Slope Stability Analysis of Highway Embankments Using Strength Reduction Method in PLAXIS 3D

Abstract

Slope stability analysis is vital in geotechnical and highway engineering due to its direct implications on infrastructure safety and serviceability. This study employs the Strength Reduction Method (SRM) in PLAXIS 3D, a finite element-based software, to assess the stability of highway embankments under various slope geometries, embankment heights, and groundwater table (GWT) positions. A total of 72 cases were analysed, involving six slope ratios (1.25H:1V to 3.5H:1V), six embankment heights (1 m to 10 m), and two groundwater conditions (?4 m and 0 m). Results show that the Factor of Safety (FOS) decreases with increasing slope steepness and embankment height, while gentler slopes consistently exhibit higher FOS values. Rising the GWT to ground level reduced FOS values by up to 35%, highlighting the destabilizing role of elevated pore-water pressures. Comparative analysis revealed that gentler slopes improved FOS by up to 50% compared to steep slopes at higher embankments. The findings underscore the importance of adopting flatter slopes and effective drainage strategies in highway embankment design to ensure long-term stability.

Introduction

Overview

Slope stability is a critical factor in infrastructure projects like highways and embankments. Traditional 2D methods (e.g., Limit Equilibrium Method) are limited by simplified assumptions. This study uses PLAXIS 3D and the Strength Reduction Method (SRM) to analyze the effect of slope geometry, embankment height, and groundwater conditions on slope stability along the Ganga Expressway in India.

???? Methodology

• Study Area: Ganga Expressway, Uttar Pradesh, India

• Software: PLAXIS 3D (v2023.2), using Mohr–Coulomb model under drained conditions

• Simulations: 72 total, combining:

  • 6 slope ratios (1.25H:1V to 3.5H:1V)

  • 6 embankment heights (1 m to 10 m)

  • 2 groundwater levels (GWT = –4 m and GWT = 0 m)

• Soil Testing: Samples collected from 5 locations; lab-tested for geotechnical parameters (e.g., cohesion, friction angle, unit weight)

???? Geometric Modelling

• Only one side of the embankment (3-lane) modeled, assuming symmetry

• The third dimension (D) in the 3D model is 5× the embankment height to ensure stability and accurate simulation

???? Key Results

A. Effect of Slope Angle and Height (GWT = –4 m)

• FOS decreases as embankment height increases

• Gentler slopes show higher FOS

• Example: At 10 m height:

  • 1.25H:1V → FOS = 1.504

  • 3.5H:1V → FOS = 2.324

B. Effect of Slope and Height (GWT = 0 m)

• FOS values significantly lower due to water pressure

• Example: At 10 m height:

  • 1.25H:1V → FOS = 1.434

  • 3.5H:1V → FOS = 2.056

C. Impact of Groundwater Table Rise

• Raising GWT from –4 m to 0 m reduces FOS by 6–14%

• Higher embankments and gentler slopes showed greater reductions

D. Comparison Between Steepest and Gentlest Slopes

• Gentler slopes significantly increase stability

• FOS improvement ranged from:

  • ~20% at 1 m height

  • ~55% at 10 m height (for GWT = –4 m)

  • Slightly lower gains for GWT = 0 m

???? Discussion and Implications

• Slope geometry and groundwater conditions are critical to slope safety

• Gentler slopes mitigate instability risks, especially under high groundwater conditions

• Groundwater at the surface can cause up to 35% loss in FOS

• The use of 3D modelling (vs. traditional 2D) provides more realistic and accurate results

• Confirms earlier findings on slope instability due to height and water table rise

Conclusion

1) Gentler slopes (?3H:1V) provide significantly higher FOS values, particularly for tall embankments. 2) FOS decreases non-linearly with embankment height across all slope ratios. 3) Groundwater rise from ?4 m to 0 m reduces FOS by up to 35%, underscoring the need for proper drainage. 4) Percentage improvement from steepest to gentlest slopes reached 54% at ?4 m GWT. 5) The study highlights the necessity of adopting flatter slopes and considering groundwater in design for safe highway embankments.

References

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Copyright

Copyright © 2025 Vaibhav Pandey, Vinod Kumar Sonthwal. 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.