Abstract

The correct application of large crater blasts in unfrozen tar sand is an effective means of resisting frost penetration during winter. This results in much higher productivity and considerably less down time with excavators during the winter operation. The resulting flatter angle of repose of the highwall has readily been accommodated with slight modification to B.W.E. operating procedure.

Other mining schemes call for large draglines to be located on top of the highwall which can be as high as 180 feet. The question now arises as to the effect that blasting might have on the stability of such a situation in which the highwall includes potential planes of weakness. This paper applies blast vibration data from large scale blasting in tar sand to the stability analysis for typical production situations likely to be encountered and indicates safe scaled distances to be employed between drag line and blast.

Introduction

The Athabasca tar sand deposit in Alberta contains a vast reserve of bitumen estimated to be greater than 700 billion barrels in place. The tar sands average approximately 170 feet in thickness and are covered with overburden varying from a few feet to more than 2,000 feet in depth. Figure 1 presents the overburden thickness versus cumulative percent of the deposit for the Athabasca field. The present price of oil, surface mining technology and economic bitumen cutoff grade of approximately 8% dictates the maximum stripping ratio of the order of 1:1 (one cubic yard of overburden per one cubic yard of tar sand). Figure 1 therefore indicates that less than 10% of the Athabasca deposit can be mined using surface mining methods. This translates into approximately 35 billion barrels of recoverable synthetic crude oil. The balance of the reserve must be recovered using alternative methods.

At present, surface mining operations are under way using bucket wheel excavators as the principle tar sand mining equipment. These wheels feed belt conveyor systems which transport the tar sand to the bitumen extraction plant. Some future mining schemes propose the use of walking draglines as the principle excavation equipment. The tar sand will be deposited in windrows on top of the overburden where it will be loaded onto a similar conveyor belt system using bucket wheel reclaim units. Some of the advantages of the dragline system include the ability of the dragline to selectively mine the tar sand; the large depth capability of the dragline making a single bench operation possible and the relatively low capital and operating costs that are associated with dragline use.

Both of these mining systems must cater for the severe winter climate which prevails in the Athabasca region. The main problem associated with the winter operation is the freezing of the tar sand faces which produces a material with properties not unlike those of concrete although perhaps not quite so brittle. The present B.W.E. operation utilizes large crater blasts in unfrozen tar sand to effectively resist frost penetration during winter and also to disrupt shale bands in the tar sand.

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