To improve the local scour protection of the pier using solid sacrificial piles, a kind of permeable sacrificial pile filled with stones is put forward in this study. Four influencing factors, including the size of the filling gravel of the permeable sacrificial pile, the distance between the pile and pier, the diameter ratio D1/D (D is the diameter of the pier and D1 is the diameter of the pile.), and the submergence rate of the pile, are studied in clear water condition with indoor tests and numerical simulations. The test results show that it has the best reduction effect on the local scour of the pier when the filled gravel size is 0.2–0.25 D, the distance between the pile and pier is 3.0 D, the diameter ratio is 1.0, and the submergence rate is 0.8. Results of the numerical simulation show that the permeable sacrificial pile has a significant weakening effect on the downflow in front of the pile and the flow velocity around the pile. In the same conditions, the reduction effects of the permeable sacrificial pile and the solid sacrificial pile on the pier are similar, but the local scour around the permeable sacrificial pile is less than that around the solid one. The pressure difference outside and inside the permeable pile causes the water to flow in the direction perpendicular to the isobars. This significantly impacts the flow velocity, the vortex system, and the shear stress around the permeable sacrificial pile and pier, which leads to a huge decrease in the local scour depths. The maximum shear stress on both sides of the permeable sacrificial pile and the pier is about 1/2 and 1/4 of those around the solid sacrificial pile respectively. And the area with lower velocity and shear stress behind the permeable sacrificial pile is larger than the area behind the solid ones.

To address the shortcomings of the solid collar, mainly the potential risk of deformation and damage due to its weight after overhanging above the river bed caused by local scour, a lighter form of a local scour reduction measure, the permeable collar, was proposed. The aim was to reduce the local scour depth around cylindrical piers, and a total of 20 tests were conducted in this study. First, the influence of the porosity of the permeable collar on the local scour reduction efficiency was studied under the condition of the constant size of the installation height, the diameter, and the thickness of the permeable collar based on indoor model experiments. Then, under the condition of optimal porosity, the influence of the installation height, the diameter, and the thickness of the permeable collar on reduction efficiency were studied in order. The results show that the porosity of the collar has a great influence on the local scour reduction. The reduction efficiency increases first and then decreases with the increase in collar porosity. When the collar porosity is 50%, the reduction effect is the best, and can reach up to 78.1%. The reduction efficiency decreases with the increase in installation height of the collar, and the reduction effect is the best when the collar is placed on the surface of the riverbed, which is close to the reduction efficiency when using a solid collar placed on the surface of the riverbed. The reduction efficiency increases with the increase in collar diameter, which increases rapidly when the diameters are less than 3.5 D (D is the diameter of bridge piers) and then slows down until tending to 100%. The reduction efficiency decreases rapidly with the increase in the collar thickness, when the thickness of the collar is 0.15 D, the reduction effect can reach up to 89.6% in this study. The results can give a perspective on the reduction of local scour depths around bridge piers.

Local scour occurring near bridge piers has become a major problem all over the world, which has caused countless bridge damage events. Explorations regarding local scour reduction measurements have become a research hotspot in the field. Much effective research has been conducted on scour reduction for single piers. However, studies on local scour reduction around multiple piers that are arranged in tandem have rarely been reported. Therefore, the effect of the span and the local scour reduction measurement (collar) on the characteristics of the local scouring behavior around two piers arranged in tandem are explored in this research, with numerical simulations in clear-water conditions. The results show that the local scour depth of the downstream pier increases gradually with an increase in the pier spacing, due to the weakened sheltering effect of the upstream pier. The local scouring of both the upstream and downstream piers can be reduced if the upstream pier is protected by a collar. The local scour reduction efficiency of the upstream pier can reach 52~55%. The local scour reduction efficiency of the downstream pier decreases rapidly from 84.3% to 8.3% with an increase in the pier spacing. If the pier spacing, G, is greater than 4.0D (D is the diameter of the pier), the local scour depth around the downstream pier is larger than that around the upstream pier. Therefore, if the local scour depth of the upstream pier is considered safe and acceptable, it is used as the reduction target of the downstream local scour depth. A collar must be adapted for use around the downstream pier when G/D > 4.0. If both the piers are protected by collars of the same size (W = 3.0D), the local scour reduction efficiency of the downstream pier is about 15% more than that of the upstream pier. The local scour depth around the downstream pier is 64.5% of that around the upstream pier. Therefore, the size of the collar around the downstream pier can be decreased to save costs. The local scour reduction efficiency for the downstream pier reduces from 66.7% to 39.8% when the downstream collar size (W) decreases from 3.0D to 2.0D. To ensure that the local scour depth around the downstream pier is no greater than that of the upstream pier, the downstream collar should be larger than 2.25D. These results can serve as a reference for the local scour reduction of two piers arranged in tandem.