What is the NATM (New Austrian Tunneling Method)?The new Austrian tunneling method process
What is the NATM (New Austrian Tunneling Method)?The New Austrian Tunneling Method (NATM), also known as the Sequential Excavation Method (SEM), is a tunneling technique developed in the 1960s by Austrian engineers. It is based on the principle of using the surrounding rock or soil as a natural load-bearing element to stabilize the tunnel during and after excavation. This method revolutionized the way tunnels are constructed, focusing on the interaction between the rock and the tunnel support. Unlike traditional tunneling methods that rely heavily on pre-designed support systems, NATM emphasizes the monitoring of the rock’s behavior and adjusting support measures accordingly.
Key Principles of NATM:
1. Utilization of In-Situ Stress: NATM capitalizes on the inherent strength of the surrounding rock or soil. By allowing controlled deformation, the method transforms the rock mass into a natural supporting structure.
2. Sequential Excavation: Tunneling is done in small sections, with each section supported as soon as it is excavated. This prevents large-scale destabilization and allows the tunnel to be gradually built up.
3. Flexible Support Systems: Unlike traditional methods that use rigid supports, NATM employs flexible and often temporary supports such as shotcrete (sprayed concrete), rock bolts, and steel ribs. These are adjusted as needed based on the rock’s behavior.
4. Monitoring and Adjustment: Continuous monitoring of the tunnel and surrounding ground is a fundamental part of NATM. Instruments are used to measure deformations, stresses, and other parameters. Based on this data, the support system can be modified in real-time to adapt to actual conditions.
5. Economic Efficiency: By utilizing the natural strength of the ground and adapting the support to the actual needs, NATM can be more cost-effective than methods that require extensive and often unnecessary reinforcement.
The new Austrian tunneling method process
1. Initial Ground Assessment:
- Geological Survey: A thorough geological survey is conducted to assess the rock or soil conditions, including strength, stability, and potential hazards.
- Classification: The ground is classified based on its behavior and support requirements. This helps in determining the necessary support systems and excavation methods.
2. Controlled Excavation:
- Sequential Excavation: Unlike traditional methods where large sections are excavated at once, NATM emphasizes the sequential excavation of smaller sections. This reduces the disturbance to the surrounding rock mass.
- Monitoring: Continuous monitoring of the ground’s behavior is essential during excavation. This allows for adjustments in the support system if unexpected conditions are encountered.
3. Primary Support Installation:
- Shotcrete Application: Immediately after excavation, a layer of shotcrete (sprayed concrete) is applied to stabilize the exposed rock or soil. This is a critical component of NATM, providing initial support.
- Rock Bolts: Rock bolts or anchors are installed to reinforce the shotcrete layer and hold the rock mass in place.
- Steel Ribs: In areas where additional support is required, steel ribs or lattice girders may be installed. These work in conjunction with the shotcrete to stabilize the tunnel.
4. Continuous Monitoring and Adjustment:
- Convergence Monitoring: Instruments are installed to monitor the deformation or convergence of the tunnel. This data is used to assess the effectiveness of the support system.
- Adjustment: Based on the monitoring data, the support system is adjusted as needed. This could involve adding more shotcrete, installing additional bolts, or modifying the excavation sequence.
5. Final Lining:
- Concrete Lining: After the primary support has stabilized the tunnel, a final concrete lining is installed. This provides a permanent structure that can withstand long-term loads.
- Waterproofing: Waterproofing membranes are often placed between the shotcrete and the final lining to prevent water ingress.
6. Drainage and Ventilation:
- Drainage Systems: To manage groundwater, drainage systems are installed behind the final lining. This helps in preventing water pressure buildup and potential leaks.
- Ventilation: Proper ventilation systems are designed and installed to ensure a safe working environment during construction and operation.
7. Final Works and Surface Restoration:
- Surface Restoration: If the tunnel has a surface entry, the surrounding area is restored to its original or an improved condition, ensuring minimal environmental impact.
- Operational Readiness: Final inspections are conducted to ensure that the tunnel meets all safety and design standards before it is opened for use.
Advantages of NATM
- Cost-Effective: By utilizing the natural strength of the rock and adjusting support measures, NATM can significantly reduce the cost of tunnel construction.
- Flexible: The ability to adapt to changing geological conditions makes NATM suitable for a wide range of tunneling projects, from soft soils to hard rock.
- Safer: Continuous monitoring and adaptation to the tunnel’s conditions can help prevent accidents and improve overall safety.
Applications
NATM is widely used in various tunneling projects, including road tunnels, railway tunnels, and underground facilities. Its adaptability and efficiency have made it a preferred method for many engineers and construction companies around the world.
NATM is widely used in tunnel construction projects around the world, particularly in areas with complex and variable geology. It combines engineering principles with on-site adaptability, making it a versatile and effective tunneling method.