Laboratory Investigation of the Effect of Plastic Clay (Ball Clay) on Reducing Local Scour Around Bridge Piers

Document Type : Research Paper

Authors

• Mahdi Majedi Asl ¹
• mohammad ghorbanzadeh ²
• Mohammad Hossein Jahanpeyma ³
• mehdi Rezaie ²

¹ Department of Civil Engineering, Faculty of Engineering, University of Maragheh, Maragheh, Iran.

² Department of Civil Engineering, Faculty of Engineering, University of Maragheh, Maragheh, Iran

³ Department of Civil Engineering, Faculty of Engineering, University of Maragheh, Maragheh Iran.

Abstract

Keywords

• Hydraulic Structures
• Local Scour
• Metashape
• Sand Bed

Main Subjects

• سازه‌های آبی

Introduction

Local scour around bridge piers is a critical factor threatening the stability of hydraulic structures in riverbeds. It occurs due to flow concentration and increased bed shear stress around the pier, leading to erosion and potential structural failure. Understanding and controlling local scour is thus essential in river engineering and pier design. Various methods have been proposed to mitigate scour, including modifying pier geometry, using protective structures such as riprap and flow guide plates, and enhancing bed properties with mineral or chemical additives. Among these, natural and environmentally friendly materials have attracted attention as effective and cost-efficient solutions. Ballclay powder, a natural material with high cohesion and low permeability, can increase bed resistance and reduce the formation of deep scour holes around piers. The present study aims to experimentally investigate the effect of different percentages of Ballclay powder on local scour reduction around a cylindrical pier in a sandy bed. Laboratory-scale experiments were conducted under various flow discharges to assess the bed response to different mixtures. Scour depths were measured over time, and equilibrium conditions were analyzed at 210 minutes. The findings provide practical insights for controlling local scour and enhancing the safety and stability of hydraulic structures, highlighting the potential of Ballclay powder as an environmentally friendly and effective countermeasure.

Method

Laboratory experiments were conducted to investigate the effect of Ballclay powder on local scour around a cylindrical pier. The experiments were performed in a flume 13 m long, 1.2 m wide, and 0.8 m high. Flow was supplied by a centrifugal pump and discharge was controlled using an ultrasonic flowmeter with ±0.1 L/s accuracy. Transparent Plexiglas sidewalls allowed visual observation of scour development. A 3-m-long sandy bed with a thickness of approximately 20 cm was prepared at the center of the flume using natural sand with a median particle size of d₅₀ = 0.78 mm. A PVC cylindrical pier with an external diameter of 9 cm was installed at the center of the sediment layer. The pier diameter and position were selected to minimize flow interference with the flume walls and to maintain geometric similarity. Ballclay powder was mixed with the dry sand at weight fractions of 5%, 10%, and 15%, resulting in four test conditions: pure sand (control) and sand with 5%, 10%, and 15% Ballclay. For each condition, experiments were conducted under five discharges: 10, 15, 20, 25, and 30 L/s, with each test repeated independently. Scour depth was measured manually at the center of the hole using a graduated ruler, and equilibrium conditions were determined at 210 minutes. High-resolution photogrammetry was employed to capture the final bed geometry around the pier. Sequential overlapping images were processed in Agisoft Metashape to reconstruct accurate 3D models of the scour hole, allowing detailed analysis of scour depth, sediment distribution, and comparison across different experimental conditions.

