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Nowadays, extraction of materials from river bed is one of the effective factors in the occurrence of scouring phenomena around bridge piers. The extent of scouring around the bridges depends on a number of factors, including the type of foundation, the Froude number, flow rates and bed granulation. In , in vitro evaluation of scouring around piers groups in two different grains was investigated. The effect of scouring and armoring of scours in scouring control was investigated in both natural and river conditions. In the present study, 44 experiments with identical laboratory conditions were tested in a rectangular channel with of 13 m, width of 1.2 m and depth of 0.8 m. In this study, 44 experiments were conducted in two models 1 (control or simple groups) and model 2 (modified groups), both scaling behavior. B = 0.7) and Bꞌꞌ (= 1.7 mm) were analyzed. In addition, to investigate the effect of material extraction on the scour rate of bridge pedestals, experiments were conducted for both pit-bed (pit-extraction mode) and pit-free bed (river natural state). A removable sand bed with a height of 22 cm was placed between the floorboards. Two series of pedestals were located upstream and downstream of the bed with a clear distance from the flooring. The pedestals were arranged in the same arrangement (consecutive tripods in the direction of flow and in the center of the channel width) with center to center 21 cm apart.
  The results showed that the armed (rough) pier group was less than the simple pier group (flat surface), due to the excavation of river materials, it increased the scour equilibrium time. The armed pier group also controlled the impact of extraction of materials on the scour rate. By examining the upstream and downstream materials mining, it was observed that extraction of materials of upstream of pier group decreased and extraction of materials of downstream pier group increased the scour. Therefore, it can be concluded that the scouring of pier is more sensitive to the downstream pit hole and even the mining of materials from upstream of the pier should be possible. In other words, the percentage of scour depth reduction in downstream bed due to extraction of materials and percentage of scour depth in upstream bed of extraction of materials in model with armed pier group were higher and lower than model with simple pier group, respectively. The percentage of scour reduction due to armed of the pier was also studied and it was observed that at best, the maximum scour depth in the aggregate with mean diameters of 0.78 mm and 1.7 mm by armed pier group were 55 and 66 percent, respectively. It is also observed that the extraction of materials from the bed increases the equilibrium time. It should be noted that scour is balanced when the driving force is approximately equal to the resistive force (particle saturation weight). The results show that the presence of the pit leads to an increase in the scour equilibrium time. In other words, when the sediments reach the pit site, due to the decrease in water velocity and increasing depth, the flow cannot carry the larger sediments and the sediments settle in the upper wall of the pit.

Keywords: scouring, river bed, extraction of materials, grouped piers, armored pier, graded bed material.

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