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The New Austrian Tunneling Method (NATM), also known as the sequential excavation method, gained recognition during the tunneling operations in the Austrian Alps. The technique proved to be very useful in sophisticated and diversified geological conditions where forecasting of the rock mass is difficult due to rapidly changing geology.
NATM is not just a construction method for performing tunneling operations, but also an approach towards a streamlined sequence of the necessary construction activities with security, safety, and economy being the pivotal elements. Versatility in the operation and construction works is the primary benefit of NATM technology as it can be used for hard rock, soft rock, and blended ground conditions.
Before applying NATM to any construction work, it is of utmost importance to understand the geology and geotechnical conditions of the ground. Thereafter, various design parameters are developed for smooth tunneling operations using NATM. The characteristic features, design criteria, and the general construction sequence of tunneling using NATM have been discussed in this article.
- 1. Characteristic Features and Philosophy of NATM
- 2. Design Criteria and Features of NATM
- 3. General Sequence of Tunnel Construction Using NATM
1. Characteristic Features and Philosophy of NATM
NATM is more of a philosophy than a construction method and is based on certain principles mentioned below:
- The surrounding rock mass is the primary load-bearing structure. Therefore, its carrying capacity should be maintained without the disruption of the rock mass.
- The fundamental strength of the soil or rock around the tunnel domain should be maintained and mobilized to the optimum possible degree.
- The mobilization can be accomplished by regulating the deformation of the ground. However, extreme deformation can lead to loss of strength and high surface area settlement. Such conditions should be prevented.
- A preliminary support group should be provided just after the excavation within the stand-up time. Generally, the preliminary support group includes rock bolts and thin semi-flexible sprayed shotcrete lining. Permanent supports are provided at a later stage.
- The shotcrete lining should be thin-walled. If extra support is needed, then mesh support, tunnel ribs, and anchors should be used instead of thickening the shotcrete lining.
- The length of the unsupported period should be as brief as possible.
- The deformation of support and ground must be monitored after the installation of primary support.
- Those who are associated with the execution and monitoring process of the NATM approach need to comprehend and accept the NATM approach and respond cooperatively to solve any issues.
2. Design Criteria and Features of NATM
The concept for a suitable design approach for NATM can be divided into two primary groups.
The first group includes the NATM technical requirement with the application in soft or hard ground.
The second group includes external constraints, such as settlement problems, environmental impacts, safety, engineering technology, and contractual and financial constraints.
As a fundamental design viewpoint for NATM, the necessary elements for design are discussed below in detail. Each of the elements is a part of the entire design procedure. If the design of these elements is not integrated with each other, the complete design of the NATM is likely to fail.
2.1 Design Criteria Related to Technical Requirements
The NATM technical design criteria include the understanding of geology and geotechnical functions of the ground, and designing of support and monitoring system. These criteria form the internal skeleton of tunneling using NATM and are discussed below:
2.1.1 Primary and Final Support Design
Design for the primary support (shotcrete, rock bolts, etc.) and the final support are the main components of the NATM technical design. The flexibility and the thickness of the primary support with the addition of steel fibre reinforcement, rock bolts, and forepoling should be considered in the design of the supports. Similarly, the design of final supports should take the deformation as the primary criteria such that the deformation around the tunnel periphery reduces to zero.
In general, the following points should be considered for primary and final supports design:
- Characteristics of ground, such as strength and stand-up time should be obtained carefully.
- The ground-support interaction curve should be determined based upon the ground characteristics.
- Detailed design for groundwater should be taken into account. For instance, the detailed design includes the design of drainage and water-proofing system.
- If the drainage system is considered, then the long-term stability of the drain holes should be determined.
- If the water-proof membrane is considered, then the water pressure should be considered in the design to compute the loads on the lining.
- Extra supports such as rock bolts, lattice girders, steel bonded mesh, and steel fiber should be utilized to increase the strength of the shotcrete.
- Continuous monitoring of the stresses and deformation on the lining should be done.
- The initial design of the preliminary lining can be performed using empirical and computational approaches, and physical models.
- The secondary lining is typically a precast concrete segment.
2.1.2 Geotechnical Design Requirements
As per the NATM’s main principle, the surrounding body of an opening is the main load-carrying structure. Therefore, the characteristic ground-support reaction curve should be established for optimizing the load-carrying capacity of the medium. To design the ground-support reaction curve, the geotechnical properties of the load-carrying medium are essential.
In general, the following points should be considered for the design of ground-support reaction curve:
- The strength of the ground i.e. the tensile, shear, and compressive strength
- Stand-up time
- Pore water and drain conditions
- Homogeneity and non-linearity of the ground
- Time-reliance or creep behavior
- Major joint-discontinuities
- The earth pressure at rest
- Magnitude of overburden pressure
In addition to ground-support curve, appropriate geotechnical design parameters must be chosen to fulfill the analytical or computational preliminary design of excavation patterns and geometry.
