Dynamic Stresses induced within Rock in Case of Blasting with one Free Face

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  • 一自由面爆破の場合の岩盤内の応力状態について
  • イチ ジユウメン バクハ ノ バアイ ノ ガンバン ナイ ノ オウリョク ジョウタイ ニ ツイテ

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In case of rock blasting, the stresses at any point near the free face are induced by the superposition of the following three stresses; the first of them is the one produced by the dilatational wave projected in the rock by an explosion, and the second and the third ones are those produced by the reflected dilatational and distortional waves which are projected from the free face by the reflection of the dilatational wave. Therefore, in this study, the stresses caused by the respective stress waves were analysed at the first time in regard to the respective favourable coordinates, by applying the elastic theory, and next, these stresses were synthesized with respect to a common coordinate, and then the values and the directions of the principal stresses were analysed as the function of time. The results obtained are briefly shown below.<BR>1) The maximum value of the compressive stress near the free face was decreased due to the existence of the free face, while that of the tensile stress was increased.<BR>2) Let's consider the polar coordinate of which origin is the center of the spherical charge. Then, at any point near free face, the directions of the principal stresses in the γθplane changed with time, but at another point near an explosion where the principal stresses were mainly composed of the stresses caused by the initial dilatational wave, the directions of the principal stresses coincided with those of the coordinate used.<BR>Now a crack will be produced in the rock mass along the line perpendicular to the direction of the maximum tensile stress. And in the present case, it can be said from the fact mentioned above that the radial cracks will be produced in the rock mass in the vicinity of the center of an explosion, but with the approach of the free face the direction of the cracks will be curved to the direction parallel to the free face.<BR>3) The maximum value of the tensile stress in the Φ direction was larger than those in theγθplane.<BR>4) The tensile stress caused by the Hoppkinson effect appeared only near the line of least resistance.<BR>5) The appearan-ce of the breakage deduced from this investigation agreed with the practical observations.

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