For the determination of the rock’s geotechnical model will be, therefore, illustrated the stages of joints survey, referring to geo-structural conditions (spacing, aperture, persistence) and to hydraulic and joint strength conditions (roughness, wall strength, degree of alteration, filling materials).
The procedure used for performing the survey is described in the ISRM recommendations, “Suggested Methods for the Quantitative Description of Discontinuities in Rock Masses”.
PROJECTIONS AND PLOTTING
• WULFF DIAGRAM
Is the stereographic projection of the terrestrial meridians and parallels on a plane passing through the center and to the two poles. It is a isogonal projection, in which the angles between the individual planes are retained on the projections, for which the areas defined by the intersection between two parallels and two meridians are strongly distorted from the center towards the edges of the net.
• SCHMIDT-LAMBERT DIAGRAM
It is used to prevent the areal distortion of the Wulff diagram and therefore suitable for statistical interpretations. The great circles are represented by arcs of ellipses.
• EQUAL AREA, EQUIANGULAR AND STEREOGRAPHIC POLAR DIAGRAM
They are similar to the previous ones, where are represented the joint poles.
• STAR DIAGRAM
Representation of joints’ measurements. the observations are represented on a circular reference marked from 0 ° to 360 ° and radial lines at intervals of 10 °. The observations are grouped in 10° sector to which they belong. The number of observations is represented in a radial direction relative to the circles corresponding to 5, 10 and 15 observations.
• ISODENSITY DIAGRAM
From the distribution on the poles grid corresponding to a significant data set can be recognized a number of joint families. To achieve this goal are plotted the isodensity diagrams, place of centers of unit areas that contain the same number of poles. The unit area is conventionally equal to 1% of the total area of the diagram. The method used to investigate the distribution of the poles’ density is developed by Denness who has divided the reference sphere in 100 elementary cells.
• PERCENTAGE OCCURENCE GRAPHICS
It is a type of graphic (histogram) where are represented, as a percentage, all occurrences of joints in the subdivision classes of the joint features to be represented (aperture, spacing or persistence).
• MODAL VALUE
In a histogram are shown, for each joint family, the distribution of the characteristic (aperture, spacing, persistence). The user can plot sequentially all the measured families.
The purpose of the Markland test is to quantify the possibility of rock wedge failure in which the sliding occurs along the intersection line of two planar joints. The test is used to evaluate critical joints, so it must be followed by more detailed stability checks. A further development of the Markland test has been implemented by Hocking. The test in fact provides for the possibility that the sliding occurs along one of the planes that constitute the wedge and not only along the intersection line of the two planes themselves.