Scour Vulnerability Evaluation of Pile Foundations During Floods for National Highway Bridges

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1 ICSE6-200 Scour Vulnerability Evaluation of Pile Foundations During Floods for National Highway Bridges Jae Hyun PARK 1, Kiseok KWAK 2, Ju Hyung LEE 3, Moonkyung CHUNG 4 1 Senior researcher/korea Institute of Construction Technology 2311, Daehwa-dong, Ilsanseo-gu, Goyang-si, Korea: - jaehyeon@kict.re.kr 2 Research fellow/korea Institute of Construction Technology 2311, Daehwa-dong, Ilsanseo-gu, Goyang-si, Korea: - kskwak@kict.re.kr 3 Senior researcher/korea Institute of Construction Technology 2311, Daehwa-dong, Ilsanseo-gu, Goyang-si, Korea: - leejh73@kict.re.kr 4 Research fellow/korea Institute of Construction Technology 2311, Daehwa-dong, Ilsanseo-gu, Goyang-si, Korea: - mkchung@kict.re.kr Researches for reducing bridge failures during floods have been concentrated on analysis, inspection and countermeasure of bridge scour for the decades. Recently, prioritization and maintenance scheme of bridge scour have become a more concern, which should be based on the reasonable evaluation and inspection. The vulnerability of bridge piers is categorized into 5 groups based on the conventional analysis of the bearing capacity of bridge foundation as well as on the analysis of the effects of floods on the foundation, on foundation type, on foundation depth, on foundation width, and on present scour condition. Performed were the evaluation of bridge scour vulnerability and prioritization on real bridges founded on pile foundations registered in the National Highway Bridge Inventory of the capital region. The case studies for twelve national highway bridges consist of site investigation including boring test, bridge scour analysis for the design flood, bearing capacity evaluation of the bridge foundation before and after scour, comprehensive evaluation of bridge scour vulnerability, and prioritization. Three of 12 pile foundation bridges showed the potential future vulnerability to scour with considerable decrease in the embedded depth of foundations due to scour and the remaining 9 bridges were expected to maintain their stability to resist the effects of scour. Key words Scour vulnerability, scour depth, bearing capacity, pile foundation, prioritization I INTRODUCTION The security of bridge safety is one of the most important issues in the management of national infrastructures and its importance has been increased continuously due to the rapid growth of traffic. There are approximately 22,000 bridges in South Korea. During the last 20 years, approximately 1,000 bridges have been damaged and most of those damages are due to scour and stream instability during floods. The average cost for flood damage repair of bridges under government control is $100 million per year [MOCT, 2003]. Researches for reducing bridge failures during floods have been concentrated on analysis, inspection and countermeasure of bridge scour for the decades. Recently, prioritization and maintenance scheme of bridge scour have become a more concern, which should be based on the reasonable evaluation and inspection; Ho et al. [2002] has developed a GIS-based bridge scour prioritization system for the efficient and economic maintenance and reinforcement of bridges in New England State (U.S) and Palmer and Turkiyyah [1997] has developed the system of assessing the bridge scour vulnerability and river stability using Bayesian module. In Korea, bridge scour management system (BSMS) was developed using GIS technique so that the efficient anti-disaster measures can be established during flood events [2005]. The bridge scour management system 1 Corresponding author 437

2 developed consists of basically two parts: a) GIS-based database system, and b) prioritization system of bridge vulnerability to scour. Bridge scour vulnerability is evaluated based on interdisciplinary concept that can include the geotechnical factors related to the bearing capacity of foundation-ground system and all the hydraulic influential factors that are necessary to scour analysis. In this study, performed were the evaluation of bridge scour vulnerability and prioritization on real bridges founded on pile foundations registered in the National Highway Bridge Inventory of the capital region. The case studies for twelve national highway bridges consist of site investigation including boring test, bridge scour analysis for the design flood, bearing capacity evaluation of the bridge foundation before and after scour, comprehensive evaluation