STEREOLOGICAL CHARACTERIZATION OF FRACTUR ED ROCK MASS - APPLICATION TO THE LIMESTONE QUARRIES OF COMBLANCHIAN (COTE-DOR, FRANCE)

The joint geometry of a rock mass located in the quarries of Comblanch ian (Cote d'or, France) is first characterized by means of stereologic al principles and next by analyzing the size and shape of two dimensio nal polygons (blocks) outlined by the fracture pattern on horizontal b enches in the quarries. Before processing a stereological estimation o f the fracture pattern surface density (B(A)), a systematic investigat ion of the lengths of the joint traces was performed on available and limited horizontal surfaces : thus we get B(A) values equal to 0.92 an d 1.16 m/m2 for two separated horizontal levels with and area equal re spectively to 912 and 2075 m2. According to stereological equations B( A) = pi/2 P(L) and S(V) = 4/pi B(A), both the length per unit area of a system of lines (joint traces) on a plane and the surface area per u nit volume of a system of non necessary plane sw-faces (fractures) can be estimated. Thus the forecoming sampling design enables us to infer B(A) with a relative error of about 3 %. A minimal surface area equal to 1000 m2 that can be split into non necessary contiguous subsets, e ach with a minimal area equal to 400 m2 must be analyzed; thus B(A) va lues are inferred by counting, on each subset, the intersections betwe en the fracture pattern and a set of forty IUR lines (Isotropic Unifor m Random). Easy to manage, this procedure is efficient in a working qu arry. Counting intersections coming both from horizontal benches and v ertical faces allows us to estimate the volumic density of fractures ( S(V) = 1.14 m2/m3); then, assuming the mean aperture of fractures is a bout 0.1 mm, we get an estimation of the macroscopic porosity of the r ock mass (e = 0.1 %). Analyzing the size and the shape of blocks, assu ming fractures are quite vertical, leads to the characterization of th e geometrical structure of the rock mass. Blocks outlined on horizonta l benches look like polygons ; for each convex hull of these polygons, four size parameters are analytically calculated : the area A, the pe rimetre B, the radius R(c) of the smallest circle containing the polyg on and the radius R(i) of the largest circle contained in it. Then two adimensional shape factors {x = R(i)/R(c); y = 32A/piB2} are used to visualize the characteristics of the blocks in an {Ox,Oy} diagram. The cluster of points with coordinates {x,y} - one point for one block -s hows that blocks are neither specifically isotropic nor particularly e longated. The middle part of the diagram (x epsilon [0, 1], y epsilon [0, 8/pi2]) covered by the cluster corresponds to shapes like rhombus, parallelograms, hexagons and/or irregular polygons with 4,5 or 6 edge s. The main shape is something like a rhombus or a parallelogram with an angle of 60 to 80-degrees designed from the two known main directio ns of fractures (N040E, N115E). The cluster dispersion is due to the t runcation of blocks by fractures with other directions (particularly N 145-150-degrees-E).