Experimental characterization of strain-dependent behavior in a jointed rock mass and its numerical application to underground structures

Abstract

Discontinuities such as cracks, joints or fractures have a significant influence on the stress-strain characteristics in a jointed rock mass. Moreover, the prediction of the deformations and stresses around excavated jointed rock masses is important for the decisions concerning construction methods and support systems. In fact, it is difficult to consider all of the discontinuity systems and their mechanical characteristics. Thus, a more reliable and reasonable theoretical model is required to characterize the behavior of jointed rock masses under excavation. Generally, the strain during an excavation is less than 0.1%. In addition, it is only locally that strains are likely to exceed this value. At this small strain range, stress-strain relationship of jointed rock masses has nonlinearity. The nonlinearity is characterized by a decrease in the stiffness of the jointed rock mass above the linear threshold strain $(10^{-5} ~ 10^{-4}%)$, which is affected by considerably the level of insitu confinement. Current design methods based on numerical simulation consider the linear behavior of jointed rock masses using the properties of a jointed rock mass determined by a conventional triaxial test which cannot properly consider the effects of confinement and small-strain nonlinearity. Therefore, a new numerical analysis method is required that takes into account the nonlinear behavior of underground structure during excavation. In this study, an equivalent strain-dependent model is theoretically derived and numerically coded using the Fish function built into FLAC 2D to estimate the behavior of underground structure surrounded by jointed rock masses during excavation. The model is evaluated via the equivalent stiffness model incorporating a hyperbolic model that is used to fit the nonlinearity of the experimental data as obtained from a rock mass dynamic test. The equivalent stiffness model applicable below the linear threshold strain has yet to be verified....