of bridge scour vulnerability, and prioritization. II BRIDGE SCOUR VULNERABILITY EVALUATION Bridge scour vulnerability is evaluated by using the predicted scour depth considering precise field inspection results, its associated change in the bearing capacity of foundation, and present scour condition, etc. Studies in the past were generally focused on formulating the scour mechanism, taking into consideration main geometrical and physical parameters involved. Thus, in assessing the vulnerability of bridge foundations to scour, the geotechnical factors are often overlooked and the scour design is not accomplished by an interdisciplinary team. Regarding the assessment of vulnerability of combined system to scour composed of bridge pier, foundation and ground, the importance of the geotechnical factors have been analyzed [De Falco et al., 1997], and the method to consider the time effect and soil properties in the analysis of scour in fine-grained soil has been developed [Briaud et al., 1999 ; Kwak, 2000]. In case of severe scour, the bearing capacity of the foundation-ground system can dramatically reduce and significant amount of displacements and rotations of foundation can be induced. More precisely, the limit state of foundation may drive from the ultimate limit states of the foundation-ground geotechnical system. Federico et al. [2003] suggested the method of evaluation of vulnerability to scour in case of spread footing considering the bearing capacity change resulting from scour. An extensive advanced method which is applicable to both of spread footing and pile foundation with sufficient preciseness is developed to assess the vulnerability of foundation to scour reasonably. The expanded applicability with preciseness of this method enables bridge designers to assess the vulnerability of foundation to scour for larger number of bridges. Bridge vulnerability to scour can be explained in the concept of bearing capacity safety factor as described in equation (1). In foundation design, the safety factor of a typical foundation-ground system is 3.0. Therefore, the safety factor of bridge foundation before scour can be defined as 3.0 and the safety factor decreases as scour progresses. Q normal Q (S.F.) S.F. u a normal normal Q scour Q (S.F.) S.F. u a scour scour (1) normal Where, is the vulnerability to scour of a foundation, Q u and Q scour u and mean the ultimate bearing capacity of the foundation-ground system before and after scour respectively, Q a means the allowable bearing capacity of the foundation-ground system, S.F. normal and S.F. scour are safety factors of foundation before and after scour, respectively. Therefore, the vulnerability ( ) can be determined from the safety factor of foundation-ground system as scour progresses. The prioritization scheme of bridge scour vulnerability developed in this study is illustrated in Figure 1. As shown in Figure 1, the representation of scour condition is described on a Cartesian plane with normalized scour depth by foundation width (B) and normalized foundation embedment depth by foundation width for X, Y domain respectively. The plane is basically divided into two areas: scour depth (Y s ) equal to or larger than foundation embedment depth (Y p ); scour depth (Y s ) less than foundation embedment depth (Y p ). The former case, actually, no longer exists as the situation contributes the immediate failure of the foundation and bridge. However, for the purpose of bridge maintenance coping with the design floods, it is necessary to consider that case. The former case is classified as Grade 1. In the latter case, scour vulnerability is determined by the ratio of bearing capacity of foundation between before and after scour. For a constant allowable bearing capacity, scour vulnerability can be defined by a function of safety factor for normal condition (before scour) to scour condition (after scour) as described in equation (1). In this case, bridges are classified into three groups: Grade 2 with 0< S.F.<1; Grade 3 with 1< S.F.<2; and Grade 4 for the stable condition with the S.F

