The intervention deals with the intensity of a joint set, to give an example of the uncertainties connected to the knowledge of the different parameters of the rock mass jointing. The basic concepts related to the intensity of a joint set, as well as the applicable statistical distributions are presented, the intrinsic uncertainties resulting from those distributions are discussed, and the formulas for the quantitative determination of the intensity of a joint set, as well as of the uncertainty connected to that determination are given.


L'intervention traite de l'intensite d'une famille de diaclases, pour donner un exemple des incertitudes liees à la connaissance des differents parametres du compartimentage des massifs rocheux. On presente les concepts basilaires relatifs à l'intensite d'une famille de diaclases, ainsi que les distributions statistiques applicables, discute les incertitudes intr insèques resultantes de ces distributions, et donne les formules pour la determination quantitative de l'intensite d'une famille de diaclases, ainsi que de I'incertitude liee à cette determination.


Der Beitrag behandelt die Starke einer Kluftschar, um ein Beispiel der mit der Kenntnis der verschiedenen Parameter der Felsklueftung verbundenen Ungewiβheiten zu geben. Die mit der Starke einer Kluftschar in Beziehung stehenden, grundlegenden Begriffe, sowie die anwendbaren statistischen Verteilungen werden vorgestellt, die sich aus jenen Verteilungen ergebenden, wesentlichen Ungewiβheiten werden erörtert, und die Formeln fuer die quantitative Bestimmung der Starke einer Kluftschar, sowie der mit jener Bestimmung verbundenen Ungewiβheit werden gegeben.

1.1 Intensity

The intensity I of a joint set describes the degree of jointing that the whole lot of the joints of that set have induced in the rock mass, independently of the individual extent of each joint. The intensity of a joint set is, therefore, quantified by the sum of the areas of the joints of the set which occur in a unit volume of the rock mass.

1.2 Spacing

The spacing s of a joint set is the inverse of its intensity, i.e., the volume of the rock mass in which the sum of the areas of the joints of that set that occur in it, corresponds to a unit area (Grossmann 1967). As is clearly shown in Fig. 1, the distance between adjacent joints of a joint set, in many cases, does not have a unique definition. The spacing of a joint set should, therefore, not be defined as the distance between adjacent joints of the set.

2. 1 Poisson process

In a homogeneous rock mass, the occurrence of the joints of the different joint sets can, usually, be modelled by a Poisson process (Grossmann 1988) (Fig, 2).


As in many practical situations the size of the rock mass volume involved is such that the mean number of joints of the considered joint set which occur in It, is small, the corresponding coefficient of variation may easily reach 100 % or even more. Fig. 2 shows a typical example of a tunnel in a homogeneous rock mass, with a joint set which can be described by a Poisson process. Although this jointing model is currently accepted as true by the majority of those dealing with jointing studies, many practitioners still are not aware that what they describe as a succession of zones with different jointing intensities, are just parts of the same homogeneous rock mass, in which the random character of the homogeneous jointing has provided a different number of Joints. Due to the random character of the jointing, it will never be possible to forecast the exact number of joints in a given volume of rock mass.

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