1 INTRODUCTION
The technology of surface mining must become more efficient and sophisticated as lower grade and deeper deposits are exploited and as the general mineral oversupply and the world economic recession has reduced product prices. To obtain a reasonable return on investment more detailed and accurate design, planning and operations have become necessary during the past 3-5 years. Many variables influence the success of surface mine operations. These include ore grade or coal quality, price, development costs, construction costs, operating costs, interest rates and foreign ex- change rates. With the potential increase in the depth of open pits to about 900m the determination of pit slope angle and control of wall stability has a major influence on the overall project economics. For strip mine operations the highwall angles, floor stability, mining cycle time and waste pile stability are critical. This paper discusses the concepts of surface mine stability and reviews recent geotechnical developments that will assist in the rational determination of slope angle to optimize economics and safety. A number of project references are included to illustrate important stability concepts. The geotechnical team must identify the variables which will have an important influence on the economics and viability of the project. Programs must then be developed to define these factors quantitatively. The program must be sufficiently flexible to allow for changes in the variables during these studies. Up until a few years ago, expediture of more than a few thousand dollars on slope stability studies was frowned upon by many mining companies. However as a result of major slope failures, such as the Chuquicamata slide in Chile (Kennedy et el, 1970) and the Chambishi pit slide in Africa (Brawner, 1974) on one hand and the excessive waste removal at the Kimbley Pit in Ely, Nevada that pit slope and highwall stability and the accurate determination of final or operating slope angles is one of the major variables that exerts a major influence on mining economics. For deep pits, savings of as much as 20 to 25 million dollars can be realized for each degree that the pit slope can be steepened. Pit slope angles can vary from as flat as 25 ° - 30 ° where the slopes consist of rock with shallow adverse dipping bedding planes, joints and faults or serpentine or weathered rock to in excess of 70 ° in competent rock with random oriented discontinuities. Since the geology will vary around the pit variable pit slope angles should be the rule. In most stability problems in other engineering fields it is usual to design for a safety factor of 1.5 or more. In surface mining such a high safety factor is too expensive and would result in many potential and existing mines being uneconomical. Good mining practice now accepts the concept that if no slope failure develops during the mining program, the slopes have been over designed and money has been wasted. This concept is only valid provided monitoring of movements in the slope is an integral part of the mining program.
