SCOUR PROTECTION BY A SLOT THROUGH A MODEL BRIDGE PIER

Transcription

1 Journal of Indian Water Resources Society, Vol 33, No. 1, January, 2013 SCOUR PROTECTION BY A SLOT THROUGH A MODEL BRIDGE PIER Baldev Setia 1 and Upain Kumar Bhatia 2 ABSTRACT A laboratory investigation has been carried out to determine the scour depth reduction for cylindrical pier models of 80mm and 82mm diameter provided with rectangular slots through them and located in two separate water flumes A and B with sediment 1 (d 50 =0.16mm, σ g = 1.38) bed and sediment 2 (d 50 =0.60mm, σ g = 1.72) bed respectively. The types of slots investigated upon were: 0 to 180 (Parallel slot), 0 to +120 (Y-slot), 0 to +90 (T-slot) and 0 to +45 (Sigma slot). Various parameters varied included height of slot, bifurcation angle and some modifications of the slot. Results suggest that a parallel slot and a Y-slot with optimum dimensions as 0.25D wide, 1D above and 0.75D below the sediment bed are able to reduce scour by 50% and 40% respectively. Some better found results were investigated on an oblong pier and in combination with a group of piles as well. Keywords: Scour, Bridge Pier, Scour Protection, Slot, Piles INTRODUCTION The phenomenon of scour around bridge piers commences with a three-dimensional separation of the flow owing to adverse pressure gradient upstream of the cylinder. This separation results in the formation of a vortex at the leading edge junction of the pier with the sediment bed, Breusers et al. (1977), Chiew and Melville (1987), Breusers and Raudkivi (1991). This vortex wraps around the cylinder in the form of a horseshoe and the system is responsible for removing sediment from around the bridge piers. Thus, reducing this adverse pressure gradient upstream of the pier provides a solution to the problem of reducing the scour around the pier. Scour prevention can be achieved by weakening of the horseshoe vortex, arresting the sinking of horseshoe vortex and modifying the horseshoe vortex to an advantage. A slot through a pier belongs to the category of devices that reduce the scour around bridge piers by reducing the strength of the horseshoe vortex due to the reduction of effective diameter of the pier. Furthermore, the passage of water through the slot reduces the intensity of adverse pressure gradient upstream of the pier. The slot helps to pass most of the flow through it and only the balance is left to cause much reduced scour damage. The geometry of the slot is simple in nature, although its field applications are fraught with other complications like structural weakening of the pier due to slot and the choking of slot due to floating debris. Tanaka and Yano (1967) and Chiew (1992) have studied the effect of providing slots through the body of circular piers on scour depth. Tanaka and Yano (1967) experimented on a 30 mm diameter pier and provided square slots of 10mm x 10mm and 20mm x 20mm in turn. The slots faced the flow and opened out to the rear. The elevation of the slot from the bed was varied. At best, scour depth reduced by 15 to 30 percent. 1. Fellow, Indian Water Resources Society and Principal, MM Engineering College Mullana (Professor on Lien, Department of Civil Engineering, National Institute of Technology, Kurukshetra, India), setia_b@rediffmail.com, rincipalmmec@mmumullana.org. 2. Member, Indian Water Resources Society and Associate Professor, Civil Engineering Department, MM Engineering College Mullana, upainbhatia@mmumullana.org. Manuscript No.:1292 Chiew (1992) conducted experiments with two distinct locations of the slots. In one case the slot was placed near the bed and in the second, near the water surface. The maximum reduction in scour was found to be 20 percent and 30 percent for width/depth ratio of 0.25 and 0.5 respectively. Based on the principle as stated above, experimental studies on the scour reduction by slots through a pier (also referred to as a slotted pier) were conducted for the types of slots shown in Figure 1. Rectangular slots were made through wooden Fig. 1: Various Types of Slots cylinders of 80mm and 82mm diameter and tested under sediment 1 (d 50 =0.16mm) and sediment 2 (d 50 =0.60mm) conditions. Flow conditions corresponding to initiation of sediment motion were maintained during the experiments. EXPERIMENTS Experiments were carried out in the Hydraulics Laboratory of the Civil Engineering Department at Indian Institute of Technology Kanpur, India. Three different flumes were employed for different types of experiments. Flume A with sediment 1 (d 50 = 0.16mm, σ g = 1.38) was a recirculating flume 27m long, 0.9m wide and 0.35m deep. It had two test sections, one at a distance of 6.0m and the other at 17.5m from the inlet. Flume B with sediment 2 (d 50 = 0.60mm, σ g = 1.72) was a recirculating flume, 5m long, 0.45m wide and 0.90m deep. It had only one test section maintained at 1.5m upstream of the outlet. Flume C was also a recirculating flume 4.0m long, 0.15m wide and 0.30m deep 9

