An Experimental Study of Local Scour Around Circular Bridge Pier in Sand Soil

Transcription

1 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol:13 No:01 23 An Experimental Study of Local Scour Around Circular Bridge Pier in Sand Soil Ibrahim H. Elsebaie 1 Abstract Study of sour bridge piers is extremely important for the safe design of the piers and other hydraulic structures. An experimental investigation of local scour around circular bridge piers in sand is presented. The principal objective of this study is to carry out a much longer duration Tests to evaluate the time development of the local scour at cylindrical pier in addition to the evaluation of the effectiveness of a pier shape and different flow rates on the depth of local scour. This study describes the variation of scour depths that may occur at bridge piers. There has been a difficulty in estimation of accurate scour depth, which includes similitude aspects of laboratory experiments on scour at bridge piers, complicate the development of reliable scour-estimation relationships. In a practical sense, the difficulties imply that estimation relationship can only be of approximate accuracy. Experimental investigations have been studied to examine the maximum depth of scour and its pattern along longitudinal as well as in transverse directions. It was found that the scour depth increases with time. In addition, the maximum depth of scour is dependent on both time as well as flow rate, it was noticed that maximum depth of scour was increasing with increase of flow and time as well. However results presented here are encouraging and are very much in the agreement with the previous studies related to scour at bridge piers. Index Term-- Bridge pier, Scour, Time dependence, Equilibrium scour depth, Sediment transport. 1. INTRODUCTION Scouring may be defined as the removal of material around piers, abutments, spur dikes, and embankments caused by flow acceleration and turbulence near bridge sub-structural elements and embankments. Scour is the removal of sediment around or near structures located in flowing water. It means the lowering of the riverbed level by water erosions such that there is a tendency to expose the foundations of a bridge. It is the result of the erosive action of flowing water, excavating and carrying away material from 1 Assistant Professor, Civil Engineering Department, King Saud University, Riyadh, KSA; phone: ; fax: ; elsebaie@ksu.edu.sa the bed and banks of streams and from around the piers and abutments of bridges. Such scour around pier and pile supported structures and abutments can result in structural collapse and loss of life and property. The amount of this reduction below an assumed natured level is termed scour depth. Pier scour is the greatest single cause of bridge failures. With the prospect of more severe and more frequent floods due to climate change, reducing the risk of bridge failure is becoming increasingly important. Scour is a worldwide phenomenon and of great concern especially to civil engineers. Any structure placed in a river, whether of natural or human origin, will tend to promote scour and deposition due to a sudden change in the flow direction or high velocity flow. Scouring has long been acknowledged as a severe hazard to the performance of bridge piers. The type of local scour is classified according to the mode of sediment transport in the approaching flow. They are clear water scour and live bed scour. Clear water scour occurs when sediment is removed from the scour hole but not supplied by the approaching flow; while live bed scour occurs when there is a general sediment transport by the approaching flow [1]. A large amount of literature has been published on the local scour at and around a bridge pier. The total scour at a river crossing consists of three components that, in general, can be added together [2]. They include general scour, contraction scour, and local scour. Cheremisinoff et al. [3] on the other hand divided scour into two major types, namely general scour and localized scour. Local scour at pier site has been subjected to many investigations throughout the world and only very limited success has been achieved by the attempts to model scour computationally, and physical model remains the principal tool employed for studying the scour at the bridges and the site of other hydraulic structures [4]. Tamer et al. [4] presented in his research that the flow depth and velocity have an appreciable effect on the local scour and the data from the physical model showed that doubling the flow depth will result in more than 200% increase in the scour depth. It is necessary to involve the hydraulic engineers in the design stage for bridges to take care of hydraulic effects of the flow on these bridges. Many methods were proposed for estimating the local scour around piers at the bridge site, but these methods were based mainly on the data collected from