Results

The longitudinal profiles of local scour depth in the clear-sand case indicate that increasing flow discharge significantly amplifies the scour hole around the cylindrical pier. This trend is attributed to higher flow energy and shear stress acting on the sandy bed, causing particle displacement and erosion. Particularly at higher discharges (25 and 30 L/s), the profiles show substantial scour depth, reflecting severe erosion under these conditions. The transverse scour profiles exhibit a symmetric distribution, with the maximum depth occurring at the pier center along the flow axis and gradually decreasing toward the sides. This pattern highlights the formation of horseshoe vortices and concentrated flow around the pier, leading to intense central erosion and lateral reduction. Higher discharges also broaden the scour width, indicating increased lateral as well as vertical erosion potential. The addition of Ballclay significantly reduced scour depth across all discharges. At 10 L/s, 5% Ballclay reduced scour depth by approximately 42%, while 10% and 15% completely prevented scour formation. The effectiveness of Ballclay is attributed to enhanced mechanical properties of the sediment bed; particle cohesion and reduced permeability limit particle displacement. At higher discharges (15–30 L/s), Ballclay still mitigated scour, although lower percentages (5% and 10%) showed reduced efficiency under stronger flows. For instance, at 15 L/s, the scour reduction reached 54%, 100%, and 100% for 5%, 10%, and 15% Ballclay, respectively, and similar trends were observed at higher discharges. Transverse profiles confirm that increasing Ballclay content decreases both the depth and lateral extent of the scour hole. At 15%, the bed remained almost flat, demonstrating full stabilization against erosion. Furthermore, downstream sediment accumulation decreased with Ballclay addition, reflecting reduced sediment detachment and transport. Short-range photogrammetry was employed to reconstruct 3D scour models ,and the manually measured longitudinal profile was compared with the 3D model for 15% Ballclay at 30 L/s. The results show excellent agreement, validating the 3D reconstruction method as a reliable tool for detailed analysis of scour patterns.

Conclusions

In this study, the effect of adding Ballclay to a sandy bed on the reduction of local scour around a cylindrical pier was investigated under laboratory conditions at different flow discharges. The results clearly demonstrated that the mixture of sand with Ballclay significantly enhanced bed stability and effectively prevented the development of scour holes. The addition of 5% Ballclay reduced the average scour depth by approximately 40% across all discharges, while 10% and especially 15% Ballclay achieved much higher reductions, up to 98%, even completely eliminating scour at low discharges of 10 and 15 L/s. This improvement is mainly attributed to increased cohesion, reduced permeability, and enhanced shear strength of the sandy bed due to the fine and plastic properties of Ballclay. Temporal analysis of scour development showed that, for both the clean sand and the 5% mixture, the scour depth reached a quasi-steady state after approximately 210 minutes, which was therefore considered as the equilibrium time for all experiments. Additionally, the formation of downstream sediment mounds, resulting from the displacement of eroded particles, was significantly reduced in the presence of Ballclay. This indicates that Ballclay not only mitigates scour depth but also prevents sediment redistribution along the flow path. Overall, the use of Ballclay as a simple, cost-effective, and environmentally friendly additive represents an efficient approach for controlling local scour around bridge piers in sandy beds. Further studies under unsteady flow conditions, long-term durability, and field implementation may provide valuable insights for practical protection of hydraulic structures.

Funding

Research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Authorship contribution

Mahdi Majedi Asl: Conceptualization, supervision, writing – review and editing.

Mohammad Ghorbanzadeh: Methodology, experimental investigation, data analysis, writing – original draft.

Mohammad Hossein Jahan Peyma: Data processing, results analysis, interpretation of findings.

Mahdi Rezaei: Three-dimensional reconstruction, visualization, preparation of figures and tables, data curation.

Declaration of Generative AI and AI-assisted technologies in the writing process

During the preparation of this work the author(s) used [Chat Gpt] in order to [Text Editing]. After using this tool/service, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the publication.

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Data availability statement

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgements

The authors would like to express their sincere appreciation to the University of Maragheh for providing laboratory facilities and scientific support for this research, and to Adak Tajhiz Iranian Company for their cooperation and support.

Ethical considerations

This study did not involve human participants or animals and therefore did not require ethical approval. The authors confirm that all ethical standards of research, including avoidance of data fabrication, falsification, and plagiarism, were fully observed.

Conflict of interest

The authors declare that they have no conflict of interest.