ABSTRACT

The rock fracture geometry data provided by Swedish Nuclear Fuel andWaste Management Company were used to develop a 3-D stochastic fracture geometry network model for a 30m cube of Äspö diorite rock mass located at a depth of 485m at Äspö Hard Rock Laboratory, Sweden. This fracture geometry network model was validated. A New procedure is developed to estimate rock block strength and deformability in 3-D allowing for the anisotropy and incorporating statistical variability associated with fracture geometry for the selected cube. The laboratory test results available on intact rock and fracture geomechanical properties, and field test results available on in situ stress for Äspö diorite were used in the said estimations. The mean rock mass strength was found to be 47% of mean intact rock strength of 297MPa at the depth of 485 m. The mean rock mass modulus was found to be 51% of the intact rock Young's modulus of 73 GPa. The rock mass Poisson's ratio was found to be 21% higher than the intact rock Poisson's ratio of 0.28. These percentages indicate the level of weakening of the rock mass due to the presence of fractures. The ratio of mean major principal rock mass strength/mean minor principal rock mass strength turned out to be 1.28. The ratio of mean major principal rock mass modulus/mean minor principal rock mass modulus turned out to be 1.21.

1 INTRODUCTION

Estimation of rock mass strength and deformability is important in evaluating rock mass stability of nuclear waste disposal sites. Swedish Nuclear Fuel andWaste Management Company (SKB) provided a limited number of fracture geometry data, in-situ stress data, and intact rock and fracture geomechanical property data applicable for the 400–500mdepth region at the Äspö Hard Rock Laboratory (HRL), Sweden to estimate rock mass strength and deformability for several 30mcubic blocks, each having a particular lithology. This paper reports estimation of strength and deformability in three dimensions (3-D) for Äspö diorite lithology, which is one of the lithologies that exist at Äspö HRL. A new procedure is developed to estimate rock mass strength and deformability in 3-D allowing for the anisotropy and incorporating the statistical variability associated with fracture geometry. In conducting this research, it was necessary to use only the information provided by SKB. For Äspö diorite, raw fracture data were available only from one borehole labeled KAS02.

2 STOCHASTIC 3-D FRACTURE GEOMETRY NETWORK MODEL FOR ÄSPÖ DIORITE ROCK MASS
2.1 Software used for modeling

Kulatilake (1988) completed a software package named FRACNTWK based on information given in more than ten journal papers that he and his co-workers published between 1984 and 1997 on the topic of fracture geometry characterization and network modeling (Kulatilake 1985, Kulatilake et al. 1990a, b, 1993b, 1996, 1997, Kulatilake & Wu 1984a, b, c, Wathugala et al. 1990).

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