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68 6.1 Introduction This chapter presents observations and data from sets of brief, preliminary laboratory experiments conducted to evalu- ate scour countermeasures for protecting abutments. The experiments were carried out using small-scale replicas of a sim- ple abutment form: a wing-wall abutment extending at a depth into a cohesionless bed of a rectangular channel. As mentioned at the outset of this report, wing-wall abutments are commonly used for short bridges, such as those that span relatively narrow channels. Given the large number of small bridges, especially in the U.S. Midwest, the great majority of abutment failures have occurred for small single- or double-span bridges that com- monly have wing-wall abutments. Accordingly, it was thought useful to expend laboratory effort exploring the responses of such abutments to various scour countermeasure concepts. In most cases of subsequent bridge repair, and increasingly for the design of new bridges, it is usual to consider use of a protective armor layer placed to prevent erosion of the chan- nel bed and bank around abutments. Also, to a lesser extent, consideration is given to wing-wall angle and abutment align- ment (relative to the channel crossed) to minimize scour. Adjustment of angle and alignment would seek to minimize local flow velocities and turbulence in the vicinity of the abut- ment, thereby reducing scour. The experiments focused chiefly on the use of armoring countermeasures and to a lesser extent on flow-altering countermeasures. The experiments investigated the performance of armor elements, aprons of riprap and geobags placed around pile-supported wing-wall abutments retaining erodible embankments, and subject to clear-water flow conditions. Also investigated were the influ- ences of wall angle and abutment alignment on scour depth. In particular, the exploratory experiments investigated the following questions: ⢠Are there simple configurations of large armor units that could be an effective scour countermeasure method for wing-wall abutments? ⢠Can aprons of smaller armor units or riprap be used as a scour countermeasure for wing-wall abutments, and, if so, to what extent should a riprap apron extend around a wing-wall abutment? ⢠How do large geobags perform as an alternative to riprap or cable-tied blocks for preventing abutment scour? ⢠How does the wing-wall angle of an abutment affect scour depth? ⢠How does the abutment alignment to a channel affect scour depth? The findings to these questions consist of general observa- tions and small-scale laboratory data about armor unit, riprap, and geobag performance at small bridges. 6.2 Program of Experiments In accordance with the set of questions enumerated above, the program of exploratory experiments consisted of the fol- lowing four series of experiments: ⢠Experiments on the scour countermeasure effectiveness of large blocks, ⢠Experiments on the use of large geobags, ⢠Experiments on the scour influence of wing-wall angle, and, ⢠Experiments on the influence of abutment alignment on scour depth. The experiments were heuristic (i.e., trial-and-error dis- covery) and exploratory in nature. They were carried out using a simple wing-wall abutment to explore the efficacy of using large armor units as a scour countermeasure. The units consisted of two sizes of concrete block, one or more large geobags, and a combination of large geobag and riprap stone. The use of large armor units held particular practical ini- tial appeal because such large blocks would not be moved by the flow and because their roughness and bulk would redirect C H A P T E R 6 Lab Results I: Preliminary Experiments
flow partially. Also, placing and positioning blocks around an abutment would seem relatively practicable, even in flowing water. The test armor units were tried in various combinations and layout extents to gage the sensitivity of scour develop- ment and depth with respect to the placement and location of individual large armor units. The experiments were heuristic, involving considerable adjustment and exploration of armor unit placement. Only a representative overview of the exper- iment results need be mentioned herein. The experiments are fully documented by Martinez (2003). 6.3 Use of Large Blocks 6.3.1 Experiment Layout A simplified configuration of wing-wall abutment was used for the experiments,which were all conducted using a laboratory flume at the University of Iowa. The overall layout and dimen- sions of the flume are given in Figure 6-1, which also indicates the location of the test region in the flume. A sand-roughened false-floor approach conveyed flow to the sediment recess mak- ing up the test section. The test abutments were placed in the sediment recess region. An overall view of the flume is shown in Figure 6-2, which also depicts the sediment recess. The preliminary experiments were done under conditions of clear-water scour, with u*/u*c 0.8, where u* is the shear velocity and u*c is the critical value of the shear velocity asso- ciated with bed-particle movement. The main hydraulic parameters for the flume flow were the following: mean velocity, V0 0.55 m/s; and flow depth, Y0 0.10 m. The sed- iment parameters were the following: median particle size, d50 0.45 mm; standard deviation of sediment size,g 1.4; specific gravity of particles 2.4; and the angle of sediment repose, r 30 degrees. Similitude between laboratory experiments and field scale was satisfied by the use of the aforementioned u*/u*c ratio, of which a value of just below 1.0 represents a condition called âclear-water scour.â This condition is extreme for scouring because the velocity is as high as possible without the move- ment of the channel bed, which causes infilling of the sedi- ment hole. The layout and dimensions of the wing-wall abutment used are given in Figure 6-3. The abutment was made of a simple approximate form, in keeping with the exploratory nature of the preliminary experiments. The figures indicate the layout extents of the armor units placed around the test abutments. Also indicated in Figure 6-3 are the two locations where scour depth was greatest. Point A is at the face of the abut- ment, and Point B is somewhat downstream of the abutment. A consequence of extensive armoring of the bed around the abutment was that the location of deepest scour was forced downstream. 69 Figure 6-1. Layout and dimensions of the flume, including the false floor and sediment recess.
