Mechanical and rheological characteristics of the siliceous mudstone at the Horonobe Underground Research Laboratory site
The Horonobe Underground Research Laboratory (URL) in Hokkaido, Japan has been constructed in diatomaceous and siliceous mudstones, which are widespread in the areas along the Sea of Japan. It is essential to understand the mechanical and rheological characteristics of these rocks for the construction of underground structures and for the assessment of their long-term stability. In this study, the siliceous mudstone of the Wakkanai Formation was applied to various laboratory tests: compression test, creep test, relaxation test, drying shrinkage test, and slaking test. The test results showed that water has a major impact on the mechanical and rheological properties of the siliceous mudstone. In addition, water content at a tunnel wall was measured in the Horonobe URL. Comparing the results of the laboratory tests and the in situ measurement, the effect of water on the tunnel stability was discussed.