ABSTRACT:

Slopes in ultrabasic rocks at the Jeffrey Mine in Canada have a history of major failures over the period 1970 – 86. Limiting stable angles are in the range 20 – 25 deg for 300m high slopes. The paper describes the major failures that have occurred and from the results of performance monitoring establishes design criteria. Details of instrumentation systems and drainage measures are also given.

RESUME:

Les pentes dans les roches ultrabasiques de la mine Jeffrey au Canada presentent un historique de mouvements importants de 1970 à 1986. Les angles pour des pentes stables sont de l'ordre de 20 – 25 degres pour des murs de 300 metres de hauteur. L'expose decrit les mouvements importants qui se sont produits et indique comment les resultats de surveillance des pentes ont permis de definir des criteres de conception. Une description detaillee d'instruments de mesure et de methodes de drainage est aussi presentee.

ZUSAMMENFASSUNG:

Boeschungen in ultrabasichem fels in Jeffrey Tagebau, Canada, haben historische, grosse rutschungen waehrend der periode von 1970–86. Die grenzwinkel fuer 300m hohe standfeste boeschungen liegen bei ca. 20–25 grad. Der artikel beschreibt die groessten rutschungen die in der vergangenen jahren geschenen sind. Die ergebnisse der messungen geben jetzt gute entwurfskriterien. Messungseinrichtungen und draenagebzw. entwaesserungsmassnahmen sind auch beschrieben.

INTRODUCTION

The south east area of the Jeffrey Mine at Asbestos, Quebec, Canada, has a history of major slope failures since 1970. In 1970 when the overall pit depth was some 240m (800ft), studies were commenced to try and determine the behaviour of the rock mass in relation to slope stability. These studies were prompted by the need for efficient ore and waste extraction and the occurrence of major instabilities in 1970–71, 1975 and in 1983, all below the adjacent town area as indicated on Fig 1. An aerial view of the 1970–71 failure is given on Plate 1. Excavation under marginal stability conditions is common to many mining operations. However at Jeffrey the proximity of the town area to the mine has led to the particular development of comprehensive monitoring systems to provide an overall control on stability conditions. The ultrabasic rocks are extremely complex structurally and in relation to strength characteristics. Groundwater also has a major influence on slope stability conditions at Jeffrey and is strongly influenced by seasonal variations. Owing to the complexity of the material behaviour, slope stability predictions at Jeffrey are based largely on observed performance of past and existing slopes, coupled with generalised back analyses of major failures. The paper summarises the geological and groundwater conditions at Jeffrey and gives an outline of past and current failures and their inferred mechanisms. A description of monitoring systems suited to the large scale slopes is provided and typical results are given. Recent developments at the mine are discussed together with results of drainage control measures.

2 GEOLOGICAL CONDITIONS

The occurrence of asbestos is often associated with weak, ultra-basic rocks such as peridotite, dunite and serpentinite that contain significant percentages of weak minerals such as olivine that can be hydrothermally altered or weathered to chlorite. A section through the most critical slopes with respect to ore recovery and town location is given on Fig 2 and shows a sequence of alternating peridotite and dunite strata dipping into the slope. Major shear zones up to 50m in overall width occur within the rock and are accompanied by many lesser sub-parallel features. These features consist of small platy rock fragments in a matrix of talcose and chloritic fines. The more competent rock strata are key elements in the overall slopes and have been the subject of several detailed appraisals to identify structural patterns that could control potential failure mechanisms. Although some localised structural patterns were identified and these in isolated cases could be linked to small scale wedge failures over one or two benches, the rock was generally devoid of any structural consistency or significant through going weaknesses that could adversely influence stability conditions. Attempts have been made to characterise the rock mass through extensive borehole drilling and surface mapping programmes. From these studies a generalised descriptive system of very poor to good rock, mainly based on drill core RQD assessments has been derived. Although such characterisations have been useful in providing a qualitative zoning for the rock mass, they have not correlated well with observed slope performance.

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