6.3.2 Observations The experiments showed that single large individual armor units, or ensembles of blocks (or such units as dolos and tetrapods), alone, are of limited effectiveness as a scour coun- termeasure. Scour of the bed sediment around the armor units diminished armor unit effectiveness as a scour counter- measure. Figure 6-4 shows the scour that formed around the wing wall without a countermeasure. Figures 6-5 and 6-6 show before-and-after photos of the 22-mm blocks and 11-mm blocks, respectively. Table 6-1 lists the scour depths from three of the experiments. Without the presence of the blocks, a scour depth (dsA0) of 140 mm developed at the face of the abutment (Point A). Flume observations showed that edge erosion of bed sedi- ment occurred around the blocks, caused the formation of a local scour hole around each exposed block, and that the block subsequently slid into the scour hole. As water flowed past the blocks, vortices were shed, which entrained bed sed- iment from around the blocks. Bed sediment particles were winnowed through the gaps of the overlying concrete blocks, causing the local scour hole to expand in area and eventually envelop the blocks. The placement of concrete blocks reduced scour depth at the abutment. Ten large concrete blocks (of side length 22 mm) that were placed around the abutment as shown in Figure 6-5 reduced the scour depth by 32.2 percent com- pared with the baseline scour depth at the unprotected 70 Figure 6-2. View of the sediment flume. Figure 6-4. View of scour hole formed at wing-wall abutment. Two sizes of blocks made of cement and sand were tested in the flume: blocks with 22-mm side lengths and blocks with 11-mm side lengths. The specific gravity of the blocks was estimated as 2.30. The blocks were placed in different arrangements to investigate as a scour countermeasure. Figure 6-3. Layout and dimensions of simple wing-wall abutment used in preliminary experiments, L 160 mm, Ha 32 mm.
abutment. The large blocks, acting as exposed large ele- ments, produced locally increased flow velocities and tur- bulence, such that bed sediment readily scoured from around the blocks. In due course, the scour hole developed around the abutment, and the blocks gradually slid toward the base of the scour hole. An important finding is that the smaller concrete blocks (of side length 11 mm) covering the same area as the large blocks performed essentially the same in reducing scour depth. The equilibrium depth of scour was identical to that conducted with large concrete blocks. In other words, pro- vided that the blocks were not entrained by the flow, block size is less important than the extent of bed covered and the presence of a filter-cloth underlay to reduce the win- nowing of bed sediment. A critical consideration that emerges from the experiments is that the size of the block 71 (a) At start of experiment (b) At end of experiment Figure 6-5. Experiment with large (22-mm) blocks placed at front of wing-wall abutment. (a) At start of experiment (b) At end of experiment Figure 6-6. Experiment with small (11-mm) blocks placed at front of wing-wall abutment. chosen must be large enough to resist shear erosion, yet small enough to substantially reduce any winnowing of bed-form sediment. To demonstrate the influence of aerial coverage on scour depth, two additional rows of the smaller (11-mm) blocks were placed upstream of the abutment and perpendicular to the flow direction, thereby increasing the coverage. The arrangement is shown in Figure 6-6. This experiment showed that dsA was reduced by 60 percent. The reduction
72 Table 6-2. Local scour depths at wing-wall abutment with geobag. Table 6-1. Local scour depths at wing-wall abutment with concrete blocks. a flat profile and rounded edges, thereby reducing local accel- eration of flow velocities around the geobag. Also, a large geobag essentially provides its own filter cloth base as well as acts as an armoring layer. A further possible advantage of a geobag is the prospect of making a geobag that conforms to a desired shape and size for particular abutment sites. Experi- ment-scale geobags of approximately equivalent weight were used as the large blocks and were sized as 90 mm à 70 mm à 18 mm. The geobags were densely filled with sand. 6.4.1 Observations Table 6-2 summarizes the results of the test with a single large geobag. While winnowing erosion did not occur between the geobag and the abutment, edge failure remained an unresolved concern. Figure 6-7 shows the formation of a large scour hole adjacent to the geobag, into which the geobag slid. Note that the geobag setup in this experiment is hinged to the abutment; otherwise, it would have slid completely into the scour hole. The experiment showed that, though the geobag protected the