Bench stability in open pit mines : a methodology for jointed rock masses
The methods used to determine the overall gradients of the slopes in open pit workings aim to sufficiently eliminate all risk of failure on a scale comparable wich the pit walls. For this the slopes musc be made shallow gradual and reinforcing measures must generally be applied. Most often these consist of ensuring the drainage of the slopes and in limiting the effects of blasting (reinforcement is generally too costly in the case of very high slopes and deep planes of failure).
Grouping discontinuities in representative sets: influence on the stability analysis of slope cuts
Based on their orientation, different methods of grouping discontinuities in sets can be used; thus, three grouping methods and their influence on subsequent stability analyses are compared in this study. The first method combines the approaches of Priest, Shanley, Mathab and Yegulalp; the second method consists of grouping discontinuities manually based on an examination of density contour plots; and the third method is based on the spectral method. Unlike the others, the first method allows for the automatic determination of the number of discontinuity sets. The first and third methods were programmed in the Mathematica software environment, while DIPS software was used for the second method. Depending on the grouping process used, each discontinuity or only a portion of all discontinuities is assigned to a set. Therefore, the results in terms of the number of sets; the mean and dispersion of the sets; and the spacing of a given set differ with different methods. The consequences of a statistical analysis of orientation on the stability analysis were studied with simulations using RESOBLOK software, which couples the construction of 3D geometric block systems and a quick, iterative, limit-equilibrium stability analysis. Different fracture networks were generated with variable input parameters that were derived from previous statistical analyses of orientation and spacing. The software provides statistical outputs, such as the number and volume of unstable blocks, that can be compared across different cases. An application in a cutting slope near Ax-les-Thermes is presented; the influence of the grouping methods and of the uncertainty of slope orientation on stability indicators is presented. A synthetic indicator of global stability is also proposed.RésuméIl existe différentes méthodes de regroupement des discontinuités en familles en fonction de leur orientation. Trois méthodes de regroupement sont comparées ainsi que leur influence sur l’analyse de stabilité. La première méthode combine les approches de Priest, Shanley, Mathab et Yegulalp. La seconde consiste à regrouper “manuellement” les discontinuités en se basant sur les contours d’isodensité de fractures et la troisième se base sur la méthode spectrale. La première et la troisième méthode ont été programmées dans l’environnement Mathematica tandis que le logiciel DIPS a été utilisé pour le regroupement “manuel”. Selon la méthode de regroupement utilisée, l’ensemble des discontinuités ou seulement une partie d’entre elles sont affectées à une famille. La moyenne et la dispersion des orientations, ainsi que l’espacement entre fractures, diffèrent d’une méthode à l’autre. Les conséquences de l’analyse statistique des orientations sur l’analyse de stabilité sont étudiées par des simulations réalisées avec le logiciel RESOBLOK. Ce code associe une génération géométrique stochastique 3D d’un massif fracturé et une analyse rapide de stabilité, itérative, basée sur l’équilibre limite. Différents réseaux de fractures peuvent être engendrés en fonction des paramètres d’entrée statistiques issus des regroupements différents en familles basés sur l’orientation et de l’espacement calculé par famille. Le logiciel fournit en sortie des résultats stochastiques tels que le nombre et le volume de blocs instables qui peuvent être comparés d’un cas à l’autre. Une application à un talus en déblai à proximité d’Ax-les-Thermes est présentée: les méthodes de regroupement et les variations possibles de l’orientation du talus influencent les indicateurs de stabilité. Un indicateur synthétique de stabilité global est également proposé.
Discontinuity frequency in rock masses
Abstract The fracture or discontinuity frequency measyred along a line through a rock mass is often quoted as one single figure, yet the value can depend significantly on the orientation of the line through the rock mass. The amount of variation that could be experienced along boreholes or scanlines in different directions is a function of the discontinuity geometry, particularly the degree to which the discontinuities tend to be oriented in certain preffered directions. In general, rock masses with a few joint sets will exhibit much greater discontinuity frequency variation than those with many sets. The basic theory of discontiuity frequency variation is presented for the general case of N discontinuity sets which can be considered as the class intervals of a rosette or stereogram. Methods for constructing loci of discontinuity frequency and for establishing the magnitudes and directions of the maximum and minimum values of discontinuity frequency are included. These methods, applying for persistent or impersistent discontinuities, are illustrated by a case example. Some practical guidance is also given on measurment of discontinuity frequency in rock masses.