2 and had no sediment. Flume C was used for preliminary tests with the help of wet paint technique. Experiments were run for uniform flow duration of 10 hours after which the results of temporal variation of scour depth and maximum scour depth were compared with those of an unprotected cylindrical pier of same diameter. Duration of 600min of the experimental run was determined after a long duration experimental run of 6000 minutes (100 hours) on the same size of pier and sediment in similar flow conditions. It has been observed that under conditions of incipient velocity, the bed forms approach the test section and the scour depth values oscillate similar to live bed conditions. The maximum scour depth value is very close to the equilibrium scour depth value. Here, maximum scour depth value refers to the largest scour depth reached in the entire duration of experimental run i.e. 600 minutes. Table 1 gives the range of variables used in the present study. Table 1: Range of Variables in the Present Study Rigid Sediment1 Sediment 2 Bed Condition Bed River River Source Ganga at Yamuna Kanpur at Kalpi d50 (mm) σg 4.24D 2.14D to 2.31D Flow Depth Flow Velocity m/s Froude No. 600 minutes Test Duration (10 hours) As a prelude to the investigation of slotted pier on mobile bed flow visualization studies were carried out on a rigid bed. The efficiency of the scour protection arrangement was judged by a parameter termed as performance potential, expressed as (1Hsm/Hsmo)x100. where Hsm is maximum scour depth around a pier with reference to the average bed level and Hsmo is maximum scour depth around an unprotected pier in similar flow and sediment conditions The details of the experiments are given in Setia (1997). Wet Pain Impressions of a Slotted Pier (bs=d/4, ls=d/2) Wet Pain Impressions of a Slotted Pier (bs=d/4, ls=d/2) Fig. 2 : Wet Paint Impressions of an unprotected and Slotted Pier q = m3/s/m, h/d = 4.24, U=0.3190m/s and Fr = FLOW VISUALIZATION STUDIES A qualitative picture of flow modification by parallel slot (0 to 180 ) is presented by means of wet paint impression technique on rigid bed, Gangadharaiah et al. (2000). As is evident from the Figure 2, two smaller regions rather than one big and concentric region mark the flow separation zone upstream of the slotted cylindrical pier. The figure also shows that because of the reduction in size of the obstruction (due to the slot), the size of the primary horseshoe vortex is also smaller. It may be recalled that the size of the primary vortex and the scour depth is primarily a function of the size of the obstruction Breusers (1972). RESULTS Height or length of slot and the bifurcation angle of the slot are among the important parameters that are likely to affect the scour protection performance of slotted piers. The effect of various parameters on the maximum scour depth is discussed under the following subheadings: Wet Pain Impressions of a representative Unprotected Pier 10

