The New Australian Tunneling Method (NATM) is also known as the Sequential Excavation Method (SEM). It is a widely used method of modern tunnel design and construction. The NATM first gained attention around 1960s by the work of Ladislaus von Rabcewicz, Leopond Muller and Franz Pacher in Australia. The name NATM was intended to differentiate the method from the old Australian tunneling method. The New Australian Tunneling Method is more of a set of principles or a philosophy rather than a technical method. It aims at maximizes the terrain’s inherent resistance and support capacity. According to Rabcewicz, NATM consists of a thin sprayed concrete lining, closed at the earliest possible moment by an invert to complete the ring called an auxiliary arc. The deformation of which is measured as a function of time until equilibrium is obtained. Around 1980, the Austrian National Committee on Underground Construction of the International Tunneling Association redefined the NATM as a concept whereby the ground surrounding an underground opening becomes a load bearing structural component through activation of a ring like body of supporting ground. Leopold Muller proposed that the NATM is a tunneling concept defined by a set of principles, suggesting that the NATM is not to be view as a method of construction as this implies a means by which to advance or drive a tunnel (Arshad 2016).
Main Principles of the New Austrian Tunneling Method
1. Mobilization of the strength of rock mass
The method relies on the inherent strength of the surrounding rock mass being conserved as the main component of tunnel support. The surrounding rock is not only the main body of the support load, but also the structure of the stratum load and the surrounding rock mass is then taken as the whole interaction. The pressure of the surrounding rock is a combination of deformation pressure and loose pressure, most of the pressure is borne by the surrounding rock and only a few of them are transferred to the supporting structure (Karakus and Fowell, 2004).
2. Shotcrete protection
Loosening and excessive rock deformation must be minimized. This is achieved by applying a thin layer 25-50mm of shotcrete immediately after face advance (Karakus and Fowell, 2004).
Every deformation of the excavation must be measured. NATM requires installation of sophisticated measurement instrumentation. It is embedded in lining, ground, and boreholes (Karakus and Fowell, 2004).
4. Flexible support
The primary lining is thin and reflects recent strata conditions. Active rather than passive support is used and the tunnel is strengthened not by a thicker concrete lining but by a flexible combination of rock bolts, wire mesh and steel ribs (Karakus and Fowell, 2004).
5. Closing of invert
Quickly closing the invert and creating a load-bearing ring is important. It is crucial in soft ground tunnels where no section of the tunnel should be left open even temporarily.
6. Contractual arrangements
Since the NATM is based on monitoring measurements, changes in support and construction method are possible. This is possible only if the contractual system enables those changes.
7. Rock mass classification determines support measures
There are several main rock classes for tunnels and corresponding support systems for each. These serve as the guidelines for tunnel reinforcement.
8. Based on the computation of the optimal cross section, just a thin shotcrete protection is necessary. It is applied immediately behind the Tunnel boring machine, to create a natural load-bearing ring and therefore to minimize the rock’s deformation. Additionally, geotechnical instruments are installed to measure the later deformation of excavation. Therefore a monitoring of the stress distribution within the rock is possible.
CLASSIFICATION OF ROCKMASS TYPE
The excavation in the rock is dependent on the class based on several factors such as compressive strength of the rock, water conditions, and number of cleavages, condition of cleavages, dip and strike of the rock. There are several approaches of classification of the rock mass and most predominantly are RQD, RMR, and Q factor of the rock mass.
RQD is used to provide a quantitative estimate of rock mass quality from drill core logs. It is defined as a percentage of core pieces longer than 100m in the total length of core.
RMR depends on the uniaxial compressive strength, RQD, spacing of discontinuities, conditions of discontinuities, groundwater conditions and the orientation of discontinuities.
Q Factor depends on the block size, inter block shear, active stress, reduction for joint water flow and presence of weakness zones. The values of Q factor range from 0.01 to 10000, that is from poor rock to exceptionally good rock.
