Tunnel Construction Methods: TBM vs. NATM

A. INTRODUCTION

Tunnels significantly contribute to the development, connectivity, and functionality of modern societies, providing critical infrastructure for transportation and utilities and enhancing economic growth and quality of life for people globally. Because of the tunnels' economic significance, construction costs, and design risks, the evaluation and selection of the best tunnel design and construction techniques are important. From safety and serviceability to construction costs, geological conditions, and construction efficiency, there are many parameters that need to be carefully evaluated by experienced tunneling professionals, engineers, and geotechnical experts when it comes to the selection of the most suitable tunnel construction method.

This article focuses on the analysis of two major tunnel construction methods – the TBM (Tunnel Boring Machine) and the NATM (New Austrian Tunneling Method), exploring the advantages, disadvantages, and limitations of each method, as well as how these are implemented in our DeepEX – Shoring & Tunnel Design Software.

B.1. TBM TUNNELS

The Tunnel Boring Machine (TBM) method is a common method for the construction of tunnels in soft soils in urban areas. It involves the use of specialized machines called tunnel boring machines, or TBMs, to excavate the tunnel. The TBM is launched from a starting point, typically referred to as the "launching shaft." The cutting head of the TBM excavates the soil or rock as it advances forward. As the TBM advances, temporary support measures, such as steel ribs or shotcrete (sprayed concrete), may be installed to stabilize the tunnel face until permanent lining is installed. Once the TBM has passed a section, permanent lining materials, such as concrete segments, are installed to provide structural stability to the tunnel.

Advantages of TBM Tunnels:

1. Speed and efficiency: TBMs are suitable for large-scale tunneling projects because of their rapid excavation rates ability. A tunnel boring machine can bore through various soil types, including rock, soil, and mixed-face conditions.

2. Precision: TBMs create smooth tunnel walls, which can reduce the need for additional lining materials. They also ensure precise alignment and reduce the risk of deviations.

3. Reduced surface disruption: TBMs minimize surface disruptions, noise, and vibrations, making them suitable for urban areas and sensitive environments.

4. Safety: TBMs offer a safer working environment for construction workers as they minimize the risk of ground collapse and exposure to hazardous conditions.

Disadvantages of TBM Tunnels:

1. Limitations in challenging ground conditions: While TBMs can be the ideal solution for softer soil layers (sands, gravel, silt, clays, and more), they may face challenges in highly fractured rock, water-bearing strata, and other complex ground conditions.

2. High acquiring, operating, and maintenance costs: TBMs are expensive pieces of machinery requiring specialized equipment, personnel, and ongoing maintenance to ensure their effectiveness.

3. Tunnel Size: TBMs are suitable for larger-diameter tunnels but may not be suitable for small tunnels or irregular tunnel shapes. The accessibility to TBM tunnels for maintenance and repairs can be more challenging.

TBM Tunnels Analysis in DeepEX Software

DeepEX software has the capability to generate all the TBM tunnels construction stages:

Stage 0: The green field conditions of the site prior to the tunnel construction initiation.

Stage 1: The cutter head section passes from the studied tunnel section.

Stage 2: The combination of soil loss at the tunnel face and along the shield is directly prescribed as a percentage area loss through the contraction option.

Stage 3: The grouting of the gaps left by the retraction of the shield tail. The grout pressure is simulated as a surface load applied in the internal walls of the excavated soil area.

Stage 4: The state of the tunnel after the installation of the final lining.

The software can fully design and analyze TBM tunnels both with the Finite Element method and with the soil springs (non-linear analysis) approach. DeepEX can design any tunnel lining (concrete, steel plates, corrugated plates, and more), including segmental concrete linings, perform all structural checks TBM checks, calculate joint stresses, design bolts and gaskets, as well as perform fire design according to Eurocode 2 specifications.

B.2. NATM TUNNELS

The New Austrian Tunneling Method (NATM), also known as the Sequential Excavation Method (SEM), is a tunnel construction method that is popular for its adaptability to varying geological conditions. Developed in the 1960s in Austria, the NATM approach revolutionized tunneling by introducing a flexible and systematic process that responds to the encountered ground conditions.

NATM involves a comprehensive assessment of the geological conditions through core drilling, testing, and monitoring. The ground is classified into different types based on strength, stability, and deformation characteristics. In NATM, initial support is provided by applying temporary support elements such as rock bolts, wire mesh, or shotcrete immediately after excavation. This initial support helps stabilize the tunnel face and prevent ground movements. Instrumentation and measurement techniques are used to assess deformations, stress changes, and groundwater conditions. According to the measurements, additional steps can be taken for the application of additional support measures.

Advantages of NATM Tunnels:

1. Adaptability: NATM allows for flexibility during construction as it can be adjusted to the encountered geological conditions. This method is particularly useful in complex ground conditions with variable rock quality.

2. Reduced environmental impact: NATM minimizes surface disruption during construction, leading to lower environmental impacts compared to other methods.

3. Cost-effectiveness: NATM can be cost-effective for smaller tunnel projects or projects with uncertain ground conditions, as it reduces the need for specialized equipment and resources.

4. Accessibility: NATM tunnels are relatively more accessible for maintenance and repairs due to their smaller size and construction.

Disadvantages of NATM Tunnels:

1. Longer construction duration: NATM tunnels generally have a longer construction duration compared to TBMs since the excavation is typically done in smaller sections, requiring more time for construction and support installation.

2. Engineering expertise: NATM tunnel construction techniques require experienced engineers and skilled workers who can accurately assess ground conditions and make informed decisions regarding the support systems.

3. Ground Stability Challenges: NATM relies on proper ground support systems to ensure stability during and after construction. If the ground conditions are not properly assessed or managed, there is a risk of ground deformation and instability.

4. Surface Settlement: NATM construction can lead to larger surface settlements compared to other methods, especially in urban areas, if not properly managed.

NATM Tunnels Analysis in DeepEX Software

DeepEX software has the capability to generate all the NATM tunnels construction stages:

Stage 0: The green field conditions of the site prior to the tunnel construction initiation.

Stage 1: Excavation of the upper part of the top heading takes place. A support core is left intact. 3D arching effects are considered through partial deactivation of the soil region.

Stage 2: The tunnel crown is supported by a shotcrete lining. The support core is excavated. 3D arching effects are considered through partial deactivation of the soil region.

Stage 3: A temporary lining is constructed at the bottom of the so-far excavated section. The specific stage corresponds further away from the excavating face where true plain strain conditions are valid.

Stage 4: Excavation of the bench part of the tunnel section. 3D arching effects are considered through partial deactivation of the soil region.

Stage 5: Excavation of the inverted part of the tunnel section. 3D arching effects are considered through partial deactivation of the soil region.

Stage 6: Construction of the remaining lining sections (left, right wall, and invert). The specific stage corresponds further away from the bench excavation location where true plain strain conditions are valid.

The software can fully design and analyze NATM tunnels with the Finite Element method, calculating the tunnel lining moment, shear and displacement diagrams, the rock bolt reactions, and more.

C. CONCLUSION

It is important to note that the selection between TBM and NATM methods depends on various factors, including the geological conditions, project requirements, budget, and expertise available. In some cases, a combination of both methods may be used, such as using a TBM for main tunnels and NATM for cross-passages or smaller sections. No matter which method you wish to use, DeepEX is the ultimate software for the design and analysis of tunnel systems. From metro stations and tunnel section design and optimization to whole metro systems design – transportation analysis, cost-benefit analysis, city buildings damages and repair costs, and more – DeepEX can assist you and help you gain a competitive advantage!

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