Introduction
Local scour, the erosion of bed material around bridge foundations, is a leading cause of bridge failures globally, accounting for approximately 60% of such incidents in the United States. This phenomenon is driven by complex hydraulic interactions, primarily the formation of a powerful downflow at the pier's leading edge and a subsequent horseshoe vortex system at its base. Square piers, due to their sharp corners, induce severe flow separation and are particularly susceptible to deep scour, making them a critical case for evaluating protective countermeasures. While conventional methods like riprap and collars are widely used, they often face limitations such as instability under high-velocity flows, edge failure, and reduced effectiveness. To address these shortcomings, this study introduces and experimentally evaluates a novel bed-armoring solution: Interlocking 3D Porous Blocks (IPBs). These blocks are designed to offer superior stability through a mechanical interlocking mechanism while dissipating erosive energy within their porous structure, representing a potentially more resilient and sustainable approach to scour protection.
Methodology
The experimental investigation was conducted in a 6-m long, 0.6-m wide recirculating flume. A uniform sand with a median particle size (d₅₀) of 1.2 mm was used as the bed material. All experiments were performed under clear-water scour conditions, with the flow intensity (U/Uc) maintained at approximately 0.95. A square pier with a width (B) of 40 mm was used, ensuring a flume-width-to-pier-width ratio (W/B = 15) that eliminates sidewall effects. The flow depth (y) was kept constant at 0.15 m.
The novel countermeasure, IPBs, are cubic units (20 mm side length) with 25% porosity and a six-sided interlocking capability. The study was performed in two phases. First, the effects of 1 to 4 rows of IPBs and three vertical placement levels (on-bed, semi-buried, and fully buried) were examined. As the fully buried configuration yielded the most favorable results, a second phase was conducted to test two advanced five-row arrangements: a single buried layer and a double-stacked buried layer.
Each test ran for 12 hours to ensure equilibrium conditions. To confirm the reliability of the findings, a repeatability assessment was conducted on five representative cases, which showed that deviations between repeated tests (0–2 mm) were within the instrumental measurement uncertainty (±1 mm). Bed topography was mapped using a high-precision laser distance sensor. The primary performance metric was the percentage reduction in the maximum scour depth (dmax) compared to the control test.
Results and Discussion
The control test with an unprotected pier resulted in a maximum scour depth (ds max) of 6.8 cm (ds max/B = 1.7) at 12 hours. The performance of the IPBs was highly dependent on both the number of rows and the vertical placement level. Increasing the number of rows consistently improved scour reduction. For instance, with four rows, the on-bed, semi-buried, and fully buried placements achieved scour reductions of 72%, 78%, and 84%, respectively, demonstrating the clear superiority of the buried configuration.
The superior performance of the buried configuration is attributed to three synergistic mechanisms: (1) elimination of form drag by being flush with the bed, (2) internal energy dissipation of the downflow within the block's porous matrix, and (3) a filtration effect from the interlocking mat that prevents the "winnowing" of underlying sediment.
The most significant finding was achieved with the optimal five-row configurations. A single buried layer of 5 rows reduced scour by 91%. By advancing to a double-layer (stacked) configuration of 5 fully buried rows, no measurable scour was observed within the accuracy of the adopted measurement system (±1 mm). This level of performance is a substantial improvement over many conventional flow-altering countermeasures, which typically offer 40–80% reduction in laboratory settings. The mechanical interlocking of the IPBs provides a monolithic, flexible mattress that resists the shear and uplift forces responsible for the failure of loose armor like riprap.
Conclusion
This study demonstrates that Interlocking 3D Porous Blocks (IPBs) are a highly effective countermeasure for mitigating local scour around bridge piers. The vertical placement of the armor was identified as a critical design parameter, with the fully buried configuration providing significantly better protection. The optimal configuration, consisting of a double-layer of 5-row IPBs buried flush with the bed, resulted in no measurable scour under aggressive clear-water conditions. The success of the IPBs is attributed to their ability to dissipate turbulent energy internally, maintain a hydraulically smooth bed boundary, and provide superior structural stability through mechanical interlocking. These findings position IPBs as a robust, resilient, and highly promising solution for the sustainable protection of critical bridge infrastructure against scour-induced failure.