The assessment of the stability performance of open pit highwalls has traditionally been based on consideration of surface movement rates. Although generally accepted in industry, this approach presents challenges for establishing appropriate threshold values to allow for safe mining operations and does not address the issue of total strain. Surface movement monitoring data from a limited number of open pit mining operations in North and South America has been evaluated in terms of highwall strain, rock mass quality and inferred mechanism of instability. Threshold strain levels for key stages of highwall stability proposed from previous work has been reviewed and are endorsed based on evaluation of additional data. An empirical highwall strain criteria for the assessment of highwall stability performance is postulated based on relatively consistent trends in highwall strain data.

L'evaluation du niveau de stabilite des hauts murs de carrières a traditionnellement ete basee sur les taux de mouvements de surfaces. Bien que cette approche soit generalement acceptee à travers l'industrie, elle presente des defis pour la definition de seuils appropries concernant la securite des operations minières et n'adresse pas la notion de deformations totales. Des donnees de mouvements de surface provenant d'un nombre limite d'operations minières à ciel ouvert d'Amerique du Nord et du Sud ont ete evaluees en terme de deformation des hauts murs, de qualite de la masse rocheuse et des mecanismes inferes d'instabilite. Des seuils de deformation pour differentes phases clefs de stabilite de hauts murs ont ete revus et sont endosses, base sur l'evaluation de donnes additionnelles. Un critère de deformation empirique pour l'evaluation de la stabilite des hauts murs est postule, base sur des tendances relativement consistantes de donnees de deformations de hauts murs.

Die Beurteilung der Stabilitat von hohen Felsböschungen basiert traditionellerweise von Oberflachenbewegungen. Obwohl dieses Verfahren im Allgemeinen akzeptiert wird, stellt es jedoch gewisse Probleme fuer die Festlegung von Grenzwerten fuer die Sicherheit dar. Die totalen Dehnungen werden bei dieser Betrachtungsweise vernachlassigt. Eine beschrankte Anzahl von Oberflachenbewegungs Daten von Bergbauwerke in Nord- und Suedamerika sind in Bezug auf die Dehnungen von hohen Felsböschungen, Felsqualitat und davon abgeleiteten Bruchmechanismen untersucht worden. Grenzwerte fuer die Belastung von hohen Felsböschungen wahrend den massgebenden Bauetappen aus bisherigen Projekten sind begutachtet und durch zusatzliche Untersuchungen bestatigt worden. Gestuetzt auf relativ konsitenten Trends, ist hier ein empirisches Verfahren der Dehnungen in hohen Felsböschungen zur Beurteilung der Stabilitat vorgestellt.

Introduction

Open pit mining results in the extraction of large volumes of rock and accompanying stress relaxation of the rock mass. The amount of stress relaxation that occurs will be governed by the overall geometry of the open pit in terms of the highwall angle, overall depth, and the rock mass quality. The stability performance of a large open pit highwall is affected by stress relaxation as the rock mass dilates and rock mass strength deteriorates. The amount of stress relaxation that occurs along open pit highwalls is commonly measured using survey-monitoring prisms.

Surface movement monitoring data can be evaluated to quantify the amount of movement as highwall strain. where the authors have been involved, and data provided by others, has been evaluated in terms of highwall strain, rock mass quality and inferred mechanism of instability. This paper discusses stress relaxation in open pit mining, defines highwall strain, outlines threshold strain levels for key stages of highwall stability and presents an empirical highwall strain criteria for open pit practitioners to consider as an additional approach for the assessment of highwall stability.

Stress Relaxation with Open Pit Mining

Open pit highwalls undergo unloading during the mining process.

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