未固結堆積物の変形挙動  [in Japanese] Deformation behavior of the unlithified sedimentary rocks  [in Japanese]

一般に石化作用や続成作用が十分に進行していない堆積性の物質を意味する, 未固結堆積物の定量的な定義を試みた。さらに未固結堆積物の変形挙動とその構成則および変形の素過程に関する主に工学分野の理論的研究を紹介した。その剪断変形挙動は岩相によって大きく異なり, さらに泥質なものでは先行圧密履歴と排水・非排水条件に, 砂質なものでは初期間隙比と排水・非排水条件に大きく支配される。また, 未固結堆積物を主要構成物質とする現世の付加体前縁部の変形構造に関する研究, および深海底掘削試料や人工試料を用いた実験的な研究を簡単にまとめた。

This paper reviews the studies on the deformation of unlithified sedimentary rocks, referred to as soils and soft sedimentary rocks in engineering science. First, the deformation behavior, constitutive laws and micro structures depend on lithology: clays differ considerably from sands. The shear deformation behavior of clays depends crucially on their previous consolidation history and whether the pore fluids are drained- or undrained; that of sands is closely controlled by their initial void ratio together with their drained or undrained condition. Cam-Clay theory and Rowe's stress-dilatancy relation, well-known constitutive laws of clays and sands, respectively, are reviewed. The microstructural differences between clays and sands are influenced by grain size, packing, and chemical properties. On the other hand, the behavior and constitutive laws for soft sedimentary rocks are similar to overconsolidated clays and dense sands, although some microstructures differ. Second, the deformation structures in the toes of the accretionary prisms such as the Barbados Ridge, the Nankai Trough and the Oregon Margin are reviewed, as they are mainly composed of incompletely lithified sediments. Third, experimental studies on core samples recovered from the ocean floor and on synthetic samples are introduced. The objects of these work are to determine quantitatively consolidation state and other physical properties, and to elucidate the mechanism by which fluid flows along faults such as the decollement zone and related thrusts. Investigating the theories, natural structures and experimental behavior of unlithified sedimentary rocks is fundamental to an understanding of their deformation.