Stress Measurement in Weak Rock by Borehole Deformation Method-A Case Study of Horonobe

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  • GHIMIRE Hem Nath
    Rock Mechanics Laboratory, Graduate School of Engineering, Hokkaido University
  • ISHIJIMA Yoji
    Rock Mechanics Laboratory, Graduate School of Engineering, Hokkaido University
  • SUGAWARA Takayuki
    Rock Mechanics Laboratory, Graduate School of Engineering, Hokkaido University
  • MATSUI Hiroya
    Horonobe Underground Research Center, Japan Nuclear Cycle Development Institute
  • NAKAMA Shigeo
    Tono Geoscience Center, Japan Nuclear Cycle Development Institute

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Other Title
  • ボアホール変形法による軟弱岩における地圧測定－ 幌延の事例

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Description

A vertical borehole of 50m depth was drilled in the diatomaceous mudstone, which is classified as weak rock, at Horonobe, Hokkaido and stress measurement by the borehole deformation method was carried out at different depths. The rock is massive with density as low as 1.48 g/cm3, reflecting high porosity of 45%. Mean value of the compressive strength is 3.21MPa. Fresh water and bentonite was inevitably used as the drilling fluid to maintain the fragile borehole wall. <BR>Stress measurements were tried at seven points along the borehole of 50m depth. At four measuring points, where the cores contained two types of fractures, longitudinal and cross, behaviors of the borehole deformations during overcoring deviated from the prediction based on the elastic theory. Results obtained at three measuring points, where cores contained little cracks, are judged to be reliable. Maximum principal stress directs EW and the stress is almost biaxial in the subsurface region of Horonobe district. This characteristic of stress condition is in harmony with the direction of the active folds distributed in the vicinity of this district.<BR>The efficiency of measurement was highly dependent on the condition of borehole and three sets of measurements were taken within 8 hours working period, at sections where fewer cracks occurred. It has been proved that the stress meter is applicable in weak rock and that the measuring system adopted is practical, although the stress meter suffered some damages at sections where the core was highly fractured.

Journal

  • Shigen-to-Sozai

    Shigen-to-Sozai 120 (10/11), 545-554, 2004

    The Mining and Materials Processing Institute of Japan

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