As the construction of bridges and tunnels on/in soft rocks increases, it has been required to make clear the strength and deformation characteristics of soft rocks. According to the previous works, soft rock is found to be regarded as strain-hardening-softening inelastic and time dependent material with dilatancy (Akai et al.(1977a), Akai et al.(1977b)). Furthermore, new failure criteria were proposed for both peak and residual strengths of the intact soft rock as shown in Fig. 1(Adachi et al.(1981)).
(Figure in full paper)
It can be also seen in the figure that the upper bound of strength of rock mass corresponds to the peak strength of intact rock, whereas the lower limit is bounded by the residual strength, and that the rock mass strength in the field at least within these limits.
However, the peak strength of material shows the following well-known time dependent behaviors. Namely, the strength increases with increase in rate of strain, and the creep failure may take place under a sustained creep stress significantly less than the peak strength obtained by a so-called standard shear test. Therefore, to precisely analyze the long term stability problems relating to soft rock mass, it is necessary to determine the long term strength of the material.
The objective of this study is to propose a method to predict the long term strength of soft sedimentary rock by using drained creep test results. This method was derived based on the empirical evidences found from the triaxial creep test results on Ohyastone (porous tuff) deposited in Tertiary Period.
Saturated samples of Ohya-stone (porous tuff) deposited in Miocene Epock of Tertiary Period were tested as in the previous study. It is felt that this material is fairly representative of and exhibits typical mechanical behaviors of soft rock.
Physical properties of the material are as follows; void ratio, e, is 0.72, dry density, γd, is1.44 tf/m 3, wet density, γw, is 1.84 tf/m3 and specific gravity, G s, is 2.48.
Cylindrical specimens were prepared in 5 cm of the diameter and 10 cm of the hight, and were saturated by applying vacuum to a vessel in which specimens were submerged in water for at least 24 hours.
Drained creep tests were carried out prescribed sustained creep stress,(σ1 -σ3), after specimens of intact rock had been isotropically consolidated under some desired confining pressure, σ3, i.e., 1, 3, 5 and 9 kgf/cm2 in this investigation.
As the test results, Fig. 2(a) shows the change of deviatoric strain, e 1(= e 1- v/3; where ε1 is axial strain and v is volumetric strain), with time, t. In the figure, it is clearly seen that the creep deformation is strongly affected by the sustained creep stress intensity. The creep deformation curves are the typical ones and in the case of 1 kgf/cm2 of confining pressure the creep failure took place within the time duration of 104 min when the creep stress was higher than 40 kgf/cm2. And the time to creep
