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37 90 in Figure 5.14. The maximum scour depth may happen either A around the pier or at some intermediate location; its location B For H/B, A>B>C was determined with the point gage system. C Scour Depth (mm) 60 5.11 PIER SPACING EFFECT ON MAXIMUM SCOUR DEPTH The pier spacing effect on the maximum pier scour depth can be incorporated in the general equation by using a correc- 30 tion factor, Ksp, equal to the ratio of the maximum scour depth of the line of piers over the maximum scour depth of the iso- lated pier. In this study, the single-pier case was the case of the single pier in the 1.5-m-wide flume. This case corresponds to 0 a C/B ratio of 4.69 as mentioned previously. Figure 5.15 shows 0 50 100 150 200 250 the correction factor. The difference of maximum scour depth Time (hr) between the single pier and a line of two piers was observed to be quite small, which gives some confidence in the use of the Figure 5.13. Comparison between scour in shallow C/B ratio of 4.69 as the single-pier case. and deep waters. In Figure 5.15, the pier spacing effect obtained in this study for a Porcelain clay is compared with existing recommenda- 5.10 PIER SPACING EFFECT: tions for cohesionless soils. Elliot and Baker (1985) used FLUME TEST RESULTS oblong piers (46 mm wide and 150 mm long) in their tests on groups of piers. Their pier spacing effect was more severe than The pier spacing effect refers to the interaction between the others, possibly due to the aspect ratio of the piers used. piers when they are closely spaced. In this case, the pier scour Salim and Jones (1998) conducted flume tests on cylindrical depth could be increased due to two reasons: (1) the inter- and square piers installed in a matrix in the middle of a chan- action and enhancement of horseshoe vortices at the base of nel. Each pier in this group configuration was more affected by the pier, or (2) the acceleration of the flow due to the contrac- other piers than in the case of a single line of piers. This may tion caused by the piers. The pier spacing effect can be exam- be the reason why Jones' correction factor is more severe than ined for two types of pier installation: (1) in a matrix and the one found in this study. The test conditions for Raudkivi's (2) in a line. The current study dealt with the effect of pier experiments (1991) are very similar to the current study, with spacing when cylindrical piers are uniformly spaced and the following exceptions: only two cylindrical piers were installed in a single row perpendicular to the flow. installed in the middle of the flume and the soil was sand. In For these flume tests, the piers were 0.160 m in diameter. Raudkivi's tests, the pier spacing effect was examined by The center-to-center distance, C, was called the pier spacing. varying the distance between the two piers. A distance equal to the space of one pier was kept between the Several attempts were made to find a prediction equation. outmost pier and the wall of the flume. Due to the flume width, First, the single equivalent pier concept proposed by Salim and the maximum number of piers that could be installed was four and the corresponding minimum spacing ratio was C/B = 1.88. The maximum pier spacing ratio was C/B = 4.69 for a single pier. Raudkivi (1991) commented that when the pier spacing 160 is larger than four, the group pier effect is negligible. The parameters and results for the four pier spacing tests Scour Depth (mm) are summarized in Table 5.4. The initial scour rate and max- 120 imum scour depth were calculated in the same way as the shallow water cases. The measured scour curves are plotted 80 TABLE 5.4 Parameters and results for pier spacing flume 1-pier, C/B=4.69 tests 2-pier, C/B=3.13 40 3-pier, C/B=2.34 Time Test H B V i z Zmax 4-pier, C/B=1.88 C/B Lasting No. (mm) (mm) (m/s) (mm/hr) (mm) (h) 0 Gr-1 375.00 160.00 0.33 (1-pier) 4.69 165.00 2.33 165.56 0 50 100 150 200 Gr-2 375.00 160.00 0.33 (2-pier) 3.13 122.50 2.83 175.44 Time(hr) Gr-3 375.00 160.00 0.33 (3-pier) 2.34 144.08 5.24 204.08 Figure 5.14. Scour depth versus time for the pier spacing Gr-4 375.00 160.00 0.33 (4-pier) 1.88 129.83 4.76 250.00 cases.