Abstract
Shirasu which is defined as the non-welded part of pyroclastic flow deposits is widely distributed in the southern part of Kyushu Island. Shirasu is regarded as soft ground when the mountain tunneling method is applied because its unconfined compression strength is about 50 kN/m2. Nevertheless, it is well known that Shirasu tunnels are comparatively stable. In order to make this mechanism clean, we have already proposed a numerical analysis method considering geotechnical nonlinear properties of Shirasu ground. In this paper, we adopted this method to a tunnel with an extremely large section in Shirasu ground and confirmed the validity of this method by comparing the numerical results with those obtained from the field measurement. Further, we analyzed the peculiar behavior of the tunnel with the extremely large section.
