Shear Characteristics of Rock Containing Weak Planes (1st Report)

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  • 弱面を含む岩石のせん断特性　Ｉ
  • ジャクメン オ フクム ガンセキ ノ センダン トクセイ 1

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In designing a foundation of dam, a tunnel or an underground structure, it is necessary to estimate the mechanical properties of rock mass. However, rock mass is not a uniform body, but contains joints and other weak planes, and these weak planes reflect on the mechanical properties of the rock mass. The strength value obtained in laboratory leads to an over-estimation of strength of rock mass, because rock specimens are almost collected from rock mass between weak planes.<BR>This study was undertaken for the purpose of estimating shear characteristics of rock mass containing weak planes, and a direct shear testing machine, which is applicable in the study, has been developed. As a preliminary research, the process of forming shear surface and the shear behaviours of rock specimens were investigated on OGINO tuff.<BR>The results in this study are summarized as follows;<BR>1) A distribution of the principal stresses in rock specimen was analyzed by F.E.M.. In this analysis, the tensile stresses appear near the shear edge of the shear testing machine. From this result, it is thought that a crack initiation in rock specimen is originated by the tensile stress, and the shear stress is then concentrated at the tip of the tension crack. Consequently, the crack may progress along the direction of shear load and the shear surface becomes undulated.<BR>Also, as normal stress is inceased in the direct shear test, peak and residual shear strengths take higher values and shear displacement at the point of the peak shear strength is larger.<BR>2) The direct shear tests were conducted on the rock specimens containing the artificial discontinuities.<BR>From the results of the shear tests which were carried out at a constant normal stress, it is found that peak shear strength takes remarkably low value in the region of θ=45°.<BR>On the other hand, it is found from the results of the shear tests done under the confined normal displacement that peak shear strength takes high value in the region of θ=45°.<BR>From the above findings, it is clear that shear behaviours of rock containing discontinuities are affected by the angle (θ) between the direction of shear load and that of discontinuities, and the condition of restraining normal displacement.

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