3 Furthermore, in Figure 1, Grade 0, derived from not an analytical approach but a conceptual approach, refers to scour critical bridges which need to be taken an urgent measure to meet the situation. In case that bridges of which the type, dimensions, reinforcing, and/or elevation of the foundation are unknown, those bridges are classified as Grade U (unknown foundation) in order to be investigated and examined independently. Y s /B Y S =Y P Grade 0 Grade 1 Grade 2 S.F.=1.0 ξ = 3.0 Y S >Y P Y S <Y P Grade 3 S.F.=2.0 ξ= Grade 4 S.F.=3.0 ξ= 1.0 Y p /B Figure 1: Prioritization scheme of bridge scour vulnerability III BRIDGE SCOUR VULNERABILITY EVALUATION III.1 Selection of Bridges and Scour Depth Calculation Among 532 bridges in the middle part (Gyeonggi and Kangwon provinces) of Korean peninsula that are under the control of the central government, 144 bridges were selected through the preliminary screening process of scour analyses. Excluded from preliminary screening were bridges: that has no pier in river; whose information such as type of foundation, embedded depth, design hydraulic/hydrologic variables are unknown; and whose foundation type is a large-scaled caisson which can withstand the effects of scour better than the other foundation types. From the 144 bridges, 12 bridges with pile foundations were finally selected for further detailed analysis including field inspection, scour analysis, assessment of bridge scour vulnerability and prioritization. The selected bridge lengths range 44m and 205m. Five of 12 bridges have maximum span lengths equal to or larger than 30 meters, and the others have smaller than 30 meters. Only one bridge has streambed slope larger than The selected bridges for scour vulnerability analysis and design flood parameters are listed in Table 1. No. Bridge code Bridge length Maximum span length Pile embedded length Stream bed slope 100-year Design flood (m 3 /sec) 100-year Design water depth 100-year Design water velocity (m/s) 1 GC HS NC DM , IW JA , JS , NP NC YA CH GE Table 1 : Bridge pile foundation dimensions and hydraulic parameters for scour analysis 439

4 In order to estimate the scour depth around a bridge pier during flood events, hydraulic and hydrological variables including discharge, velocity, and depth for the design flood should be established by considering potential future bad conditions. In this study, hydraulic and hydrological variables were determined by using database available from the Water Management Information System web site ( The hydraulic parameters for scour analyses were calculated for 100-year frequency suggested by Korean river design standards [KWRA, 2009]. The hydraulic design flood parameters are listed in Table 1 More information for scour analysis is needed to get precise assessment of geotechnical and structural factors as well as hydraulic factors affecting scour characteristics. Therefore field inspection was conducted on each bridge site to obtain the general structural condition of the bridge, the present degree of scour damage around bridge foundation and embankment, geomorphic properties of the watershed area, bed material properties (size, gradation, distribution and soil classification), and boring log information at the pier site. The geotechnical and structural design parameters are listed in Table 2. The scour depth of each bridge foundation was calculated using the parameters that are summarized in Tables 1 and 2. The expected scour depths were determined from the average value calculated using the CSU equation of HEC-18 [Richardson and Davis, 2001], Froehlich's Equation [1988], Laursen's Equation [1960] and Neill's Equation [1973] that are adopted by Korean river design standards [KWRA, 2009]. Furthermore, in case rock exists in the expected scour depth that was calculated from the scour equation, the scour depth of the subject bridge was decided taking the rock depth into account. The determined scour depth for each pile foundation is summarized in Table 2. No. Bridge code Streambed particle size (mm) Rock depth Pier width Pier length Calculated scour depth Determined scour depth D 10 D 50 D 60 D 95 CSU Froehlich Laursen Neill 1 GC HS NC DM IW JA N/A JS N/A NP N/A NC N/A YA N/A CH N/A GE N/A Table 2 : Scour depth calculation results In Table 2, expected scour depths of the bridges ranged from 1.6m to 4.7m. Compared to the average scour depth, the CSU equation, widely-used equation for pier scour depth on coarse-grained soils, predicted smaller scour depths in case large particle size existed at river bed due to the armoring effect. The bridges showing this tendency were NC, DM, JA, GE. Scour depth calculated by the Froehlich's equation considering inflow angle showed relatively larger value than the results from Laursen's equation or Neill's equation. As Neill's equation considers only water depth and bridge pier width in calculating scour depth, the effect by the bridge pier width is relatively large. The four equations incorporate different variables with varying importance. Thus they gave inherently different scour depths as expected, and no equation gave consistently larger or smaller values. The variation of scour depth from the four equations, however, did not exceed 1.5m in all the bridges except one (JA). Figure 2 illustrates the calculated and determined scour depths. The scour depths were estimated assuming that scour would not occur within the design period of bridges in case of rock due to its high resistance to scour. In all the cases, scour depths were calculated smaller than average scour depths due to the depth of rock. 440

