Suitable Method (Empirical Analytical and Numerical Method) to Evaluate the Requirements for the Elephant Feet, Temporary Invert and permanent Invert

Abstract

Introduction

The New Austrian Tunneling Method (NATM) emphasizes controlling deformation during tunnel construction to ensure stability. In soft ground conditions, specialized techniques such as Elephant’s foot, micro-piles, and temporary or permanent invert closures are employed during the excavation of the tunnel’s top heading to control deformation and provide support, preventing excessive settlement and inward squeezing of tunnel walls.

Elephant’s foot refers to a widened base in the support system that distributes loads effectively, while micro-piles are small, high-strength steel reinforcements drilled into the ground. Temporary and permanent invert closures at the tunnel bottom resist inward ground pressures, crucial for maintaining stability during excavation phases.

Although these techniques improve safety and effectiveness, they bring challenges including higher costs, longer project timelines, and technical complexity requiring specialized expertise. The thesis aims to evaluate the actual need for these supports in soft ground tunneling using empirical, analytical, and numerical methods.

The project includes a detailed tunnel cross-section design accommodating walkways, driving lanes, and ventilation, considering geological conditions such as unconsolidated to well-consolidated pebbly-boulder conglomerates and friable mudstone/claystone layers. Due to the complexity of bimrocks and bimsoils (rock-soil mixtures), standard rock mass classification methods are inadequate, and empirical relations are used to estimate strength parameters critical for design.

Three methods to determine support requirements are discussed:

• Empirical: Based on past project data and classification systems.

• Analytical: Theoretical stress and load calculations.

• Numerical: Advanced computer simulations for detailed ground behavior.

Combining these methods yields a reliable design for tunnel supports.

Empirical methods, notably Hoek’s guidelines, address challenges like squeezing ground conditions by recommending support strategies tailored to strain levels and tunnel size, reflecting regional tunneling practices developed in Europe.

Conclusion

The necessity and functionality of the elephant foot, temporary invert, and permanent invert in engineering practices depend significantly on the depth of ground cover and the associated geological conditions. The choice of method depends on several factors such as the complexity of the ground conditions, the availability of data, and the resources available for the project. Often, a combination of these methods is used to achieve a reliable and robust design. Empirical Method Suitable for preliminary design and in cases where quick decisions are needed. Analytical Method Useful for straightforward conditions and where detailed theoretical understanding is beneficial. Numerical Method Ideal for complex ground conditions and when detailed, accurate predictions are necessary. By combining these methods, engineers can develop a comprehensive understanding of the support requirements for a tunnel, ensuring safety and stability throughout the construction and operational phases.

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Copyright © 2025 Ajit Kumar. 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.