3 Effect of Height of Slot on the Maximum Scour Depth In order to evaluate the influence of height of slot, experiments were conducted in sediment 1 (d 50 =0.16mm) on a 80mm diameter wooden cylinder. The width of the slot for all experiments was maintained as 0.25D. The height of the slot, to begin with, was half the pier diameter and the base of the slot was kept flush with the average sediment bed. In subsequent experiments, the height of the slot was increased up to 2.5D in increments of 0.5 times the pier diameter. The upper limit of the height of slot above the bed was 2.5D because the depth of flow had been maintained between 2D and 2.5D. In other words, free water surface flow existed through the slot. Figure 3 shows the results plotted as scour depth verses the height of the slot, both non-dimensionalised with the diameter of the pier. The range of non-dimensional scour depth is from to 0.6 which for all practical purposes may be taken as almost same. In the following set, for all other arrangements remaining constant, the base of the slot was shifted further down by half the pier diameter location of slot base. Therefore, in order to provide a slot that would not show above the sediment bed, it was taken up to 0.75D below the bed. It was observed that for a slot of length 2.5D above bed and 0.75 D below bed, the maximum scour depth, H sm decreased to 0.475D, registering a saving of 56.8% in scour depth with respect to an unprotected pier (H smo =1.1D). It may be concluded that the slot is effective when placed near the bed. It has been demonstrated by the present set of experiments that variation of height of slot from about half the pier diameter above the sediment bed up to the depth of flow is responsible for less than 15% of variation in scour depth. Therefore, it was decided to have only one pier diameter equivalent height of slot above the sediment bed and 0.75 times the pier diameter below the bed for a constant width of slot equal to 0.25D. Effect of Slot Angle on the Maximum Scour Depth Influence of height of slot was investigated on a slot that faced the flow and opened on the rear i.e., running diametrically through the body of the pier. Such an arrangement of slot with optimum specifications would be able to bring about substantial reduction in scour depth. But a slot that runs through across the body of the pier is a dangerous proposition in terms of structural stability and in the event of getting blocked by floating debris. In order to obviate these bottlenecks, the slot was modified so as to have one inlet at 0 facing the flow and two outlets at the following locations: (a) ± 120 (b) ± 90 (c) ± 45 The results have been compared in terms of scour depth non- Fig. 3: Effect of Height of Slot Through a Pier on Maximum Scour Depth 11

4 dimensionalised with the diameter of the pier and presented in Figure 4. Modifications of Slots Y-Slot (0 to ±120 ) A slot facing the flow and opening at the rear is able to reduce the scour depth to 0.475D and 0.43D accounting for a saving in scour depth of about 56% and 61% with respect to an unprotected pier, in sediment 1 and sediment 2. However, such a long slot is fraught with complications like structural weakening and choking due to floating debris. Among the various arrangements that were tried, the next most effective arrangement of slot is a Y-slot (0 to ± 120 ). In sediment 1, the scour depth non-dimensionalized with size of pier (henceforth also being referred to as relative scour depths) is 0.66, that is 60% of the scour depth of an unprotected pier (Performance Potential being 40%). Its effectiveness may be Fig. 4: Effect of Slots of Different Orientations on Maximum Scour Depth Fig. 5: Modifications of Y - Slot 12

5 Fig. 6: Modification of a T Slot Fig. 6: Modification of a T Slot marginally reduced but at the same time the risk involved in terms of its getting choked or blocked is also less. Therefore, it was decided to investigate the Y-slot in greater detail. In the first change, a 45 ramp was provided in the slot facing the flow up to the center of the pier. Thereafter, a horizontal platform followed at the level of top of the ramp in the two arms of Y - pattern. However, the change did not bring any improvement in the scouring extent upstream of the pier. The second modification was in the form of attaching a web, triangular in shape, 1.5D long and 0.5D deep. It was similar to the web of a passive device, Setia (1997), without the top plate. It was expected that such a plate would serve as a guide wall and not allow flow separation to take place at the leading edge of the pier. The maximum scour depths non-dimensionalised with the diameter of the cylindrical pier for the modifications of Y-slot have been shown in Figure 5. The results showed an improvement over the protection by a Y-slot alone. The overall saving in scour depth with respect to the scour depth of an unprotected pier was as high as 52.70%. T-Slot (0 to ±90 ) Referring to Figure 6, it is observed that the slot in a pier facing the flow, extending to the center of the cylindrical pier and then bifurcating into two, to finally open at +90 and - 90, (T-slot), appears at number three, as far as its scour reduction efficacy is concerned. For an oblong pier with a semi-circular nose, the T-pattern would actually be confined to the curved portion only. In prototype, it need not go beyond the section where bearings transfer the load of the superstructure (deck) to the substructure (pier and foundation). In the first modification, the slot from 0 to the center of the pier was provided with small plates push-fitted at an angle of 45 to the base. The plates were spaced vertically by half the pier diameter. Five such plates, referred to as slanting plates, were inserted creating four open compartments, two each of which were positioned above and below the sediment bed. However, the end result, after running the flow past this kind of slotted arrangement, for a net duration of 600 minutes, was not encouraging. To improve this arrangement, horizontal strips were inserted in the exit slots, running through and through from +90 to -90, at the levels of the top of the slanting plates this created compartments or conduits throughout the slot length from entry to exit thus streamlining the flow. Results of the modified T-slot have been shown in Figure 6. Slots through an Oblong Pier An oblong pier with semi-circular ends, 54 mm wide and an aspect ratio (L/D) of 3.5 was used to observe the effect of shape and size under the same sediment and similar flow conditions. Here L is Length of oblong pier along the flow and D is diameter of its semicircular nose. Two orientations of the slot i.e., 0 to 180 (parallel slot) and 0 to ±120 (Y-slot) besides an unprotected pier, were tested. In sediment 1 (d 50 =0.16mm), which formed the test case, maximum relative scour depths of 0.75 and 0.84 were obtained for the parallel and Y-slots, respectively. Corresponding values in the sediment 2 (d 50 =0.60mm) were 0.63 and For oblong piers, structurally, a Y-slot will have advantages over the parallel slot. 13

