Abstract

For the purpose of capturing the long-term displacement behavior of a fracture, seasonal changes in fracture aperture were continuously monitored with 0.01 mm accuracy for an open fracture. The site is Yoshimi Hyaku-Ana, a nationally designated historic site, on a hill with a relative height of about 30 m. The target fracture found at the southwestern slope composed of a porous tuffaceous rock in Miocene epoch is open about 7 cm due to physical weathering caused by the intrusion of tree roots. The block above the fracture is unstable and is in risk to start sliding and falling. Therefore, three displacement sensors were arranged almost linearly in the direction of inclination of the fracture to measure the change in aperture width with the surrounding environment of temperature, relative humidity, air pressure and rainfall.

As a result of monitoring over one year, it was shown that the fracture is closed in the summer and opened in the winter, showing a high correlation with the temperature. After the rainy season of the year of observation, the width of fracture aperture was gradually shifted to open by less than 4 mm. Although sensor drift must be considered, the three sensors respond in the same way, but with different magnitudes.

We considered the change in aperture width with respect to temperature in two ways. One is the relationship between the daily average temperature and the aperture width. From this consideration, it was found that there is a correlation between the temperature and the aperture width before the rainy season, and that the aperture width becomes narrower as the temperature rises. It was also found that after the rainy season, all three sensors showed the same trend of aperture change.

Another consideration examined changes in temperature and aperture width during the day. As a result, it was found that the gradient in aperture width to temperature change at one day became smaller as the temperature decreased, and that the changes were clearly different before and after the rainy season.

These results imply that the upper block is moved to the dangerous side. In addition, we can propose that the observation of the diurnal change of the gradient is one of the effective indices for the signs of the change in the properties of the fracture at an early stage.

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