COMPONENTS AND SEQUENCE OF EXCAVATION IN NATM
1. Sealing shotcrete
2. Fixing of Lattice Girder
3. Fixing of wire mesh
4. Primary lining with shotcrete
5. Rock bolting
6. Pipe Fore poling
First the profile and the face needs to be washed down so that there will be a good connection between rock and shotcrete. Areas where there are over break situations should be sprayed from the bottom upwards to have a good abutment for upper shotcrete. After a while it needs to go back to “over break” area for the next thin layer. Two thirds of the face needs to be sprayed to act as face support (SEKER Z. 2012).
Figure 1 showing sealing shotcrete .
Fixing of lattice girder
The Lattice Girder is used when the rock is loose.
Functions of Lattice Girder
• Steel Rib or Lattice Girder composite structure of lattice girder and concrete confined to load distribution
• Carrying of “green” shotcrete
• Profile control
• Support for fore poling
Figure 2 showing lattice girder being installed.
Fixing of wire mesh
• It prevents the shotcrete portion from dropping after cracking or failing of the lining
• It reduces and limits the shotcrete from cracking due to creep and overstressing
• It reinforces construction joints
• It increases shear strength
• It stabilizes applied shotcrete until setting and hardening
Figure 4 showing installed wire mesh with rock bolts and cable bolts on the side wall of an excavation.
Primary lining with shotcrete
The function of primary lining with shotcrete is
1 As sealing shotcrete
• To avoid loosening of the surrounding ground
• Closes joints and prevents fall downs, so activating the rock arch
2 As main shotcrete
• To carry the load introduced by the ground in the lining which is mainly normal forces
Figure 5 showing shotcrete being sprayed on the walls of the tunnel.
Rock bolting is a primary means of rock reinforcement used to stabilize excavated rock in tunnels. Rock bolts are systematically arranged in such a manner as to transfer the load from the unstable surface or exterior of the rock, to the stronger interior part of the rock (Bower 2004). The rock mass can be reinforced by rock bolting in one of the following ways: building beams, erecting a pressure arch, support of discrete blocks, and suspension of weak, fractured ground to more competent layers. Mechanical-anchor bolts, resin bolts, and cement-bonded bolts are the common types of rock bolts used. Mechanical-anchor bolts utilize an expansion shell to secure an anchoring point at the end of a drilled hole in stable ground. Installation entails simply pushing the bolt up inside a drilled hole and tensioning it. Resin bolts and cement-bonded bolts differ in that they rely on the bonding property of resin or cement to transfer rock loads. The installation of the rock bolts can be achieved either manually with stoppers and impact wrenches, or mechanically with pneumatic bolting equipment. In general the direction of rock bolts should be radial that is perpendicular to the lining (Hustrulid 2001).
Figure 6 showing radial placement of rock bolts in different shapes of tunnels.
Forepoling and lagging are support measures installed in the tunnel longitudinal direction prior to excavation. They shorten the free span of the unsupported excavation surface. Forepoling and lagging are support aids for excavation and will have less function after installation of initial support (Rib, shotcrete, wire mesh and rock bolts).
NATM: ADVANTAGES AND LIMITATIONS
• The primary advantage of NATM is the economy resulting from matching the amount of support installed to the ground conditions as opposed to installed support for expected worst case scenario throughout the entire tunnel.
• The safety of the work is more easily assured because the sizes and configurations of the headings making up the total tunnel cross section can be adapted to the degree of instability of the working face.
• Suitable for a wide range of geometry (shafts, junctions, non-circular tunnels and tunnels with variable shapes)
• One of the major problems is the need to decide on the amount of support to be installed from day to day. It is not easy to achieve this in the adversarial conditions often encountered.
• Its suitability diminishes in softer ground, which can subside when excavated
• Not suitable below water table in highly permeable soils
• Highly skilled manpower is needed and expert engineers.
The New Austrian Tunneling Method was of design and excavation of tunneling is ground is advantageous and scientific way in comparison to the old way of tunnel. The NATM also has an advantage of monitoring the rock mass around the tunnel during its life span for deformation and design and design a support system according to the conditions of the rock mass
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