Effect of clay content on local scouring downstream of a submerged horizontal jet at maximum bed compaction

Document Type : Research Paper

Authors

• Mohsen Nasrabadi
• Aref Jabbari-Zahra
• Javad Mozaffari

Department of Water Science and Engineering, Faculty of Agriculture and Environment, Arak University, Arak, Iran.

Abstract

Keywords

• Local scouring
• Sedimentary bed compaction
• Submerged horizontal jet
• Clay content

Main Subjects

• سازه‌های آبی

Introduction

Previous studies have demonstrated that the application of clay in sediment mixtures may reduce scour downstream of hydraulic structures such as bridge piers and stilling ponds. In addition, the addition of clay to sediment mixtures increases the dry specific density of the soil, improves its compaction, and increases its mechanical strength, which is of great importance in the design of foundations, embankments, and geotechnical structures. However, unlike traditional compaction methods that examine clay and sand separately, the combined compaction approach for sand and sand-clay mixtures simultaneously increases the interpenetration and adhesion of particles, leading to improved resistance to deformation and erosion, which is particularly useful for hydraulic and geotechnical applications, but has not been addressed in any laboratory study to date. For this purpose, in this research, an attempt was first made to bring three non-cohesive sediments to the maximum dry specific gravity, then experiments were conducted by adding different clay contents to the sediment mixtures.

Methodology

Experimental tests were conducted in a laboratory flume (see Fig. 1), with a rectangular cross-section, with 9 m length, 0.8 m width and a depth of 0.6 m. Walls and bottom of the flume were made from Plexy-Glass and metal, respectively, and slope was set to zero. For modeling the scour process, a sedimentation basin was built with a depth of 11.6 cm, length of 1 m and a width equal to the flume width. two uniform sands (S1 and S2) and one non-uniform sand (S3) were used. According to ASTM-D6913, sieve analysis was conducted on the samples to obtain gradation curve of the sediments. In addition, the Hydrometer Analysis was done based on ASTM-D7928 for used clays as cohesive particles.

Results and Discussion

For sand S1 and Froude number 4, the scour hole dimensions decrease with increasing clay content. So that at bed compaction of 100% and for 5% clay, the scour depth decreased by 59% compared to the control experiment. Interestingly, no measurable scour was observed in the sedimentary bed at 100% density and for 10 and 15% clay contents. Also, for experiments with Froude number 6, the scour hole dimensions decrease with increasing clay contents. So that at bed compaction of 100% and for 5% clay, the scour depth decreased by 37% compared to the control experiment. For sand S2 with Froude number 4 and with increasing clay content, the hole dimensions did not change significantly. Also, for experiments with Froude number 6, the scour hole dimensions decrease with increasing clay percentage. So that at bed compaction of 100% and 5% clay, the scour depth has decreased by 20% compared to the control experiment. For sand S3 at bed compaction of 100% and Froude number 4, no measurable scour was observed in the sedimentary bed. For experiments with Froude number 6, the scour hole dimensions decrease with increasing clay content. So that at 100% density and 5% clay, the scour depth has decreased by 57% compared to the control experiment.

Conclusions

Within the experimental conditions of this study (sediments with a diameter of less than about 2 mm and a maximum Froude number of 6), the optimal clay ratio in the sediment mixture is between 10 and 15 percent, and with this ratio, bed protection is appropriately achieved. Of course, each type of cohesive sediment has its own behavior that should be considered in future research.

Therefore, clay, as a low-cost, effective and environmentally friendly approach, plays a key role in increasing bed resistance and reducing the dimensions of the scour hole downstream of hydraulic structures by increasing the adhesion and stabilization of the sedimentary bed. It can be used as an alternative or complement to traditional scour control methods.

Author Contributions

“Conceptualization, M.N. and J.M.; methodology, A.J.Z.; experiments, A.J.Z.; validation, M.N.; formal analysis, M.N.; investigation, J.M.; resources, A.J.Z.; data curation, M.N. and A.J.Z.; writing—original draft preparation, M.N.; writing—review and editing, M.N.; supervision, M.N.;

All authors have read and agreed to the published version of the manuscript.”

Data Availability Statement

Not applicable

Acknowledgements

The authors would like to thank all participants of the present study.

Ethical considerations

The authors avoided data fabrication, falsification, plagiarism, and misconduc

Conflict of interest

The author declares no conflict of interest.