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  Optimal Profile Pattern for NATM Tunneling in Soft Ground

  The New Austrian Tunnelling Method (NATM), as named by Professor Rabcewicz, is defined as a tunnelling method that consists in stabilising the ground around the excavation in the most safe and economic manner possible by making extensive use of the bearing capacity of the ground with the help of sprayed concrete and other support elements as well as through the use of measured data. Making use of the ground bearing capacity helps saving in support elements. Thus, the NATM is known as an efficient and economical tunnelling method [1]. It has been successfully used in a variety of geological conditions ranging from hard to soft rock, soft stable ground to weak, friable and unstable ground. Its main advantage, over other tunnelling methods, is flexibility [2]. It can be applied for varying profiles. The design of NATM tunnel profile is based mainly on designer experience. This experience is not expected to cover all possible tunnelling conditions: ground type, difference in mechanical properties for the same ground type, difference in tunnelling depths, ground water existence, surrounding structures effect, etc. Matching possible alternatives for these parameters gives infinite number of tunnelling conditions. This research aims to introduce a scientific based procedure to draw patterns for NATM tunnel optimal profile in different soft ground conditions. In 2013, the authors have presented a new tool to search for the optimal NATM tunnel profile in different soil types. Herein, the developed tool, which combines the finite element analysis with a modern search technique, is applied to find the optimal profiles for different soft ground types and tunnelling depths. Comparing the resulted profiles can help in drawing the target pattern.

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