6 Combination of Slots with Upstream Piles In order to obviate the problems of angle of attack of flow to the slot and the floating debris blocking the flow, experiments were carried out with the two types of slots being combined with a group of small diameter piles. A group of piles comprising of nine piles each of diameter 6.25mm, in a tworow staggered pattern, spaced laterally and longitudinally three times the diameter of the piles and located four times the diameter of the pier at the upstream front of the pier had been found to be the best arrangement, Setia (1997). The lateral extent of the piles would take care of small changes of angle of attack and the piles by their presence would be able to trap any floating debris. Table 2 gives the results of the combined effect of slots and pile group in reduction of scour around the pier. Table 2: Performance Potential of Two Device Combinations Combination Performance Potential (%) 1. Piles + Parallel slot 2. Piles + Y- slot Sediment 1 (d 50 =0.16mm) Sediment 2 (d 50 = 0.60mm) CONCLUSIONS i. A parallel slot (0 to 180 ), 0.25D wide, 1D above and 0.75D below the sediment is able to reduce the scour by more than 50% in both, sediment 1 (d 50 =0.16mm, σ g = 1.38) and sediment 2 (d 50 =0.60mm, σ g = 1.72). ii. A Y-slot (0 to ±120 ), with other specifications as in (i) above, is responsible for about 40% reduction in scour depth in both sediments. A Y-Slot confined to the front semicircular portion of an oblong pier will not sacrifice the structural safety as the slot need not run through the body of the pier to open only at the rear. iii. The other slots, namely T and sigma slots show far too small improvement to be of any significance. iv. The performance of a slot depends upon its position with respect to sediment bed and is more effective nearer the bed than away from it. ACKNOWLEDGEMENT The authors wish to acknowledge the help and guidance received from Dr. T. Gangadharaiah, Professor in Civil Engineering at the IIT, Kanpur, India. REFERENCES 1. Breusers, H.N.C "Local scour Near Offshore Structures. Delft hydraulics laboratory, Publication no Breusers, H.N.C. and Raudkivi A.J., Scouring. Hydraulic Structures Design manual No.2 Balkema Rotterdam-Brookefield, The Netherlands. 3. Breusers, H.N.C., Nicollet, G. and Shen, H.W., Local Scour around Cylindrical Piers. Journal of Hydraulics Research, Vol. 15, No. 3, pp Chiew, Y.M and Melville, B.W., Local Scour Around Bridge Piers, JHR, IAHR, Vol.25, No. 1, pp Chiew, Y.M, Scour Protection at Bridge Piers J. Hydr. Engg., ASCE, 118(9), Gangadharaiah, T., Setia, Baldev and Muzzammil, M., Flow Visualization In Hydraulic Engineering, Proc. Of International Symposium on Recent Advances in Experimental fluid Mechanics Indian Institute Of technology Kanpur, India, Dec Setia, Baldev, Scour Around Bridge Piers, Ph.D Thesis submitted to I.I.T kanpur, India. 8. Tanaka, S. and Yano, M., Local Scour Around A Circular Cylinder, Proc. 12 th Congress, IAHR, Vol. 3 Fort Collins, USA. 14

7 15