2 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol:13 No:01 24 physical models and field data need to be collected to verify these methods [4]. Richardson [5] indicated in his study that bridge foundations should be designed to withstand the effects of scour without failing for the worst conditions resulting from floods equal to the 100-year flood, or a smaller flood if it will cause scour depths deeper than the 100-year flood. Bridge foundations should be checked to ensure that they would not fail due to scour resulting from the occurrence of a super flood in order of magnitude of a 500-year flood [5]. Oscar [6] presented an experimental investigation on time variation of three-dimensional scour-hole geometry at a circular pier in sand. Time-dependent scour-hole geometry has been measured by a new high-resolution non-intrusive method. Experimental results provide information for a quantitative definition of the different scour phases, namely initial, development, stabilisation and equilibrium phase. The obtained data from the study can be used in improving bridge scour monitoring and testing results of numerical simulations [6]. Jau-Yau Lu et al. [7] conducted a research for proposing a semiempirical model to estimate the temporal development of scour depth at cylindrical piers with unexposed foundations. Acylindrical pier with a foundation is considered as nonuniform pier. The simulated results obtained from the proposed model are in good agreement with the present experiment results with the experimental data. He concluded in his study that model agrees satisfactory with experimental data. Qiping [8] indicated in his recent study that scour prediction methods developed in the laboratories and the scour equations based on laboratory data did not always produce reasonable results for field conditions. Recent research indicates that laboratory investigations often oversimplify or ignore many of the complexities of the flow fields around the bridge piers. Patrick D. A. [9] conducted a research; the use of collars for reducing the effects of local scour at a bridge pier is presented together with the time aspect of the scour development. The adoption of a collar is based on the concept that its existence will sufficiently inhibit and/or deflect the local scour mechanisms so as to reduce the local scour immediately adjacent to the pier. The overall objective of the research is to study the temporal development of the scour for a pier fitted with a collar and a pier without a collar. Many researchers have conducted various studies to predict the maximum depth and diameter of scour hole. An attempt has been made to review few previous studies related to scour ( [10]. Scour has been the major concern for safety of marine and hydraulic structures. A large number of hydraulic structures failed as the local scour progresses which gradually undermines the foundations. It is important to control the local scour depth at downstream of hydraulic structures to ensure safety of these structures [10]. Recent study by Guney et al. [11] showed that the Local scours around bridge piers influence their stabilities and play a key role in bridge failures. In his study local scours around bridge piers resulting from unsteady flow was measured. Sabita and Maiti [12] performed study in the field of Local scour around a cylindrical pier in a channel with an erodible bed or natural bed. It was concluded that the highly unsteady complex flow field around a circular pier produces scour hole mainly for the presence of vortices. The main mechanism that drives the formation and evolution of the scour hole around bridge pier is horse shoe vortex motion. Failure of bridges due to local scour has motivated many investigators to explore the causes of scouring and to predict maximum scour depth at bridge piers [13 &14]. In this work, an experimental study was conducted to investigate the effect of the pier shape, discharge and time on the main scour hole dimensions. Also, the maximum depth of scour and its pattern along the longitudinal as well as in transverse directions were investigated. 2. EXPRIMENTAL SETUP 2.1 Experiment Apparatus Experiments were conducted in a rectangular transparent glass flume in the hydraulics laboratory, College of Engineering, King Saud University. The overall length of flume was measured to be 9.45 m. This length includes inlet, outlet and the working section. The length of flume was found to be sufficient to provide stable flow conditions in the flume. The flume was 45 cm deep with a bed width of approximately 30 cm. The flume was constructed on an adjustable steel frame, 1.3 m above the laboratory floor. The flume is provided with two controlling gates, one vertical gate upstream of the working section and a tailgate downstream of the flume. Water into the channel was supplied from a sump tank constructed below the floor level of the laboratory. Centrifugal pump, having a maximum capacity of 27 l/s, serve the purpose of lifting water and supplying it to the channel. Discharge was measured by a V- notch fitted at the end of the flume. Two point-gauge mounted on a sliding aluminum frame was utilized to measure surface elevations at upstream and downstream of the pier. 2.2 Experimental Procedure A cylindrical pier is placed at the middle of the channel section of the flume. The cylindrical shape of the pier is similar to the circular bridge pier. The wooden circular pier of 5 cm diameter was fixed on the flume bed at 2.5 m from the upstream. The bed was leveled thoroughly with the sand and initial level (elevation) of sand bed was taken with the sliding point gauge prior to the start of flow in the channel. All the levels of bed with different time intervals were taken with the same moving gauge installed at upstream as well as down stream separately. Every time runs were started by allowing the water to flow over horizontal bed with a defined flow rate.