abutment, scour continued at a location shifted away from the abutment to a location downstream of the abutment. Accordingly, two values of scour reduction need to be considered: one at the abutment, dsA, and the other at the in scour depth is attributable to the increased area of bed protection around the abutment. The placement of two rows upstream of the abutment helped to minimize ero- sion of bed sediment from around the leading edge of the abutment, thereby resulting in an enlarged extent of scour hole, but a shallower depth of scour. Furthermore, when the scour hole eventually developed fully around the abut- ment, the larger number of blocks provided greater cover- age of the base of the scour hole, thereby reducing scour depth. The two mechanisms of scour reduction explained above produce a much shallower scour hole. These experimental results agree with prior observations on riprap stones as a pier-scour countermeasure (e.g., Chiew 1995), where suffi- cient riprap stones could significantly reduce winnowing failure. 6.4 Use of Large Geobags The main problems concerning the use of large armor units, such as concrete blocks, for scour reduction are the winnowing of bed sediment around blocks and edge erosion around the blocks. To reduce these problems, experiments were carried out to examine the use of a large geobag formed from geotextile fabric and filled with sand. A large geobag has Layout X +/L X â/L Z +/L Z â/L dsA (mm) dsA/dsA0 (%) No blocks 0 0 0 0 140 100.00 10 blocks a1/L = 0.13 0.33 0.33 0.27 0 94 67 40 blocks a1/L = 0.13 0.33 0.33 0.27 0 94 67 70 blocks a2/ L= 0.07 0.33 0.33 0.27 1 56 40 dsA = scour reduction at the abutment with scour countermeasure. dsAO = scour depth at the abutmnet without scour countermeasure. Layout X +/L X â/L Z +/L Z â/L dsA (mm) dsA/dsA0 (%) dsB (mm) dsB/dsA0 (%) No bag 0.00 0.00 0.00 0.00 140 100 140 100 bag 0.69 0.69 0.50 0.00 84 60 102 73 bag+rock 0.69 0.69 0.50 0.00 60 43 102 73 bag 0.69 0.69 0.50 1.00 52 37 100 71 bag+rock 0.69 0.69 0.50 1.00 58 49 104 74 bag 0.69 0.69 1.00 1.00 0 0 112 80
maximum deepest point of scour, dsB. The values for these two locations were 0.40dsA0 and 0.27dsA0, respectively. 6.4.2 Geobag and Riprap Stone In an effort to control edge erosion, simulated riprap stones (median diameter d50 8 mm) were placed around the geobag. Minor improvements resulted such that dsAR at the abutment was increased from the original 40 percent to 60 percent, though the dsB at the deepest point of the scour hole remained almost the same. Therefore, placing loose riprap stones around the geobag in order to prevent edge failure had marginal success. Figure 6-8 shows how the geobag was at risk of sliding into the scour hole. When the area of geobag protection was enlarged around the abutment so that the geobag covered the bed beneath the large-scale turbulence structures generated by flow around the abutment, the geobag completely prevented scour devel- opment at the nose of the abutment, but the scour hole downstream of the abutment persisted. A further experiment investigated whether the placement of riprap stones on the geobags would reduce the depth of the scour hole down- stream of the abutment. The idea explored in this experiment was whether the riprap on the geobag would roll into the developing scour hole and consequently retard its deepening. The results from both tests show that the deepest point of the scour hole is about 0.48dsA0. No scour occurred at the nose of the abutment. While the formation of the scour hole down- stream of the abutment seems to be unavoidable, the present test shows that armoring the bed would be able to control its development and protect the scour countermeasure. 6.5 Wing-Wall Abutment and Geobags Experiments with a wing-wall abutment and geobags entailed the same flume conditions as those used for the experiments described in Section 6.3. However, now the abut- ment was of wing-wall shape. The wing-wall abutment form used for the experiment replicated, at a scale corresponding to approximately 1:40, the width of abutments typical of two- lane roads in the United States when the road width is about 12 m (40 ft). The abutmentâs wing-walls were set at an angle of 45 degrees. Figure 6-9 shows the dimensions of the model abutment used. Table 6-3 shows the ratio among geometric variables as well as scour depths for four of the experiments conducted. The scour at the unprotected abutment is shown in Figure 6-10. Scour was deepest at the abutment face. When a large geobag was placed around the wing-wall abutment, scour did not occur at the abutment face; that is, 73 (a) At start of experiment (b) At end of experiment, view from above (c) At end of experiment, side view Figure 6-7. Experiment with a single large geobag placed at front of wing-wall abutment.
