This paper describes the methods and techniques used at the Laboratório Nacional de Engenharia Civil to characterize quantitatively the geometric parameters defining jointing in rock masses. In particular statistical methods used to establish and analyse the homogeneity of rock masses are explained, and the procedures used to determine the areas of and spacings between joints of the occurring sets are presented. Examples illustrating the application of these methods are given.
On decrit les methodes et les procedes employes par le Laboratório Nacional de Engenharia Civil pour caracteriser quantitativement les paramètres geometriques definissant la fissuration des ma ssifs rocheux. On indique, en special, les conditions que les massifs rocheux ou leurs parties doivent satisfaire, pour permettre l'etude de leur fissuration par des methodes statistiques, ainsi que les procedes employes dans la determination des aires et des distances entre les fissures d'un même système. On presente des exemples d'application.
Es werden die Methoden und Ausfuehrungsverfahren beschrieben, die vom Laboratório Nacional de Engenharia Civil zur quantitativen Charakterisierung der geometrischen Parameter der Gebirgsklueftung angewandt werden. Besonders werden die Bedingungen angegeben, die erfuellt werden muessen, un Gebirge oder Gebirgszonen Objekt einer Klueftigkeitsuntersuchung durch statistische Methoden zu machen; auβerdem werden die zur Bestimmung von Flache und Abstand der Kluefte in den auftretenden Kluftscharen benutzten Verfahren angegeben. Es werden zwei Anwendungsbeispiele gegeben.
The consideration of rock masses as discontinuous solids may have many practical applications. However, these applications are limited until the characteristics of the discontinuities can be determined quantitatively. Therefore, it is not surprising to observe in the literature a general trend towards improving procedures and theories used to establish these characteristics. This paper describes the methods developed at the Laboratório Nacional de Engenharia Civil (LNEC) to quantitatively characterize the geometric parameters of joints in rock masses. However the word «joint» is used in a broad sense. It includes, regardless of their origin, all discontinuities subdividing the rock mass, which require statistical methods for their analysis. Large discontinuities, such as faults, generally have to be considered on an individual basis and are not dealt with in this paper. Nevertheless, much of what follows will also apply to them.
Joints can be considered as plane surfaces, or preferably, as flat prisms having a very small height with respect to their base dimensions. Their orientation in space is, for practical purposes, defined by the attitude (*) of their bases. Thus, each joint (j) is geometrically defined, with respect to the rock mass, by the following parameters: - strike (σί) - the angle, measured clockwise from North, between a vertical North-South plane and a horizontal line in the joint plane. The strike is described as «geographic» or «magnetic» depending upon the north reference, dip (δί)- the angle of a steepest descent line of the joint plane with respect to a horizontal plane; together with the direction towards which the joint dips, - area (Ai) - the area of the joint surface, - thickness (ti) - the mean height of the joint prism, - co-ordinates of the joint center. A series of subparallel joints having a similar origin and type of infilling are defined as a set (S). A joint set is geometrically defined by the following parameters: - strike (σS) - the mean strike of the component joints, - dip (δS) - the mean dip of the component joints, - area (As) - the mean area of the component joints, - thickness (ts)-the mean thickness of the component joints, - spacing (ss) - the mean distance between the component joints. It is defined as the ratio of any given volume of the rock mass to the sum of the areas of the component joints of this set contained in that volume.
For each detected joint in the rock mass the following parameters are determined: σi, δi, ti, as well as the intercept of the joint with the surface of the rock mass (e. g., ground surface, drift walls, trench walls, etc.). This intercept is located in plan and in elevation. However, experience has shown that this value has little practical interest. Similarly, the field determination of the distance between joints of a given set seems to be of little interest. In fact, due to the normally limited areas of the joints, the projections of the joints of a given set on a plane parallel to this set generally do not coincide, so that it is not possible to say which joint is adjacent to which. Consequently the distance between joints cannot be accurately defined. As the joints present in a rock mass are usually very numerous, field operations are as a rule restricted to small areas. Thus, it is usual to resort to observing sample areas of the rock masses, taking care to secure a representative sampling.
