Evaluation of Shear Strength Characteristics of Root-Reinforced and Non-Reinforced Soils Using Direct Shear Test

Abstract

Slope instability of mine overburden dumps is a major geotechnical concern, particularly in tropical regions subjected to intense rainfall and erosion. Vegetation-based soil reinforcement offers an eco-friendly and cost-effective solution for improving dump stability. This study experimentally evaluates the shear strength behaviour of root-reinforced and non-reinforced soils using the direct shear test. Clay– dominant soil collected from coal mine overburden dumps was reinforced with Acacia mangium roots at a controlled Root Area Ratio (RAR) of 3.58%. Direct shear tests were conducted under varying normal stresses (0.5, 1.0, and 1.5 kg/cm²). Results indicate a significant increase in shear strength for root-reinforced soil compared to unreinforced soil. Cohesion showed substantial improvement, whereas changes in the internal friction angle were marginal. The findings confirm that root reinforcement primarily enhances apparent cohesion through tensile resistance and soil–root interlocking. The study demonstrates the effectiveness of Acacia mangium as a bioengineering solution for shallow slope stabilization in mine reclamation projects.

Introduction

This study investigates vegetation-based reinforcement as a sustainable method to stabilize open-cast mine overburden dumps, which are prone to failure due to rainfall infiltration, erosion, and weak mechanical properties. Conventional stabilization methods (retaining walls, geosynthetics) are expensive and environmentally intrusive, whereas plant roots offer a cost-effective, eco-friendly alternative.

Key Points:

1. Vegetation-Based Soil Reinforcement:

   • Roots enhance shear strength, apparent cohesion, and soil structure, providing natural tensile resistance.

   • Acacia mangium is particularly effective due to rapid growth, deep roots, and adaptability to degraded soils.

2. Literature Insights:

   • Bio-engineering improves dump stability and reduces pore water pressure through evapotranspiration.

   • Studies report shear strength improvements up to 413% for Leucaena leucocephala and root tensile strength up to 75 MPa for Vetiver grass.

   • Species selection is critical: survival rate, root diameter, biomass, and architecture strongly influence stabilization.

   • Miyawaki plantation and vegetation growth increase factor of safety (from 1.2 to 1.4).

   • Shear strength also depends on soil moisture and matric suction, highlighting the importance of environmental conditions.

3. Experimental Method:

   • Soil from Singareni Collieries (sandy clay) was used.

   • Roots of Acacia mangium (6–12 months) were incorporated at a Root Area Ratio (RAR) of 3.58%.

   • Direct shear tests were conducted on non-reinforced and root-reinforced soils under normal stresses of 0.5, 1.0, and 1.5 kg/cm².

4. Results:

   • Root-reinforced soils exhibited higher peak shear stress than non-reinforced soils at all stress levels.

   • Cohesion increased significantly due to root tensile resistance and improved soil–root bonding, confirming that roots act like natural fibers reinforcing the soil.

Conclusion

The experimental investigation confirms that Acacia mangium roots significantly enhance the shear strength of mine overburden soils. The major findings are: 1) Root-reinforced soil exhibits substantially higher shear strength than non-reinforced soil. 2) Cohesion increases considerably due to root tensile resistance and soil–root interlocking. 3) Internal friction angle remains largely unaffected by root presence. 4) Root reinforcement is most effective under low to moderate normal stresses, typical of shallow slopes. The study supports the application of vegetation-based bioengineering techniques for stabilizing mine dump slopes and promotes sustainable mine reclamation practices.

References

[1] A Chaturvedi, Sanjay k. Sharma. S. P. Singh, stabilization of mine waste dump through bio-engineering, July 2023, IIT BHU VARANASI. [2] Wu, T.H., McKinnell, W.P., & Swanston, D.N. (1979). Strength of tree roots and landslides on Prince of Wales Island, Alaska. Canadian Geotechnical Journal, 16(1), 19–33. [3] Bischetti, G.B., et al. (2005). Root strength and root area ratio of forest species in Lombardy (Italy). Plant and Soil, 278, 11–22. [4] Docker, B.B., & Hubble, T.C.T. (2008). Quantifying root-reinforcement of river bank soils. Geomorphology, 100, 401–417. [5] Mao, Z., et al. (2023). Mechanical reinforcement of soil by plant roots. Engineering Geology, 315, 106987.

Copyright

Copyright © 2026 Amma Sreedhar, Sharma K. V., Naik M. T.. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.