ABSTRACT

A number of technical treatments of the general subject matter regarding the New Austrian Tunneling Method (NATM) approach to tunneling have been published in recent years and have dealt with many facets of the subject. These publications have documented the relatively wide acceptance of the philosophy of NATM throughout the world. However, for a number of reasons that could be considered both technical and philosophical, NATM has been slow to receive much acceptance in the United States. The authors have spent time in recent years trying to understand both the strengths and weaknesses of NATM as a design process and to consider some of the rationale for its relative lack of favor in United States design and construction practice. This paper is an attempt to present our findings and conclusions as to the benefits of NATM as it might be applied to United States practice. As a starting point of this discussion, we would like to present our definition of what we believe constitutes the NATM philosophy. We believe it to be a common sense geotechnical approach to the design of underground support systems that can embody a variety of support elements and excavation techniques. It also makes extensive use of monitoring of ground behavior during construction to corroborate design assumptions. The support elements generally include combinations of shotcrete, wire mesh, rock bolts, and light steel ribs, as applied to construction in rock. These are the most common elements used in relatively poor ground conditions. However, other elements can be used within the context of the general philosophy. Being a geotechnical approach, the design methods incorporate controlled deformation to permit mobilization of the inherent strength of the earth medium. This point will be developed further in the discussion. On occasion, the literature has called it a "shotcrete method". It certainly can embody shotcrete as a support method, but it is obviously not restricted to shotcrete alone as a support element. It has also been called an "observational method". Observation during construction is one element of the approach, but this description is also limited. The philosophy has certain historical negatives, particularly relative to application in the United States. However, we submit that NATM constitutes a valuable tool in the designers' arsenal of design concepts which should be favorably considered on projects where it is advantageous to do so. The authors hope by this discussion to develop the strong points of the philosophy as it applies to United States practice and consider possible limitations. To do so, it is necessary to turn back the clock and examine the philosophical development of the basic concepts. A convenient point of initial reference to an American engineer, would be to consider the work of Karl Terzaghi. It is understood that part of the early research and study of Terzaghi was accomplished in Europe and that he spent some time at the University of Graz in Austria. His early work in the United States was accomplished in about the late 1920's and early 1930's, when he conducted some of his classical experiments relative to pressures on retaining walls. From this early work, Terzaghi used Rankine theory to evolve an understanding of the nature of stresses on retaining walls.

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