ABSTRACT:
The finite element program CRItical State Program (CRISP) has been used to model the New Austrian Tunneling Method (NATM) in London Clay. The non-linear behavior of the London Clay was modeled by a Strain Dependent Modified Cam Clay (SDMCC) model and the tunnel lining was modeled by constant and time-dependent classic models. The construction process was modeled in two and three dimensions by removing soil elements in sequence. The tunnel lining was either assumed wished-in-place or introduced after the excavation of each panel. The results obtained from plane strain and three dimensional analyses are compared to assess the importance of arching of soil ahead of tunnel race.

INTRODUCTION
The New Austrian Tunneling Method (NATM) is a technique in which ground exposed by excavation is lined with shotcrete to form a temporary lining. Rapid and consistent support of freshly excavated ground, easier construction of complex intersections, and lower capital cost of major equipment are some of the advantages of NATM. The successful use of the method is reliant upon high quality working by a skilled work force under continuous engineering supervision. Some of the limitations of this method are that it is slow compared to shield tunneling in uniform soils, dealing with water ingress can be difficult, and it demands skilled man power. In particular, instability at the tunnel face, unless positive support is provide^ can endanger the work force. Kuhnhenn| carefully analyses the typical collapses of NATM tunnels, constructed in hard and soft rocks in Germany. He highlights the importance of workmanship and limiting the length of the unsupported section ahead of the shotcrete. Although NATM was primarily developed for rocks, h is now being used in clayey soils. Therefore, it is important to understand this method in clayey soils.


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