SYNOPSIS:

Open pit coal operations in the Alberta foothills will encounter difficult geomechanical conditions including steeply dipping coal bearing strata and inner weak coal and bentonite seams. The variation of the rock mass rating can be considered as a model of the variation of the geomechanical conditions of the mine, and was achieved by using a site specific classification. A stress-strain approach to shear strength evaluation of the rock behaviour allows the utilization of the full strength along the discontinuities in the design of slopes for the mines where important stress relief is expected.

RESUME:

Les mines de charbon à ciel ouvert des contreforts des Rocheuses de l'Alberta vont rencontrer des conditions geomecaniques difficiles, y compris des interlits de bentonite, des niveaux charbonneux à pendage raide, ainsi qu'un charbon à faible resistance. La variation de la valeur totale pour la masse rocheuse (RMR) peut être consideree comme un modèle de la variation des conditions geomecaniques dans la mine. Ce modèle est realise en utilisant une classification specifique, developpee pour le site. Une approche par contrainte-deformation pour evaluer la resistance au cisaillement permet l'utilisation de la pleine resistance le long des discontinuites lors de la conception des pentes pour les mines où un important relachement des contraintes est envisage.

ZUSAMMENFASSUNG:

Der Übertagekohlenabbau im Vorgebirge Albertas stößt auf schwierige geomechanische Bedingungen, einschließlich steil eintauchender Kohleschichten mit Weichkohle- und Bentonitsaumen. Die jeweiligen Gebirgsgueten stellen die geomechanische Grundbedingung eines Kohlebergwerks dar, was durch eine spezielle Gebirgsklassifizierung erreicht wird. Spannungs-Verformungsmessungen zeigen, daß der Gesamtwiderstand entlang Trennflachen in Rechnung gestellt werden kann. Beim Abböschen sind bedeutende Spannungsentlastungen zu erwarten.

Future large open pit coal mines in the Canadian Rocky Mountain foothills will extend for several, kilometers (15 to 25 km) along the, seam sub-crops. Prior to development, studies must, be carried out to evaluate the technical and economic feasibility of the project. The results of these studies-are submitted to the Alberta Energy Resources Conservation Board to obtain approval of a mine development permit and subsequent license to mine under the Coal Conservation Act. The licensing process ensures that-the proposed mine operation will maximize resource extraction while minimizing environment and social-economic impact. Geotechnical evaluation of the operation is required during this process to allow proper evaluation of the concerns. Assessment of the viability of the coal exploitation by open pit methods requires definition of the geomechanical condition in the field and design of slopes, mine dewatering systems, and dumps. The geomechanical parameters have a significant impact on both the technical and economic feasibility of the proposed development. Proposed open pit coal operations in the Alberta foothills will affect large areas. During development of these coal deposits mine operations will encounter difficult geological conditions including steeply dipping coal bearing strata of low strength which will control. the design of both the highwall and footwall slopes. In evaluating the stability of the pit slopes the geological structure and its influence on kinematics and rock mass strength should be assessed. This requires the determination of both the shear strength of discontinuities and the strength of the intact rock. Geomechanical evaluation of such large areas makes any assessment of rock structures and extrapolation of rock properties very difficult. Any field observation or laboratory test data could be challenged as being non-representative. Certainly a variation in rock strength is as important as the variation in structural factors. As the mining method employed tends to parallel the geological structure, the possible modes of failure tend to be similar along the entire pit wall. However the most critical failure mechanism in anyone section of the pit could vary with the local geological conditions. The stratigraphy of the Alberta foothills consist of,a complex sequence of interbedded sandstones, siltstones, and mudstones, with local bentonite seams associated with the coal. The intense fracturing and related system of discontinuities dictates that evaluation of the strength along joints be fully understood and assessed prior to final mine design. Because both the geologic conditions and the expected rock behavior will vary within the stratigraphic sequence, pit slope design may vary within individual operations. Ideally, under such conditions, more than one mining method would be recommended. However, it is generally not practical to utilize more than one mining method (ie. dragline vs. truck and shovel) in an actual operation. Design of highwall and footwall pit slopes involves the identification of the principal kinematically possible - failure modes including buckling, translation failure and non-circular slip surfaces. This is based on the evaluation of the distribution of discontinuities in areas of similar geological conditions. After identification of the principal mode of failure, an estimate of the stability of the slope is made through the factor of safety as a ratio of the maximum shearing resistance that can be mobilized in the direction of movement to the shearing forces active in the slope.

This content is only available via PDF.
You can access this article if you purchase or spend a download.