3 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol:13 No:01 25 For every run a different discharge was maintained and the water was allowed to flow for successive time periods of 5, 10, 15, 30, 60 and 120 minutes. After each and every defined time interval the elevation of the sand bed was gauged with the same moving gauge. Scour depth measurements were taken along three directions namely X 1 (in the center line of pier/longitudinal), X 2 in the same longitudinal direction but near to channel boundary and parallel to center line of pier, variation of scour depth was also taken in transverse direction(y), as shown in Fig. 1. The arrangement of the pier in the flume is shown in Fig. 2 (a and b). Fig. 1. Circular bridge pier 2.3 Sediment Material The bed material was a mixture of sand with grain sizes ranging between and 2.00 mm. The sand was filled in the working section of the channel up to a layer of approximately 11 cm in thickness. The variation of size of bed material (sand) has been presented in table I. T ABLE I SIEVE ANALYSIS Sieve Weight Dia (mm) No. ret. % Retained % Passing # # # # # # # RESULTS AND DISCUSSIONS The variation in the depth of scouring along the channel and in the vicinity of the pier can be directly observed through a scale attached at different sections on the Plexiglas s each at 10 cm interval and at the pier face. The Scour pattern at bridge pier and the Scour in the vicinity of pier are shown in Fig. (3 and 4). (a) (a) (b) (b) Fig. 2. (a & b) Arrangement of the cylindrical pier in the flume Fig. 3. (a & b): Scour pattern at bridge pier

4 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol:13 No:01 26 (a) Fig. 5. Variation of scour depth with longitudinal direction (X 1 ) (b) Fig. 4. (a& b): Scour in the vicinity of pier The results obtained show the variation of scour depth with distance X 1, X 2 in longitudinal direction and with y in transverse direction respectively, as shown in Fig. (5-7) for one of the flow rates used. In this set of run the flow rate was maintained at flow rates range between 5.82 and liter/second. The observations obtained from all runs with different flow rates in the longitudinal directions X 1 & X 2 showed that the depth of scour is comparatively higher in upstream while it is less in down stream side of the pier. It was found that the maximum depth of scour was attained approximately after one hour of run and there was a very little increase in scour depth in next one hour of run. Scour depth variation in the longitudinal direction X 1 was gauged in the center line of pier while scour depth variations in the longitudinal direction X 2, for different time intervals (starting from 5, 15, 30, 60 & up to 120 minutes), was gauged near to the channel boundary. As evident from the graphs of X 1 series that the level of the sand bed was bit higher than the initial elevation of bed in the down stream of pier which was subsequently reducing with the higher time interval, the reason behind it is the deposition of eroded material from the vicinity of upstream of the pier. The variation of scour depth with distance Y in transverse direction was found to be almost same towards both the sides of pier. Centre line of pier is located at X 1 = 32.5 cm. Series I data were recorded by putting switch on and off for flow with every reading while series II data were recorded in continuation of flow. Fig. 6. Variation of scour depth with longitudinal direction (X 2 ) Fig. 7. Variation of scour depth with transverse direction (Y) All the results depicted here show an increase in maximum depth of scour with the increase of flow rate in longitudinal as well as in transverse direction. The variation of scour depth with distance Y in transverse direction was found to be almost like same trend towards both side of the pier but maximum depth of scour was noticed to be little more compared to results presented with smaller discharge. The maximum scour depth around the cylindrical pier is measured for different flow rates and the experimental results are presented in graphical form to predict the equilibrium scour depth around the cylindrical pier. The variation of depth of scour hole is presented in Fig. (8 and 9). It is observed that as the depth of flow increases the scour hole depth increases but

