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The New Austrian Tunnelling method (NATM), also known as Sequential Excavation Method (SEM), describes a popular method of modern tunnel design and construction. This technique first gained attention in the 1960s based on the work of Ladislaus von Rabcewicz, Leopold Müller and Franz Pacher between 1957 and 1965 in Austria. The name NATM was intended to distinguish it from the old Austrian tunnelling approach. The fundamental difference between this new method of tunneling, as opposed to earlier methods, comes from the economic advantages made available by taking advantage of the inherent geological strength available in the surrounding rock mass to stabilize the tunnel . NATM/SEM is generally thought to have helped revolutionise the modern tunnelling industry. Many of the most famous modern tunnels have used this excavation technique. ==Principles== The NATM integrates the principles of the behaviour of rock masses under load and monitoring the performance of underground construction during construction. The NATM has often been referred to as a "design as you go" approach, by providing an optimized support based on observed ground conditions. More correctly it can be described as a "design as you monitor" approach, based on observed convergence and divergence in the lining and mapping of prevailing rock conditions. It is not a set of specific excavation and support techniques. NATM has seven elements: * Exploitation of the strength of native rock mass - 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. * Shotcrete protection - Loosening and excessive rock deformation must be minimised. This is achieved by applying a thin layer of shotcrete immediately after face advance. * Measurement and monitoring - Potential deformations of the excavation must be carefully monitored. NATM requires installation of sophisticated measurement instrumentation. It is embedded in lining, ground, and boreholes. In the event of observed movements, additional supports are installed only when needed, with a resultant overall economy to the total cost of the project. * Flexible support - The primary lining is thin and reflects recent strata conditions. Active rather than passive support is used and the tunnel is strengthened by a flexible combination of rock bolts, wire mesh and steel ribs, not by a thicker concrete lining. * Closing of the invert - Especially crucial in soft ground, the quick closing of the invert (the bottom portion of the tunnel) which creates a load-bearing ring is important, and has the advantage of engaging the inherent strength of the rock mass surrounding the tunnel. * Contractual arrangements - Since the NATM is based on monitoring measurements, changes in support and construction method are possible, but only if the contractual system enables them. * Rock mass classification, ranging from very hard to very soft, determines the minimum support measures required and avoids economic waste that comes from needlessly strong support measures. Support system designs exist for each of the main rock classes. These serve as the guidelines for tunnel reinforcement. Based on the computation of the optimal cross section, only a thin shotcrete protection is necessary. It is applied immediately behind the tunnel boring machine, to create a natural load-bearing ring and minimize the rock's deformation. Geotechnical instruments are installed to measure the later deformation of excavation. Monitoring of the stress distribution within the rock is possible. This monitoring makes the method very flexible, even if teams encounter unexpected changes in the geomechanical rock consistency, e.g. by crevices or pit water. Reinforcement is done by wired concrete that can be combined with steel ribs or lug bolts, not with thicker shotcrete, The measured rock properties suggest the appropriate tools for tunnel strengthening. Since the turn of the 21st century, NATM has been used for soft ground excavations and making tunnels in porous sediments. NATM enables immediate adjustments in the construction details, but requires a flexible contractual system to support such changes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「New Austrian Tunnelling method」の詳細全文を読む スポンサード リンク
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