INTRODUCTION

It is especially important to reach the characteristics of deformation and permeability of fractured rock for the stability analysis in the rock mechanics. Many studies have been done on the failure condition of rock-like materials and the post-failure behavior of rock has been investigated using the various types of stiff compression testing machines. However, it is very difficult to explain systematically the characteristics of deformation including the failure condition and the post-failure behavior such as the strain hardening and softening obtained by the experiments based on the theory of elasto-plasticity.

The authors have investigated the deformation behavior and the mechanics of failure of rock-like material such as sand stone, Oya-tuff, Kobe-tuff, marble and cement mortar considering the various states of stress and strain using the true triaxial compression testing machine(Kawamoto & Tomita 1970, Kawamoto & Saito 1974). The factors, such as strain hardening, strain softening, anisotropy, change of deformation modulus at unloading and reloading, residual strength and change of elastic and plastic strains, can be considered to understand the change of characteristics of deformation of rocks after failure in engineering practice.

In this paper, a new type of stiff compression testing machine is introduced and the experimental results obtained by using this testing machine under uniaxial and tri-axial stress states of some rock-like materials are shown. Some of the relations mentioned above are discussed based on the experimental results, and the relations between the strain history and the permeability of rock after failure are discussed as well.

A NEW TYPE OF STIFF TESTING MACHINE AND LOADING PATTERN

Many of the stiff testing machine, hitheto, are controlled by the servocontrol system (Runnel & Fairhurst 1970, Peng & Podnieks 1972, Kawamoto & Tomita 1970). The servocontrol system has the difficult points about the very small instantaneous control of oil pressure at the failure of specimen and the decrease of stiffness of machine due to the compressibility of oil. Therefore, in this study, we used a new type of highly stiff machine which control the strain rate of specimen by the mechanical control system (Tanigawa, Nishikawa & Kosaka 1977, Kawamoto & Tokashiki 1979).

(Figure in full paper)

As shown in Fig. 1 the special feature of the testing machine is that its stiffness is increased by arranging two steel stiff columns parallel to the specimen and the length of these columns is controlled mechanically by the displacement of the wedges installed under the columns. The testing machine used has the maximum loading capacity of 100 ton and the deformation rate of specimen controlled in the range of 0.01 – 1.0 mm/min. The triaxial cell (maximum confining pressure of 400 kg/cm 2) for the specimen having the diameter of 50 mm can be installed between the platens of the machine.

The loading patterns used are as follows (cf. Fig. 2);

  1. static loading with monotonic increase of strain,

  2. cyclic loading under constant stress level and

  3. cyclic loading under incrementally increased strain level.

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