Experimental investigation of granular bulk density effect on bridge local scour under clear-water conditions

The effect of granular bulk density on local scour around an oblong pier under clear-water conditions is studied. A total of four open channel flume tests were conducted, in which the sediment beds were prepared at different relative states of compaction. Uniform Ottawa sand was used for all cases. Flow conditions, i.e., flow velocity and depth, were maintained the same for all cases to isolate the bulk density effect. Time evolution of the maximum scour depth and the scoured bed morphology were compared among all cases. For tests on sediment beds of the lowest and highest density, three miniature pressure transducers were installed at various depths within the bed upstream of pier nose to monitor the pore pressure response. Experimental results indicate that the erosive resistance of the sand bed is a slightly decreasing function of bulk density. The possible underlying mechanism, responsible for such different scour behaviors between test cases, is also discussed.