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[SCG44-01] Consolidation anisotropy of Mio–Pleistocene sedimentary soft rocks in the eastern part of the Boso Peninsula, central Japan
Keywords:Consolidation, Anisotropy, Sedimentary soft rock
Consolidation generally progresses with an increase in the overburden by the thickness of the sediment. In tectonic stress field, such as a subduction zone, folding also progress with sedimentation, therefore it is necessary to consider the effects of tectonic stress on consolidation. In this study, consolidation tests were conducted using siltstone distributed on the Boso Peninsula, central Japan which is located at the subduction zone of the Sagami Trough to investigate the anisotropy of the consolidation.
The Boso Peninsula is located at the northern part of the Sagami Trough and had formed with the subduction of the Philippine Sea Plate. The Boso Peninsula is composed of the accretionary prisms, trench slope break, and forearc basin, in this study, siltstones were taken from the eastern part of the Boso forearc basin as samples for the consolidation tests. The Boso forearc basin is composed of the Miura Group (15-3 Ma) and the Kazusa Group (2.5-0.3 Ma) and the boundary of both groups is the Kurotaki Unconformity. The large scale folds are distributed in the Miura Group, but the Kazusa group is monocline to the north. In this study, siltstone block samples were taken from the eastern part of the Boso forearc basin composed of both groups to consider the characteristics of anisotropy of consolidation based on geological structure.
We made two types of specimens from a block sample: height direction is perpendicular or parallel to the bedding plane. Cylindrical specimens were put into a consolidation ring, loaded at a constant strain rate, and the consolidation yield stress (pc) was obtained from the consolidation curve. As a result, the consolidation yield stress in the parallel direction was the same or larger than that in the perpendicular direction. K0pc value which is the ratio of the stress of the parallel to the perpendicular was estimated as an indicator of the anisotropy of consolidation using the pc of perpendicular and parallel directions. K0pc of the Miura Group is approximately 1.1 and that of the Kazusa Group is approximately 1.2. Compared to the characteristics of anisotropy of consolidation between the two groups, there is no difference between the two groups. The K0 value of sediment is generally 0.5-0.8 in the silence basin without tectonic stress. Therefore, considering the setting of the Boso forearc basin, it was suggested that the consolidation of the Boso forearc basin is affected by the tectonic compaction.
The Boso Peninsula is located at the northern part of the Sagami Trough and had formed with the subduction of the Philippine Sea Plate. The Boso Peninsula is composed of the accretionary prisms, trench slope break, and forearc basin, in this study, siltstones were taken from the eastern part of the Boso forearc basin as samples for the consolidation tests. The Boso forearc basin is composed of the Miura Group (15-3 Ma) and the Kazusa Group (2.5-0.3 Ma) and the boundary of both groups is the Kurotaki Unconformity. The large scale folds are distributed in the Miura Group, but the Kazusa group is monocline to the north. In this study, siltstone block samples were taken from the eastern part of the Boso forearc basin composed of both groups to consider the characteristics of anisotropy of consolidation based on geological structure.
We made two types of specimens from a block sample: height direction is perpendicular or parallel to the bedding plane. Cylindrical specimens were put into a consolidation ring, loaded at a constant strain rate, and the consolidation yield stress (pc) was obtained from the consolidation curve. As a result, the consolidation yield stress in the parallel direction was the same or larger than that in the perpendicular direction. K0pc value which is the ratio of the stress of the parallel to the perpendicular was estimated as an indicator of the anisotropy of consolidation using the pc of perpendicular and parallel directions. K0pc of the Miura Group is approximately 1.1 and that of the Kazusa Group is approximately 1.2. Compared to the characteristics of anisotropy of consolidation between the two groups, there is no difference between the two groups. The K0 value of sediment is generally 0.5-0.8 in the silence basin without tectonic stress. Therefore, considering the setting of the Boso forearc basin, it was suggested that the consolidation of the Boso forearc basin is affected by the tectonic compaction.