Newly constructed tunnel is a two-lane road tunnel with a total length of 2,995 m. Shortly after the breakthrough of the tunnel, the concrete structures of the tunnel invert upheaved up to 101 mm in two locations at an overburden depth of approximately 300 m, forcing re-excavation of these two locations. The results of boring explorations found that the upheaval was caused by the expansion of hydrothermally altered autobrecciated lava that appeared under the invert structures.

We set the tentative support specification using a thick cylinder model culcurus, and confirmed the soundness of the supports under the impact of swelling pressure through finite element analysis for prediction of strength by applying a heterogeneous model that assumed the distribution of the altered zone. By conducting various measurements, boring explorations, and laboratory testings of the peculiar deformation caused by the hydrothermally altered autobrecciated lava, it was possible to design rationally and re-excavate in a safe and efficient manner.

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