INTRODUCTION

The mechanical properties of soft sedimentary rock, which is extensively distributed in Japan, vary according to the constituent grains and their degree of consolidation, nevertheless the influence of the pore water contained in voids is common to virtually all soft rocks. They also display strain softening and creep behavior which are the causes of progressive and delayed failure. The fundamental studies on these characteristics have been carried continuously on. Above all, Akai & Adachi et al. have continued experimental research into characteristics of porous tuff, and have released results of interes (1979, 1980 etc.). This paper takes relatively homogeneous mudstone of soft sedimentary rock as its subject and discusses its mechanical behavior, based on the results of various triaxial tests which have been performed up to the present. In order to determine suitable methods for testing mudstone, several preliminary investigations were undertaken, concerning with minute testing methods and techniques and size effect of mudstone's sample. The main items of the experiments were triaxial compression tests under wide range of strain rate and creep tests with different stress levels, in which the confining Pressure was applied from 0 to 100kgf/cm2. Actually, due to contemplations from the Point of engineering view, regions of overconsolidation state below a consolidation yield stress have here been made the major focus, where consideration was given to the mechanical properties of mudstone and their time-dependent characteristics.

MATERIALS AND APPARATUS FOR TESTING
Testing materials

The samples used in this study were Neogene silty mudstone of which physical properties are listed in Table 1.

(Table in full paper)

It had a uniaxial compressive strength of approximately 28kgf/cm2 and a modulus of deformation about 5000kgf/cm2. Its yield stress shown by a high pressure oedometer test averaged 50kgf/cm2 due to be over-consolidated to the overburden conditions. In order to examine the differences of characteristics on size effect, the diameters of specimens varied beween 35mm and 300mm. No large difference was noticed in the strength or coefficients of creep deformation except the result by 35mm diameter specimen. Then, this mudstone is considered to be isotropic, homogeneous and massive without size effect. So the cylindrical specimens were prepared in 50mm of the diameter and 100mm of the height in this experiments.

Apparatus used and preparation for Testing

The stain controlled. loading apparatus was used in the triaxial test. The pressure to confine a specimen was applied up to 100kgf/cm2 and initial back pressure of 3 to 6kgf/cm2 was determined according to the testing conditions. As illustrated in Fig. 1,the triaxial cell was constructed of inner and outer cylinders with the aim of measuring volumetric strain. The developed device employing a non-contact deformation sensor is also installed on it, in order to make continuous measurements of the specimen's volumetric changes and drained water volume over a long period of time(Tanaka 1979).

(Figure in full paper)

In addition to volume of water, the values of displacement, stress and pore pressure are recorded together by a data logging system, and a computer is used to process the data.

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