ARMA 16-XXXX Application of discrete fracture networks ( DFN ) in the stability analysis of Delabole Slate Quarry

The failure mechanism of rock slopes is mainly controlled by the strength and orientation of discontinuities within the rock mass. A realistic representation of the joint network within the rock mass is therefore an essential component of stability analysis of rock structures (e.g. rock slopes, tunnels etc.). Discontinuity persistence and connectivity are significant parameters which control the stability of rock slopes. A small percentage of rock bridges on the discontinuity surface can significantly increase its strength and prevent slope failure. Discontinuities within the rock mass are rarely fully connected. In practice, however, discontinuities are often assumed fully persistent due to the difficulties both in mapping and simulation of non-persistence. Discrete fracture networks (DFN) provide a rigorous and convenient tool for the simulation of joint systems within a rock mass. Utilizing statistical methods, DFNs consider the stochastic nature of some key parameters (e.g. persistence and orientation) within numerical models. Discrete fracture network engineering is increasingly used due to recent developments in discontinuity data acquisition techniques (e.g. ground-based digital photogrammetry and laser scanning). Recent development in geomechanical modelling codes and increased computing power have also allowed to either import DFN’s into models or to generate DFN’s within the numerical modelling code itself (e.g. 3DEC). This paper describes the use of photogrammetry at the Delabole slate quarry in Cornwall, UK for remotely acquiring key discontinuity parameter data (orientation, intensity and length) and its subsequent use in developing statistically validated discrete fracture network parameters. The 3D distinct element code, 3DEC, is used for the DFN generation and subsequent stability analysis. Several realizations of the 3DEC-DFN models are run to investigate the stochastic nature of discontinuities within the quarry and their potential influence on the stability of the pit. Finally the simulation results are used to determine the slope instability mechanisms and determine the most likely areas of potential instability. geotechnical investigations have also highlighted the controlling influence of discontinuities on both the stability of the quarry and quarrying operations (Clover 1978; Coggan and Pine 1996; Costa et al. 1999). Terrestrial digital photogrammetry has significantly improved our ability to measure geometrical properties of rock mass discontinuities. This technique provides a promising supplement to the traditional rock engineering scanline or window mapping methods by overcoming some of their constraints such as accessibility and safety (Sturzenegger and Stead 2009). Many authors have demonstrated the application of this method in characterizing rock masses in underground (Preston et al. 2014) and surface applications (Sturzeneger et al. 2011). If properly georeferenced, a photogrammetry model can provide a valuable discontinuity data set which contains the spatial location of individual discontinuities. This data set then can be used to provide key discontinuity information (e.g. length, orientation and fracture intensity) as a discrete fracture network (DFN) to be incorporated into 3D numerical simulation codes such as 3DEC (Itasca 2014). Discrete fracture networks have been used in a wide range of geomechanical problems (e.g. large open pits, tunneling, block caving, reservoir geomechanics, etc.). Elmo and Stead (2010) incorporated a discrete fracture network within the FDEM code, ELFEN (Rockfield 2009), to realistically simulate the behaviour of fractured rock pillars. Vyazmensky et al. (2010) used a combined FDEM/DFN modelling approach to investigate the pit wall instability triggered by caving operations at Palabora mine, South Africa. In this paper long-range terrestrial photogrammetry is used to acquire geometrical properties of discontinuities as well as 3D geometry of the North-East wall of Delabole slate quarry. Discontinuity properties are then used to develop a realistic statistical representation of the fracture system using discrete fracture network. Several realizations of the DFN are then incorporated into 3DEC and the stability of the slope comprehensively investigated. In this paper, the process of digital data acquisition and 3DEC model setup is briefly explained, further details are provided in Havaej et al. (2015). Figure 1. The 2012 planar failure on the upper bench of the north-east Delabole Quarry slope showing the Floors, Grain and Shortahs discontinuity sets that caused the failure; the strike length of upper section of the failure is 50 m