scour reduction was 100 percent (dsA 0). However, turbu- lence generated by flow around the abutment and over the geobag eroded the sand bed immediately downstream of the geobag, thereby shifting the scour and creating a deeper scour hole. Figure 6-11 depicts the initial state and the eventual scoured state of the bed. The erosion of the bed at the down- stream edge of the geobag gradually propagated upstream around the edge of the geobag. It is noteworthy to point out that this process of edge erosion was observed to occur for all the experiments with geobags. The deepest scour hole for this experiments was dsB 143 percent of dsA0. Its location is shown in Figure 6-11(b) and (c). As the scour hole reached the downstream edge of the geobag, an additional row of geobags was used to further reduce the scour. Figure 6-12 shows the initial condition and the eventual scour condition. Although scour was eliminated at the abutment, the scour hole immediately downstream of the abutment and geobags remained, though it was somewhat shallower. Figure 6-12 shows that the extra row of geobags diminished the erosion attributable to wake vortices. The maximum deepest scour was 119 percent of the scour depth at the unprotected abut- ment (dsA). In addition, when a fringe of riprap stone was placed around the geobags in an effort to limit edge erosion, the maximum scour depth was reduced further to 97 percent of dsA. The stones provided partial armoring of the scour hole. Dune-bed conditions pose the severest test for the stability of an armor cover, such as riprap or geobags, because the pas- sage of dunes may dislodge portions of a cover. This certainly was found in the present study, and it is amply shown for efforts at armoring beds around piers (e.g., Chiew, 2000). It is of interest to note that existing guidelines for riprap design are based on laboratory experiments performed exclusively in clear-water scour and do not account for the dislodging effects of bed forms passing the riprap. 74 (a) At start of experiment (b) At end of experiment, view from above (c) At end of experiment, side view Figure 6-8. Experiment with a large geobag placed around the wing-wall abutment and with stone placed along geobag edges. Figure 6-9. Dimensions of simple wing-wall abut- ment used in preliminary experiments; L 160 mm, b 160 mm, 45 degrees, ra 160 mm, thickness of geobag layer 20 mm.
