NATM Constructive Method:
- Creation of a first suport on the surface of the tunnel opening (loosenig zone) to avoid deterioration;
- Opening of a tunnel traject not very long ;
- Projection of concrete in all the circular perimetre of the excaved zone (25 at 50 mm);
- Introduction of metalics suports (cambots);
- Fixation of the sustention reinforcement (usualy 6mm);
- First hand of shotcrete, in a layer with thickness not superior at 150 mm;
- Perfuration of the formed layer for the inserection of the anchorages;
- Introduction of the suport for the next excavation cicle.There are eight features on which NATM is based:
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. Primary support is directed to enable the rock to support itself.
2- Shotcrete protection - Loosening and excessive rock deformation must be minimised. This is achieved by applying a thin layer of shotcrete immediately after face advance.
3- Measurements - Every deformation of the excavation must be measured. NATM requires installation of sophisticated measurement instrumentation. It is embedded in lining, ground, and boreholes.
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.
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.
Advantages of NATM
1- Eliminates the need for using some expensive TBM equipment during excavation
2- Suitable for a wide range of geometry (shafts, junctions, non-circular tunnels and
tunnels with variable shapes)
Disadvantages of NATM
1- Its suitability diminishes in softer ground, which can subside when excavated
2- Not suitable below water table in highly permeable soils
