Nowadays, the hydromechanical stability analysis and design of various workings in jointed rock masses are numerically performed by computing some more or less stochastic models. It is an increasing evidence that modeling/or simulating a natural fracture network needs to be as representative of the rock mass structure as possible. Not only average structural, geometrical and geostatistical characteristics have to be taken into account but also their variability. We propose an analysis of a fracture network, using size and shape of blocks outlined by a fracture pattern. Five size parameters and six shape factors of blocks outlined by quite vertical fractures on horizontal benches are computed ; their values were derived from 2095 traces resulting in 401 blocks. The statistical and multivariate analysis of these parameters give rise to an efficient classification of the blocks with respect to the chronology of the rock mass geological fracturation.
The hydromechanical stability analysis and design of various working in jointed rock masses, such as rock slope, underground openings or dam foundations require the knowledge of the fracture pattern of the rock mass. It is a common purpose, analyzing ground water flow, to study a fracture network by the mean of structural and geometrical characteristics of the rock mass discontinuities (orientation, position, length, aperture, densities, spacing between joints, etc.). But, from a mechanical point of view, it is also possible and interesting to consider a jointed rock mass as a set of space filling blocks looking like polyhedra ; the faces, edges and vetices of the polyhedra being respectively distinct individual sub-planar fractures and intersection of two or three of them. So a rock mass is considered as a 3D mosaic of blocks, fractures being faces. In this paper we present an analysis of a fracture network of a rock mass located in the quarries of Comblanchien (Côte d'Or, France) using the size and shape of blocks outlined by the fracture pattern. As fractures are quite vertical, polyhedra become prisms and joint geometry is characterized by analyzing the size and shape of the two dimensional polygons out- lined on two partially superimposed horizontal benches -vertically 2.4 m apart-(Figure 1).
Data acquisition (orientation and position) has consisted in a systematic manual in situ survey of the fracture traces. Co-ordinates of a set of points distributed on the fracture traces were measured with reference to an arbitrary orthogonal system (Gervais, 1993). Then 564 and 1531 traces were surveyed on 912 and 2075 m² area surfaces with densities LA (length per unit area) respectively equal to 0.92 and 1.06 m/m² (Gervais et al, 1993). Blocks are defined as closed domains outlined by fracture traces; if any ending part of a trace exists in such a domain, it is canceled. Finally we have 126 and 275 blocks (Figure 1). Principal orientation of polygon edges are parallel to the main fracture directions, so they are expressing the regional geological history N020ºE, N040ºE(1)¸ N110-115ºE (2), N145-150ºE (3) and N060-080ºE (4), Figure 1.
