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39 5.13 PIER SHAPE EFFECT: 70 FLUME TEST RESULTS 60 The shape of a bridge pier can strongly affect the flow pat- tern around it. In this study, only rectangular piers were con- 50 Scour Depth (mm) sidered. Bridge piers are most often installed with the longer side parallel to the major flow direction; therefore, the length 40 over width ratio, L/B, is kept greater than one for all piers in this study. The rectangular pier was installed with a 0-degree 30 Circular attack angle in the middle of the soil tank. Major scour Square always occurred around the four corners of the rectangular 20 L/B=12 pier but only the time history of the maximum scour depth L/B=8 was used in the analysis. The shapes of the scour holes for 10 L/B=4 different rectangular piers were recorded and compared. In addition, cylindrical piers with a diameter equal to the width 0 of the rectangular pier were used as the reference case. Para- 0 50 100 150 meters and major results for the flume tests for pier shape Time(hr) effect are summarized in Table 5.5. Again, the maximum Figure 5.18. Scour depth versus time curves for pier scour depth and the initial scour rate were calculated in the shape effect tests. same way as in the case of the other flume tests. The scour depth development curves are plotted in Figure 5.18. 2 .0 5.14 PIER SHAPE EFFECT ON MAXIMUM H E C -1 8 , K s h = 1 .1 1 .6 SCOUR DEPTH fo r S q u a re N o s e The cylindrical pier test, SP-1, was chosen as the reference 1 .2 case. The correction factor, Ksp, is the ratio of the maximum Ksh scour depth for a given shape over the maximum scour depth 0 .8 for the cylinder (Figure 5.19). The results on Figure 5.19 indi- cate that there is no noticeable effect on scour depth due to the pier shape. Indeed, the correction factor varies from 1:1 to 1:12. 0 .4 This conclusion is consistent with the correction factor for sand listed in HEC-18. Therefore, it is concluded that a pier shape 0 .0 correction factor of 1.1 is a good approximation for the maxi- 0 5 10 15 L /B mum scour depth around rectangular piers in both clay and sand as long as the L/B ratio is larger than 1. The case of the L/B Figure 5.19. Correction factor for pier shape effect. ratio smaller than 1 was not covered in this research project. 6 5.15 PIER SHAPE EFFECT ON INITIAL SCOUR RATE 5 Initial Scour Rate (m m / h) The initial scour rate for the flume tests on the rectangular Rectangular 4 piers having the same width but different lengths are com- Circular pared in Figure 5.20. As can be seen, the rectangular piers 3 TABLE 5.5 Parameters and results for pier shape effect 2 flume tests Time Test H B V i z Zmax 1 L/B Lasting No. (mm) (mm) (m/s) (mm/hr) (mm) (h) Sp-1 375.00 61.00 0.33 Circular 151.92 1.45 68.03 0 Sp-2 375.00 61.00 0.33 1:1 129.50 5.00 73.53 0 5 10 15 Sp-3 375.00 61.00 0.33 4:1 124.42 2.05 72.99 L/B Sp-4 375.00 61.00 0.33 8:1 131.58 1.93 74.63 Figure 5.20. Initial scour rates for the shape effect flume Sp-5 375.00 61.00 0.33 12:1 131.50 1.84 75.19 tests.