YIELD LOCUS OF A DIATOMACEOUS MUDSTONE WITH ARTIFICIALLY MADE PLANE OF SEPARATION

Abstract

Studied is the geometrical character of the yield locus of a diatomaceous mudstone which has an artificially made plane of separation in itself. The diatomaceous mudstone behaves in such a way that overconsolidated clays typically behave, although the diatomaceous mudstone is chemically bonded and has its post geological history totally different from that of overconsolidated clays. The location of the yield points in the stress space was searched by applying the loads to the specimens of diatomaceous mudstone being brought from the elastic state into the elasto-plastic state under the load beyond the yielding stress. The diatomaceous mudstone used in this investigation is a uniform material carefully taken from a mining pit. Thus taken specimens are under undisturbed, saturated and unweathered conditions. The diatomaceous mudstone has a distinct yielding point on the stress-strain curve obtained from the CD (consolidated-drained) tests. The yield locus of the diatomaceous mudstone is well described by the Cam Clay model as is already reported. In this paper, the yielding of the intact diatomaceous mudstone is firstly introduced. The yielding of discontinuous specimens of diatomaceous mudstone having a plane of separation artificially made at three different angles is then investigated. Major conclusions obtained in this study are as follows. (1) The patterns of yielding behaviour are classified into two categories depending both on the angle θ of the plane of separation and on the confining pressure. (2) The yield locus of the specimen which slides along the plane of separation is represented by q = β(θ)·p', while the yield locus of the specimen which fails without sliding along the plane of separation is represented by q/p'+Mln(p'/p_c')=0, i.e., the Cam-Clay model. (3) The yield locus of the specimen having the plane of separation at an angle of 37.1°≤θ≤90° to the horizontal plane is represented by the combination of two types of yield functions mentioned above depending the magnitude of confining pressure. (4) The shape of p' vs volumetric strain ε_vy curve coincides with that of intact specimen regardless of the pattern of yielding.