6.6 Influence of Wing-Wall Angle A series of experiments was conducted in which the wing- wall angle was varied. No additional scour countermeasure was used in these experiments. The angle (Figure 6-13) was set at 15, 30, 45, 65, and 90 degrees to the flow. Figure 6-13 and Table 6-4 show the resulting trends for the variation with of equilibrium scour depth at the abutment, dsA. The values of dsA are normalized with dsA obtained for the 90-degree wing wall (i.e., the vertical wall). Scour depths reduced as decreased. As is to be expected, a smaller angle of wing-wall produces less velocity component normal to the wall. Consequently, the strength of the horseshoe vortex in the scour hole was reduced. Also, the intensity of wake turbulence was reduced. Figure 6-13 shows that the reduc- tions in scour depth are substantial, at least for the length of abutment used in the experiments; for instance, the scour depth using a 15-degree wall angle was only 23 percent of the scour depth that developed for a 90-degree (vertical-wall) wing-wall abutment. The findings on wall angle presented here indicate the scour-reducing merit of (a) decreasing the bluffness of an abutmentâs upstream profile and (b) streamlining the down- stream profile to greatly weaken wake vortices. The findings do not necessarily imply that angling the approach of a wing- wall abutment produces the same extent of scour depth reduction, because the downstream side of an angled abut- ment may still produce strong wake vortices. Also, as pointed out by Dongol (1994), reducing the scour at one abutment by reducing its angle to the flow may aggravate scour at the opposite abutment on the river bank; the opposite abutment has an adverse angle to the approach flow. This concern, how- ever, applies to long abutments that substantially contract the flow at a bridge crossing. It is not a concern that affects short abutments, such as wing-wall abutments. 6.7 Influence of Abutment Alignment A brief further set of exploratory experiments examined the influence of abutment alignment on scour depth. These experiments, conducted for the present study but using a dif- ferent flume than that shown in Figure 6-1, are reported by Martinez (2003). The corollary question addressed by these experiments is whether scour depth is minimized or aggra- vated by aligning a bridge at some angle other than 90 degrees to a channel. The experiments were conducted with a thin wall replicating a simplified abutment. Figure 6-14 shows that the scour depth, dsA, increased as alignment angle increased from 15 to 90 degrees, and then the scour depth decreased as the angle further increased from 90 to 150 degrees. The variation of dsA with angle appears to be almost symmetrical for alignments upstream or downstream. For all angles, the deepest scour occurred at the end of the abutment. Dye observations from the present experiment indicate that the downflow and horseshoe vortices around the end of the abutment weakened as the abutment pointed upstream, as they also did when the abutment pointed down- stream. These flow features play major roles in scour, and weakening them is one way to minimize scour. 75 Table 6-3. Local scour depths at wing-wall abutment with geobag. Figure 6-10. Scour development at the unprotected wing-wall abutment. Test Layout dsA (mm) dsA/dsA45 (%) dsB (mm) dsB /dsA (%) W1 no geobag 65 100 0 0 W2 1 geobag row 0 0 93 143 W4 2 geobag rows 0 0 77 119 W5 2 geobag rows plus stone at edge 0 0 63 97
76 Figure 6-11. Experiment with a large geobag placed around the wing-wall abutment. (a) At start of experiment (b) At end of experiment, view from above (c) At end of experiment, view from the side Figure 6-12. Experiment with two rows of large geobags placed around the wing-wall abutment. (a) At start of experiment (b) At end of experiment, view from above (c) At end of experiment, view from the side
77 Figure 6-13. Influence of wall angle on scour depth at a wing-wall abutment. Table 6-4. Influence of wing-wall alignment on scour depth. Figure 6-14. Influence of abutment alignment on scour depth at a wing-wall abutment. α (degrees) dsA (mm) dsA /dsA0 (%) 90 140 100.00 65 75 53.57 45 65 46.43 30 46 32.86 15 32 22.86
78 6.8 Summary of Findings from Preliminary Experiments The results from the preliminary experiments led to the following findings in answer to the questions posed at the outset of this chapter. The findings are of significance for the more detailed sets of experiments that were conducted subsequently for the project: ⢠Large concrete blocks placed around an abutment are insufficiently effective as a scour countermeasure for reducing scour depth at an abutment. The winnowing of the bed material from around the blocks enables scour to progress, though possibly not as deep as may occur if the blocks were not present. Sediment winnowing, edge ero- sion, and local scour around the blocks are processes that need to be addressed in order for armoring to function as an effective scour countermeasure. ⢠Once a critical block size is attained (with respect to resistance to entrainment by flow), increasing block size does not result in reduced scour depth. Of greater impor- tance than block size is aerial coverage of blocks. Smaller concrete blocks closely arranged were more effective than the larger blocks because they caused less winnowing of sediment. ⢠A large geobag or a continuous mat of relatively small geobags holds promise of functioning as an effective scour countermeasure for wing-wall abutments when the mat extends over an area defined approximately as ra/La 1, where ra is radial distance out from the end of the abutment and La is abutment length. Edge erosion remains a concern because the geobag is thick. However, edge erosion likely can be reduced by use of riprap stone, or smaller geobags, placed around the geobags. ⢠The results obtained show that decreasing wall angle (from 90 degrees) to flow reduces the scour depth under either live-bed or clear-water scour conditions. Decreasing the wall angle at an abutment was observed to weaken down- flow and the horseshoe vortex. Accordingly, an approach- flow guide wall likely can be effective in reducing scour depth at a wing-wall abutment. ⢠The brief ancillary experiments on scour at various align- ments of abutment show that scour depth is a maximum when an abutment is perpendicular to the channel crossed.
