Several researchers have done on soft rock in experiment and numerical modeling. They were commonly known that the mechanical behavior of soft rocks resented elasto-plastic, dilation, strain hardening- softening at low confining stress level and time dependency. In soft rock, dilation occurs in the mechanical process, and it is assumed that permeability does change. Therefore, when the stability of the tunnel and the cavern in soft rock is discussed, both influences of groundwater and dilation should be considered. In this study, the authors will discuss the long term tunnel stability considering the permeability and volumetric strain using the hydro – mechanical coupled analysis. Consequently, it is confirmed that the difference of permeability is strongly affected to the mechanical behavior in the tunnel excavation.
In general, the mechanical behavior of soft rock is elasto-plastic, dilational, strain-hardening/-softening and time dependent. Moreover, during the shearing process, the mechanical behavior of soft rock is largely dependent on the confining stress and the pore water pressure. When a tunnel and/or underground cavern is excavated in the soft rock, the remarkable mechanical behavior of soft rock should be considered. The pore water pressure depends on the permeability of soft rock and the permeability also depends on the porosity of material. In the shear process, the various volumetric strain appears and it is thought that the porosity of soft rock is changed. Therefore, the change of the permeability affects the response of the pore water pressure. It is thought that the mechanical behavior of soft rock is changed. The relation between the mechanical behavior and the pore water pressure is interactive and we must consider both factors when the geotechnical issues on soft rock are discussed. Many researchers have described the mechanical behavior of soft rock both in experiment and numerical modeling.