5 Scour Depth Bridge No. CSU Froehlich Laursen Neill Determined Figure 2: Scour depth calculation results III.2 Bridge Scour Vulnerability Evaluation and Prioritization The assessment of the bridge vulnerability to scour is enabled by the analysis of bearing capacity of foundation before and after scour. The Korean Design Standards for Foundation Structures [KGS, 2009] adopts two static bearing capacity analysis methods: (a) general static bearing capacity method, and (b) empirical method which is revised after Meyerhof [1976]. In this study, the general method was used to estimate the ultimate bearing capacity (Q u ) of pile foundations because the method predicts Q u more accurately than the other method [Kwak et al., 2010]. The equation for the general static bearing capacity is: Q u ( vnq cn c)ap fsas (2) where Q u is the ultimate bearing capacity; σ v ' is the effective overburden pressure at pile tip with the depth limit to 20D (D = pile diameter); N q is the bearing capacity factor as a function of friction angle ( ) of soil (U.S. Department of the Navy, 1982); N c is the bearing capacity factor for cohesion; A p is the cross-section area of pile; f s is the unit frictional resistance along the shaft of each layer (= K s σ v tanδ for cohesionless soils); K s = 1.4(1-sinϕ); σ v is average effective overburden pressure along the shaft ; δ=20 degrees; f s = αc u for cohesive soils ; α is the adhesion factor, c u is the undrained shear strength of soil, and A s is the pile shaft surface area of each layer. Static bearing capacities of the twelve bridge pile foundations were calculate with dimensions of foundation, predicted scour depth, and subsurface conditions of the bridge sites. The bridge scour vulnerability priorities are initially divided into two parts by comparing expected scour depth with foundation embedment depth. When expected scour depth is larger than the foundation embedded depth, its scour vulnerability is categorized as Grade 1 with potential severe scour condition, while when expected scour depth is smaller than the foundation embedment depth, bridge vulnerability to scour is categorized into three groups (Grade 2 to 4) based on the reduction ratio of the bearing capacity of foundation due to scour. Result of the assessment of bearing capacity of the foundation before and after scour and the evaluation of scour vulnerability prioritization for the 12 bridges are summarized in Table 3. As a pile foundation, in general, is used to overcome the difficulties of founding on soft and erosive soils, it has commonly large embedment depth. Hence, it is known that a pile foundation is more resistible to scour than a spread footing. Table 3 listed the calculated bearing capacities of 12 pile foundations before and after scour. 441

6 No. Bridge code Pile embedded length Scour depth Bearing capacity(tonf) Before scour After scour Bearing capacity reduction (%) Scour vulnerability ( ) S.F. after scour Scour vulnerability prioritization 1 GC HS NC DM IW JA JS NP NC YA CH GE Table 3 : Scour vulnerability evaluation for pile foundations In Table 3, three (YA1, CH, GE) of 12 bridges exhibited considerable decrease in their bearing capacity after scour and categorized as Grade 3. In those cases, the expected scour depth-to-the pile embedded depth ratios were relatively high and then the piles were exposed to the flow which would cause severe negative effects such as lateral displacement of the pile and flow impact. Therefore, the evaluation of bridge scour vulnerability by scour monitoring including detailed site investigation as well as analytical approach is very necessary to prevent the situation of pile foundations exposed to the flow in advance. The remaining 9 of 12 pile foundation bridges were expected to maintain stability against scour for design floods. The bearing capacity reduction [%=1-(bearing capacity after scour/bearing capacity before scour)] of each pile foundation due to scour is presented in Figure Bearing capacity reduction(%) Bridge No. Figure 3: Bearing capacity reduction (%) due to scour After scour vulnerability evaluation, the status of scour progress should be checked through the detailed site investigation. And the vulnerability to scour obtained from aforementioned analytical approach, should be changed from Grade 4 to Grade 3, Grade 2 or even Grade 1 according to the field condition, if necessary. In this procedure, the potential vulnerability of a bridge to scour should be evaluated by an interdisciplinary team of geotechnical, hydraulic, and structural engineers who can make the necessary engineering judgments. 442

