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This page is a start point of our Knowledge Management (KM) for the Project. We are intending to establish our system for 1) Techinical Knowlegde, 2) Administrative Knowlegde, 3) Project Management Knowledge, and so on. If you have any further useful question and information, please email us. Thank you.
1 . Introduction of NATM
New Austrian Tunneling Method (NATM) is applied to the Project for its design and construction method of the tunnel. The NATM was developed by Austrian engineers such as RABCEWICZ from 1957 to 1965. The principles of the method is that observations should be undertaken to confirm that the support is adequately stabilizing the surrounding ground, so that the rate of convergence towards the tunnel is perceived to be approaching an asymptotic value. Where, after initial support, this is not assured, further support may be applied incrementally. Thus, the principle of the system may be described as that of ‘incremental support’. The essence of design of a system of incremental support is to understand the stress-strain properties of the ground and of the tunnel support since, essentially, controlled strain of the ground has to occur in order to develop a changed stress field in the ground around the tunnel compatible with the degree of support. The NATM has now gained general acceptance in the engineering world through the years of construction practices in various field such as highway, railway, and waterway.
2. What is NATM?
NATM represents for New Austrian Tunnelling Method. The NATM may be defined as a method of producing underground space using all available means to develop the maximum self-supporting capacity of the rock or soil to provide the stability of the underground opening. This is achieved by the application of a smooth and appropriately resistant initial support and a final lining which should accept the necessary deformation but guard against the development of rock load. The initial lining may consist of shotcrete, steel arches, and rockbolts, either singly or in combination.
Simplified definition of the principles and effects of the NATM
3. Basic theory of soft ground tunnelling
Soft ground can be compared to a highly viscose liquid with a limited stand up time when excavated. This fact leads to the most important requirements of the NATM:
Continuous (and, if feasible) symmetric excavation of drifts avoids inhomogeneities in stiffness of the initial lining, changing load conditions and, therefore, provides smooth stress redistribution in both, lining and the surrounding ground.
NATM is an observational method. Therefore monitoring (in-situ-measurements) of deformation within the ground and opening as well as stress development on and in the initial lining (shotcrete) is essential.
The success of execution of the NATM is based on three premises:
Typical support for NATM in unstable ground conditions
Typical elements of support
1. What is "top heading" and "benching" in tunnel excavation?
Tunnel excavation consists of "top heading" - excavating and supporting the upper two-thirds of the tunnel - followed by "benching" - excavating and supporting (stabilizing) the lower one-third of the tunel
"Top heading" construction sequence consists of the following:
- Holes are drilled in the top two-thirds of the tunnel and exposives are loaded and detonated in a carefully designed sequence to acieve the proper balance between sufficient breakage of excavated material and minimal damage to surrounding rock.
- Following the blast, the tunnel is ventilated to clear the explosion gases, and air quality measurements are taken to ensure safe return to the tunnel.
- The exposed top part of the tunnel arch("crown") and face are "scaled" (scraped off) using drill equiment or hand tools to remove loose rock. The crown area is then reinforced with rock bolts, rock dowels, or lattice girders (if needed), and shotcrete, creating a rock arch over the tunnel. The decision to use either bolts, dowels or lattice girders and the thickness of shotcrete (2-6 inches, plain concrete or reinforced with steel fibres) depends on the category of rock and the necessary support required.
"Benching" construction sequence consists of the following:
- First, the center portion (within 5-feet of each sidewall) of the lower one-third of the tunnel is drilled, blasted, and mucked out, following the same procedures as for the "top heading". The 5-foot spacings on both sides of the lower one-third of the tunnel ("side slashed") are left to act as buttreses and to prevent the blast pressure from damaging the retianing walls.
- Second, the "side slashes" are drilled, blasted, mucked out, and stabilized by rock bolts and shotcrete, corersponding to the ground support used for the "top heading"
2. What is a "round"?
The tunneling process is the excavation of the tunnel shape through the rock in small increments - each increment representing a "round". The "round" measures the forward advancement (in feet) of the excavation process, which consist of drill, blasting, ventilating, mucking and installing the support. A "cycle" is the time (in hours) to complete the "round". The quality of the rock determines how many feet can be extracted in each "round" and the amount time ("cycle") it takes to install the necessary support before a subsequent "round" can be excavated.
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