An excavation damaged zone (EDZ) develops under an excavation-induced redistributed stress in a fractured rock mass. Natural fractures further develop and new fractures are formed due to a tunnel excavation. As one of the intrinsic properties of a fractured rock mass, seismic velocity such as P-wave velocity will change when a tunnel is excavated in fractured rock mass. To our knowledge, there is limited research on seismic velocity change due to tunneling in a fractured rock mass. A novel numerical study of seismic velocity (such as P-wave velocity) change due to a tunnel excavation in a fractured rock mass is carried out in this paper. The stress in a fractured rock mass will redistribute after a tunnel is excavated. Fracture apertures change accordingly. Using discrete fracture network (DFN) modeling approach, fractures are generated in a rock mass. Fracture lengths, orientations, apertures, etc. are mathematically represented. Based on empirical equations, fracture apertures under the in-situ stress (before and after a tunnel excavation) can be calculated. To verify our proposed method, a case study is carried out. It shows that our proposed method can be used to estimate seismic velocity change due to a tunnel excavation in a fractured rock mass.

1. Introduction

An excavation damaged zone (EDZ) may be developed during a tunnel excavation in a rock mass where there are pre-existed or excavation-induced fractures (Lei et al., 2017b). It is observed that several physical or mechanical properties (e.g., hydraulic conductivity, elastic moduli, seismic velocity, electrical resistivity,) may change in an EDZ formed in an intact or fractured rock mass. The EDZ is formed or further developed because of fracture initiation and evolution due to a tunnel excavation in a rock mass. Various numerical algorithms have been focusing on estimating fracture initiation and evolution in a fractured rock mas based on continuum approaches (e.g., finite difference methods, finite element methods, boundary-element methods), discrete element methods and combined finite-discrete element methods (Lisjak and Grasselli, 2014). Based on those algorithms, it shows that EDZ is formed and developed due to a tunnel excavation. A lot of research has been carried out to estimate hydraulic conductivity and elastic moduli quantitatively after a tunnel excavation in a rock mass. Hydraulic conductivity and elastic moduli are necessary parameters for mechanical or hydro-mechanical simulations for tunneling. There are still quite limited research talking about estimating seismic velocity or electrical resistivity quantitively after a tunnel excavation in a fractured rock mass. There are in-situ surveys available to get seismic velocity tomography or electrical resistivity tomography before and after a tunnel excavation in a rock mass. Through these data, the EDZ size can be estimated. Thus, it is interesting to estimate seismic velocity (e.g., P-wave velocity, S-wave velocity) after a tunnel excavation in a fractured rock mass. The paper focuses on showing a novel method which estimates seismic velocity change after a tunnel excavation in a natural fractured rock mass. It is organized as follows. A natural fracture network generated by the discrete fracture network (DFN) modeling approach is described. The empirical equations which calculate seismic velocity of a rock mass which include fractures are presented. At last, a case study is used to show our proposed method can be applied to estimate seismic velocity change due to a tunnel excavation in a natural fractured rock mass.

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2019. Japanese Society for Rock Mechanics. Permission to distribute - International Society for Rock Mechanics and Rock Engineering
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