7 IV CONCLUSIONS In recent years, the increasing demand for minimizing damage to the bridges requires more sophisticated research into the analysis, inspection, countermeasure and maintenance with prioritization of bridge scour. In this study, a reasonable bridge scour vulnerability prioritization was introduced with multidisciplinary concept using the correlation between bridge scour and bearing capacity of bridge foundation and twelve pile foundation case studies were comprehensively conducted. Scour depths around bridge foundation for each bridge were estimated applying four well established equations: the CSU equation; the Froehlich s equation; the Laursen s equation; and the Neill s equation. Most of the bridges showed the expected scour depths ranging between 1.6m and 4.7m. The different kinds and degree of effects of the parameters considered in scour equations contributed to the variation of the scour depths. The static bearing capacity analysis methods adopted in the Korean Design Standards for Foundation Structures were used to evaluate the vulnerability of bridge foundation to scour considering geologic bstrata at the site. Three of 12 pile foundation showed the potential risk of failure due to scour. It is noted that even pile foundations, having large embedment depth, showed considerable decrease in their bearing capacity resulting from scour in some cases. After prioritization, the field conditions of foundations are conducted to manage scour investigation schedule for bridge maintenance and in timely decision-making with respect to foundations vulnerabilities to scour. V ACKNOWLEGMENTS AND THANKS This work was supported by a grant (10 CTIP E04) from Offshore Wind-energy Foundation System (OWFS) program and by a grant (No. 12CCTI-A ) from Super Long Span Bridge research project program funded by Korea Ministry of Land, Transport and Maritime Affairs. VI REFERENCES Briaud, J.-L., Ting, F., Chen, H.C., Gudavalli, S.R., Perugu, S., & Wei, G. (1999). - SRICOS: Prediction of scour rate in cohesive soils at bridge piers. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 125: De Falco, F., Gentile, M., & Mancini, M. (1997). - Hydraulic vulnerability due to scour at bridge piers and monitoring systems. The Italian railways experience. C.I.S.M. Course on Hydraulic Phenomena in Proximity of Bridges. Federico, F., Silvagni, G., & Volpi, F. (2003). - Scour vulnerability of river bridge piers. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 129: Froehlich, D. C. (1988). - Analysis of onsite measurement of scour at piers. Proceedings of National Hydraulic Engineering Conference, ASCE. Ho, C. L., Di Stasi, J. M. & Rees, P. (2002). - GIS-Based bridge scour prioritization. Proceedings of the First International Conference on Scour of Foundations, ICSF-1, 2: Korea Institute of Construction Technology (KICT) (2005). - Bridge scour management system. Ministry of Construction and Transportation, Korea. Korean Geotechnical Society (KGS) (2009). - Design Standards for Foundation Structures. Ministry of Land Transport and Maritime affairs, Korea. Korea Water Resources Association (KWRA) (2009 ). - The River Design Standard. Ministry of Land Transport and Maritime affairs, Korea. Kwak, K. (2000). - Prediction of scour depth versus time for bridge piers in cohesive soils in the case of multi-flood and multi-layer soil systems, Ph.D. Dissertation, Texas A&M University, Texas, USA. Kwak, K, Kim, K.J., Huh, J, Lee, J.H., & Park, J.H. (2010). - Reliability based calibration of resistance factors for static bearing capacity of driven steel pipe piles. Canadian Geotechnical Journal, 47: Laursen, E.M. (1960). - Scour at bridge crossings. Journal of Hydraulic Division, ASCE, 86: Meyerhof, G.G. (1976). - Bearing capacity and settlement of pile foundations. Journal of the Geotechnical Engineering Division, ASCE, 102: Ministry of Construction and Transportation (2003). - Bridge management system. Korea. 443

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