2.1.3 Monitoring System
The design of the monitoring system is essential to know the ground and support deformation after the excavation of the tunnel. Particularly, there are two main objectives of a monitoring system: design monitoring and construction monitoring. In brief, monitoring should be carried out to make sure the safety of design and construction. Thus, the data assessment and analysis should be done by the geological/geotechnical professionals, tunnel designers, and construction supervisors.
2.2 Design Criteria Related to External Constraints
Application of NATM in soft ground can unfortunately lead to the collapse of a tunnel. As a result, the surrounding structure may get damaged. It may also trigger an ecological effect by developing sinkholes in cities. Therefore, certain external constraints should be included in the design of NATM so that such situations can be avoided. These criteria form the external skeleton of tunneling using NATM and are discussed below.
2.2.1 Engineering Technology
The technological enhancements in tunneling devices should be considered to take advantage of them. New technological developments should be used for enhancing the construction procedure of tunneling. Therefore, a contrast between previous and new innovations should be carried out to choose the best among them.
2.2.2 Safety of the Tunnel
The tunnel heading is the part of the tunnel that is excavated ahead of the finished face of the tunnel. The majority of failures happen during the excavation of this part of the tunnel. Therefore, to protect those who work within the tunnel and in structures above the tunnel, the stability of the face should be maintained by using extra support such as fore-poles, draining pipes, rock bolts, etc.
2.2.3 Ground Settlement Control Procedures
Settlement due to tunnel excavation should be managed by appropriate construction of the tunnel heading, underpinning of the existing structures, and compensating grouting to reduce the threat of damage to surface area structures.
3. General Sequence of Tunnel Construction Using NATM
NATM is an approach that includes various construction activities performed in a proper sequence giving heed to flexibility and adaptability. The sequence of these activities is discussed below:
3.1 Marking of Drill Holes
The drill holes are marked on the face of the tunnel based upon the blasting pattern. The following requirements are needed to design the blasting pattern:
- Diameter of tunnel
- Ground conditions
- Properties of rock, such as discontinuity, joint set, etc.
3.2 Drilling of Holes
For soft strata, a manual excavator is used for drilling of holes. For hard strata, boomer machines are used for drilling of holes.
3.3 Charging with Explosives
Explosives are placed into the drilled holes and are connected to detonators, that are linked to one prime connection.
3.4 Charging and Blasting
The prime connection where all the detonators are connected is exploded which consecutively enables the surge of each explosive positioned in each drilled hole.
The mucking process starts after the complete removal of dust and hazardous gases through a proper ventilation. In this process, all the broken material and debris around the tunnel face are removed.
3.6 Installation of Primary Support
Stand-up time is a period for which the unsupported ground remains stable on its own after blasting or manual excavation. Installation of primary support should be within the stand-up time. Mainly, shotcrete and rock bolts are used as primary support.
3.7 Arrangement to Prevent Ingress of Water
Water ingress inside the tunnel is a significant issue as it might lead to failure of the structure. To prevent such situations, waterproofing membrane is used after the installation of primary support. Dimplex and Nyllex strip are the most commonly used waterproofing membranes in tunneling construction.
3.8 Secondary Support
If the strata need more support for the structural stability, supports can be provided in the form of lattice girders, steel ribs, another layer of shotcrete, etc.
3.9 Final Lining
Finally, the precast segments of concrete are installed as lining on the periphery of the tunnel area. Such precast segments are known as kicker beams.
NATM is not just a construction method for performing tunneling operations, but also an approach towards a streamlined sequence of the necessary construction activities with security, safety, and economy being the pivotal elements
Following are some of the circumstances where NATM is preferred over TBM machines:
1. If the geology of ground is extremely varying, in such cases, the approach can be modified according to the modifications in the geology.
2. NATM is a versatile technology. It can be used to construct the various profiles of the tunnels. This can be achieved by altering the blasting patterns as per the need of the tunnel’s geometry.
3. TBM setup is often challenging due to the congestion and insufficient space at the site. In such cases, NATM is preferred as the tunneling method.
Application of NATM in soft ground, specifically in soils, is relatively recent. The following are the major issues with using NATM in soft soil:
1. The stand-up time of soft ground is low because the bond between soil particles is weak and the cohesion is also lower than rocks.
2. Basically, in the near-surface area of soft ground case, the in-situ stress will be relatively low. Therefore, the ground will be reasonably weak to support redistributed loads.
3. In addition, the stability of the working-face is very low due to the lower support strength of the surrounding load-bearing structure.