FÖRFATTARE
Sjöberg, Jonny

DATUM
1996-05-10

INSTITUTION / AVDELNING
Väg- och vattenbyggnad / Bergmekanik

SAMMANFATTNING
Design of open pit slope angles is becoming more and more important as the mining depths of open pits continuously increase. Small changes in the overall pit slope angle have large consequences on the overall economy of the mining operation. A case in particular is the Aitik open pit mine in northern Sweden, which currently faces the design of the overall slope angles for continued mining toward a depth of around 500 meters. This report constitutes the first phase in a research project aimed at developing design methods for large scale pit slopes. In this report, the stability and design of large scale pit slopes in open pit mining is reviewed, with special reference to slopes in hard, jointed, rocks, similar to the rock types found at the Aitik mine. The review covers the mechanics of pit slopes, existing design methods for large scale slopes, remedial measures and mining strategy to cope with slope failures, and a compilation of case studies from open pits worldwide. Finally, suggestions for future research in this area are presented.

The factors governing large scale slope stability are primarily: (1) the stress conditions in the pit slopes, including the effects of groundwater, (2) the geological structure, in particular the presence of large scale features, (3) the pit geometry, and (4) the rock mass strength. Observed failure modes in rock slopes are of a wide variety. On a bench scale, structurally controlled failures such as plane shear and wedge failures are common. However, as the scale increases, simple structurally controlled failures are less dominate, and more complex failures such as step-path failures start to develop. From observations, it appears that for large scale slopes, two failure modes are especially important to consider. These are (1) rotational shear failure, and (2) large scale toppling failure. Rotational shear failure in a large scale slope involves failure both along pre-existing discontinuities and through intact rock bridges, but where the overall failure surface follows a curved path. Large scale (or deep seated) toppling failures have been observed in several large scale natural slopes and high open pit slopes.

The mechanisms behind large scale failures are, however, not well known, in particular for hard, strong rocks. Criteria for the shape and location of the failure surface are lacking, as is detailed knowledge regarding failure through intact rock versus failure along discontinuities. Knowledge of the kinetic behavior of failing rock slopes is mostly empirical and requires more studies, in particular for hard and brittle rock masses in which rapid failures can be expected.

This review has shown that the strength of a large scale rock mass is very difficult to assess. At the same time, the required accuracy for the strength parameters which are needed for the design is very high. For large scale rock masses, back-analysis of previous failures proves to be the only practical means of obtaining relevant strength parameters. However, the interpretation and translation of such data from one geological environment to another, is very cumbersome and lined with problems.

Design methods for rock slopes can divided into mainly four categories, namely: (1) limit equilibrium methods, (2) numerical modeling, (3) empirical methods, and (4) probabilistic methods. The advantages and disadvantages of each of these methods are discussed in the review. For the design of large scale slopes, it appears that the choice of design method is less important than the choice of input parameters to the design, in particular the rock mass strength parameters.

Remedial measures for controlling the stability of slope include support and drainage. While support can work for small scale slopes, only drainage is feasible for increasing the stability of large scale slopes. Monitoring of displacements, preferably using survey networks, should be carried out routinely in all open pit mining. Provided that the failure is slow and stable, it is also possible to continue to mine a failing slope. This requires that contingency plans are being made at an early stage in the mine planning process.

From the collection of a number of case studies from North and South America, Africa, Asia and Europe, several examples of large scale failures were found, although mostly occurring in weak rocks. There are much fewer examples of slope failures in hard, brittle rocks. The few cases found indicate that failures in this type of rock is more uncontrollable. A compilation of slope height, slope angles, rock strength, and stability conditions for the studied cases concludes this chapter.

Future research in the field of large scale slope design must be focused on quantifying the mechanisms for large scale slope failures. Once the mechanisms are better known, design methods based on the actual slope mechanics can be employed. Also, better and more reliable methods for determining the strength of large scale rock masses are important to develop.

ISSN 0349-3571 / NR 1996:10 / ISRN: HLU-TH-TR--1996/10-T--SE

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