5 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol:13 No:01 27 rate of increment is not linear. It is clear that the maximum depth of scour is dependent on both time as well as flow rate. Also, it was observed that as increasing the flow in the channel initial depth of scour hole is increasing. As time increases saturation comes and the equilibrium scour depth reaches. stabilisation and equilibrium phase. Performing the sequence of experiments and analyzing the results presented here in this study for local scour at bridge piers, following conclusions can be drawn: The depth of scour increases with time, however it was found that the rate of increase of scour depth was decreasing for a longer time interval. Rate of flow does affect the depth of scour; scour depth was more with higher flow rate. Maximum scour depth was observed to occur at the upstream of the pier. The maximum depth of scour is dependent on both time as well as flow rate, it was noticed that maximum depth of scour was increasing with increase of flow and time as well. Fig. 8. Variation of maximum scour depth with time Fig. 9. Variation of maximum scour depth with time The results obtained by this study confirm the results obtained by [6, 7, 9, 13], where it was observed that the maximum depth of scour is highly dependent on the experimental duration. The depth of the scour hole increases as the duration of the increased flow that initiates the scour increases. The extent of scour observed at the pier also increases as the duration of the tests increases. It was found by Patrick [9] that the temporal development of the scour hole at the pier was dependent on whether or not the pier was fitted with a collar placed at the bed level. 4. CONCLUSION Local scour monitoring is very important to avoid major damages that may occur. An experimental investigation on time variation of three-dimensional scour-hole geometry at a circular pier in sand has been presented. The experimental results provided information for a quantitative definition of the different scour phases, namely initial, development, It is observed that the coarse portion of the sediment is deposited at downstream zone of the pier. However scour hole dimensions in the transverse direction was found to be almost same. Result indicates that scour may take a relatively long time to reach an asymptotic state. The presented data can be used in improving bridge scour monitoring and testing results of numerical simulations. Thus, it is clear from the study that the scour at bridge pier is very important for design of protection works and hence sufficient provisions should be made during designs against expected scour at bridge piers. ACKNOWLEDGMENT The experiments for this study were carried out in the Hydraulics Laboratory of the College of Engineering, King Saud University, Kingdom of Saudi Arabia. The author is grateful for the support provided by the Laboratory staff. REFERENCES [1] Dey, S., Bose Sujit K. and Sastry, L. N., Clear water scour at circular piers: A model,, Journal of Hydraulic Engineering, 1995; 121(12), [2] Richardson, E.V. and Davies, S.R., Evaluating scour at bridges. Rep. No. FHWAIP (HEC 18), Federal Administration, U.S. Department of Transportation, Washington, D.C., [3] Cheremisinoff et al., Hydraulic mechanics 2. Civil Engineering Practice, Technomic Publishing Company, Inc., Lancaster, Pennsylvania, U.S.A., 1987; 780 p. [4] Thamer A. M., Megat J. M., Mohd N., Abdul Halim G., Badronnisa Y.and Katayon S., Physical Modeling of Local Scouring around Bridge Piers in Erodable Bed J. King Saud Univ., Vol. 19, Eng. Sci. (2), 2007; [5] Richardson, E.V., "Instruments to Measure and Monitor Bridge Scour", First International Conference on Scour of foundations, ICSF-1, College Station, Texas, Nov, Vol. 2, 2002;

6 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol:13 No:01 28 [6] Oscar Link, Time Scale of Scour around a Cylindrical Pier in Sand and Gravel, Third Chinese-German Joint Symposium on Coastal and Ocean Engineering National Cheng Kung University, Tainan November 8-16, [7] Jau-Yau Lu, M. Asce, Zhong-Zhi Shi, Jian-Hao Hong, Jun-Ji Lee, and Rajkumar V. Raikar, Temporal Variation of Scour Depth at Nonuniform Cylidrical Piers, Journal of Hydraulic Engineering, [8] Qiping Y., Numerical Investigations of Scale Effects on Local Scour around A Bbridge Pier. A Thesis submitted to the Department of Civil and Environmental Engineering In partial fulfillment of the Requirements for the degree of Master of Science, [9] Patrick D. A.., Time Development of Local Sscour at A Bridge Pier Fitted with A Collar. A Thesis Submitted to the College of Graduate Studies and Research in Partial Fulfillment of the Requirements for the Degree of Master of Science in the Department of Civil and Geological Engineering University of Saskatchewan Saskatoon, Saskatchewan, Canada, [10] Padmini K. and Asis M., Local Scour Around Hydraulic Structures, International Journal of Recent Trends in Engineering, Vol. 1, No. 6, May [11] Guney M.S., Aksoy A.O. and Bombar G., Experimental Study of Local Scour Versus Time Around Circular Bridge Pier, 6th International Advanced Technologies Symposium (IATS 11), May 2011, Elazığ, Turkey [12] Sabita Madhvi Singh #, P. R. Maiti * Local Scouring around a Circular Pier in Open ChannelInternational Journal of Emerging Technology and Advanced Engineering Website: (ISSN , Volume 2, Issue 5, May 2012) [13] Melville, B.W. and Chiew, Y.M., Time scale for local scour at bridge piers. Journal of Hydraulic Engineering, ASCE, 1999; 125(1): [14] Ting et al., Flume tests for scour in clay at circular piers. Journal of Hydraulic Engineering, ASCE, 2001; 127(11):