertaken by way of diving inspections whereby divers would manually inspect the condition of critical bridge foundations using crude depth-measuring instrumentation, see Fig. 3(a). This practice is dangerous and outdated. In general diving inspections cannot be undertaken during heavy flood flow, when scour risk of occurrence is highest and also the fact that scour holes tend to refill upon the subsidence of flood waters means that the scour depth measured may often not be that of the maximum depth attained during the flood. There are a number of systems available that aim to remotely monitor the presence of scour around bridge foundations. These systems range from sound monitoring devices and electrical conductivity devices to physical probes resting on the riverbed. In general these devices aim to detect changes in the depth of a scour hole around a foundation and can either be operated continuously or discretely as part of standard maintenance procedures. An
Luke J. Prendergast et al. / Transportation Research Procedia 14 (2016) 3982 – 3989
example of a number of these devices is shown schematically in Fig. 3(b). This figure is reproduced from Prendergast & Gavin (2014) and shows a Magnetic Sliding Collar (MSC), Float-Out Device (Briaud et al. 2011), Time-Domain Reflectometry (TDR) (Yankielun & Zabilansky 1999; Yu 2009), Sonic Fathometer and Ground Penetrating Radar (GPR) (Anderson et al. 2007) systems. (a
(b
Fig. 3 (a) Schematic of diving inspections for scour measurement; (b) Schematic of scour-measuring instrumentation (Prendergast&Gavin 2014).
2. Vibration-based scour detection 2.1. Background By virtue of the process of eroding away soil from around foundation elements, scour causes a reduction in the stiffness of a foundation. Traditional Structural Health Monitoring (SHM) utilizes losses in stiffness of a structural element to detect damage as stiffness loss typically manifests itself as a change in modal characteristics of a structure. The same is true for the detection of scour. Losses in soil contact with a foundation can manifest itself as a change in modal properties (natural frequencies, damping ratios and mode shapes etc.). The key question with relation to the detection of scour is how sensitive these parameters are to the scour process. Several authors have investigated vibration-based scour detection methods, see Briaud et al. (2011); Elsaid & Seracino (2014); Foti & Sabia (2011); Prendergast et al. (2013); Prendergast et al. (2015); Klinga & Alipour (2015); Ju (2013); Chen et al. (2014). The detection and monitoring of scour is arguably more complex than crack damage detection in bridge beams for example as there are a significant number of parameters at play in the problem. Foundation geometry, bridge superstructure properties, soil type and stiffness nonlinearities are all issues which affect the sensitivity of scour detection using vibrations. In particular, the nonlinear and inelastic nature of foundation soils at intermediate operational strains can alter the dynamic response of a foundation system (and by extension, a superstructure). This is shown by the change in secant stiffness of a shallow pad foundation element due to scour undermining the foundation (Prendergast & Gavin 2014). The loss of soil contact leads to an increased stress (and strain) on the remaining soil, and since soil stiffness is strain-dependent, a loss of stiffness can occur. This adds a further degree of complexity to the global issue of trying to identify foundation scour using vibration-based damage detection methodologies.
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V
V
Bridge Pier
Bridge Pier
Scour Hole
No Scour
E1
E2
Stress
E1 = Stiffness before scour E2= Stiffness after scour E2 < E1
Strain
Fig.4. Stiffness loss due to scour (Prendergast & Gavin 2014).
2.2. Modelling of foundation scour In this paper, a numerical model of a two-span integral bridge is created to highlight the sensitivity of structural vibrations to scour of the foundation. A schematic of the bridge is shown in Fig. 5. The bridge model is loaded by a point load (representing the passage of a vehicle along the bridge deck). The dynamic response of the bridge due to this point load traversing is calculated in the model and outputted as bridge dynamic displacement, velocity and acceleration from the top of the pier.
Fig. 5. Two-span integral bridge schematic with moving point load.
The bridge properties correspond to a typical two-span integral bridge, see Prendergast et al. (2015) for case study properties adopted. The soil is modelled using a Winkler spring philosophy whereby the soil stiffness is discretised into individual springs for ease of modelling (Winkler 1867; Dutta & Roy 2002; Prendergast et al. 2013). The method for deriving soil stiffness is available in L J Prendergast et al. (2015).
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2.3. Results of analysis To highlight the sensitivity of the dynamic response of a typical integral bridge to pier scour, a brief analysis is conducted herein. The load (50 kN) traverses the bridge at 50 km/hr (13.88 m/s) and the lateral dynamic displacement, velocity and acceleration is calculated at the pier top (see Fig. 5 for sensor location in schematic). 10 seconds of free vibration (damped at 2%) is included in the analysis. This is shown in Fig. 6.
Fig. 6. (a) Pier top lateral displacement; (b) Pier top lateral velocity; (c) Pier top lateral acceleration.
The results from Fig. 6. show a typical lateral bridge response due to the passage of a load along the bridge deck. For completeness, the displacement, velocity and acceleration signals generated are shown. Both the forced vibration component (when the load is on the bridge) and the free vibration component (when the load has departed) are highlighted. In reality, the dynamic response will be measured by an accelerometer; therefore it is of interest to assess the effect of scour of the pier on the lateral pier top acceleration response. This is shown in Fig. 7. For this analysis, the load traverses the bridge deck at 50 km/hr for zero scour and for 10 m of scour around the central pier foundation and the change in the acceleration response is shown.
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-3
12
x 10
zero scour 10m scour
-3
1
10
x 10
8 0
acc (m/s 2)
6 4
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0 -2 -4 -6 -8
0
2
4
8
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time (s) Fig. 7. Zero scour and 10m scour lateral pier top acceleration responses due to passage of a vehicle over the bridge.
Fig. 7. shows the lateral pier top acceleration for zero and 10m scour affected the bridge pier. As is evident, scour has the effect of increasing the period of the vibration for the scoured signal (see the insert on the figure). This is sensible as an increased effective length leads to a reduced flexural stiffness therefore an increased period is expected. The scoured signal also has a lower amplitude in the free vibration than the zero scour signal. Scour therefore has a noticeable effect on the dynamic response of an integral bridge system. 3. Conclusions Scour is the number one cause of bridge failure in bridges located over waterways. It leads to a rapid loss in foundation stiffness and can result in sudden catastrophic collapse. Traditionally, diving inspections were used to detect and monitor scour around critical sub-structure components of bridges. More recently, a range of automated and manually operated instrumentation has become available that aims to detect the change in scour depths around bridge foundations either continuously or discretely. These instruments have associated advantages and disadvantages. This paper focusses on the topic of applying vibration-based Structural Health Monitoring techniques to the detection and monitoring of scour. Preliminary results show that the lateral pier top acceleration response is highly affected by the presence of scour around the pier and that the method may be applicable as a lowmaintenance scour monitoring alternative to the more intensive and expensive instrumentation available on the market. Further research is required to assess the potential of the approach on actual structures as the results in this paper were derived from numerical simulations.
Acknowledgements The authors would like to acknowledge the support of the Earth and Natural Sciences (ENS) Doctoral Studies Programme, funded by the Higher Education Authority (HEA) through the Programme for Research at Third Level Institutions, Cycle 5 (PRTLI-5), co-funded by the European Regional Development Fund (ERDF), the European Union Framework 7 project SMART RAIL (Project No. 285683) and the University College Dublin (UCD) Earth Institute.
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