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Journal of science of the Hiroshima University. Series C, Geology and mineralogy 9 巻 2 号
1990-07-20 発行
Behavior of Geologic Bodies Under Paleostress Field and Groundwater Environment at the Foundation of the Nanakita Dam, Northeast Japan
KASHIWAGI Hideharu
Abstract
Miocene tuffaceous rocks are developed at the foundation and adjacent ground of the Nanakita Dam in Miyagi Prefecture. During the detailed surveys for the construction of the dam, a large number of open cracks and clay veins were found in tuff breccia and sandy tuff, respectively. In addition, formation of open cracks was observed in the sandy tuff at the time of Miyagi-ken-oki Earthquake on June 12, 1978. Since these open cracks and clay veins will surely cause serious problems for the dam foundation, detailed investigations have been carried out on the formation processes of these geological phenomena, and appropriate foundation treatments have been performed during the construction based on the results obtained.
Shear fractures in the tuff breccia and extension fractures in the sandy tuff were concluded to be formed under the paleostress field at the period of late Tertiary. The direction of the principal compressive stress axis of the paleostress field was estimated horizontal and N77°E which well coincides with the direction of E-W compressive stress field of the late Miocene to Pliocene ages. The latter is commonly recognized in the southern part of the Tohoku District. It is concluded that the difference between the shear fractures and extension fractures is attributable mainly to the differences in the confining pressure and the mechanical strength of the respective rocks caused by the overlain sediments. Shear fractures in the tuff breccia became gradually open cracks by weathering process, mainly due to the groundwater agents. Extension fractures in the sandy tuff, on the other hand, became clay veins by deposition of clay materials suspended in ground water. This difference probably originates from the difference of resistivity against weathering.
As the result of proper foundation treatments, the dam has been confirmed as a safe structure with no problems for the past four years.
Shear fractures in the tuff breccia and extension fractures in the sandy tuff were concluded to be formed under the paleostress field at the period of late Tertiary. The direction of the principal compressive stress axis of the paleostress field was estimated horizontal and N77°E which well coincides with the direction of E-W compressive stress field of the late Miocene to Pliocene ages. The latter is commonly recognized in the southern part of the Tohoku District. It is concluded that the difference between the shear fractures and extension fractures is attributable mainly to the differences in the confining pressure and the mechanical strength of the respective rocks caused by the overlain sediments. Shear fractures in the tuff breccia became gradually open cracks by weathering process, mainly due to the groundwater agents. Extension fractures in the sandy tuff, on the other hand, became clay veins by deposition of clay materials suspended in ground water. This difference probably originates from the difference of resistivity against weathering.
As the result of proper foundation treatments, the dam has been confirmed as a safe structure with no problems for the past four years.
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PP. 313 - 327