Abstract
In the design of tunnel support, the behavior of the rock mass around a tunnel and the stress acting on the tunnel support may be predicted using a numerical analysis. However, in such a numerical analysis, it is common to assume that each stratum comprises a homogeneous material, ignoring the heterogeneity of the geomechanical properties inherent to the rock mass. For this reason, it is not unusual for the results of the numerical analysis to differ from the actual behavior. We performed a tunnel excavation analysis considering the heterogeneity of the geomechanical properties in the rock mass to investigate the local increase in the tunnel support stress obtained in the 350 m gallery at the Horonobe Underground Research Laboratory. The results revealed that, in order to predict the locally increased support stress in advance, it is necessary to carry out a tunneling excavation analysis considering the heterogeneity of the geomechanical properties. It was also revealed that the scale at which the geomechanical properties fluctuate is an important factor.
In the design of tunnel support, a numerical analysis such as the Finite Element Method (FEM) or the Finite Difference Method (FDM) may be applied. At that time, the behavior of the rock mass around the tunnel and the stress acting on the tunnel support are generally predicted. However, in such a numerical analysis, it is common to assume that each stratum comprises a homogeneous material, ignoring the spatial variation (i.e., heterogeneity) of the geomechanical properties (e.g., Young's modulus, uniaxial compressive strength, etc.) inherent to the rock mass. For this reason, it is not unusual for the results of the numerical analysis to differ from the actual behavior and, in some cases, the differences between reality and the predictions are large enough to cause serious accidents. For example, some of the measurement results of the support stress in the 350 m gallery at the Horonobe Underground Research Laboratory (URL) (See Fig. 1), which was constructed in a rock mass where the values of the geomechanical properties vary, greatly exceed the results of the numerical analysis assuming that the rock mass is homogeneous (Okazaki et al., 2016). Hence, to ensure adequate safety during a tunnel construction, it is necessary to consider the heterogeneity of the geomechanical properties, and to predict and evaluate its effects on the behavior and stability of the tunnel. However, there are currently very few examples of predictions/evaluations of the effects of the heterogeneity of the geomechanical properties on the behavior and stability of tunnels (e.g., Su et al., 1969 and Kumasaka, 2005).
