The calculation principles and interpretation of results are discussed for three alternative methods for rock slope stability analysis:

  1. The traditional deterministic principle,

  2. The partial factor principle, and

  3. The probabilistic approach.

To illustrate the different principles, stability analyses of a proposed open spillway at the planned Paunglaung hydropower project in Burma are used as example. The traditional deterministic approach, with calculation of one single factor of safety, has the general advantages of being well established and easy to understand. To conform with new standards and guidelines, it has to be replaced, however, by the partial factor principle, with partial factors on actions and material strengths. Advantages and disadvantages of the partial factor principle compared to the old approach are discussed. Due to the uncertainty and often variability of input parameters, the probabilistic approach has obvious advantages. In important analyses it is recommended, therefore, in order to obtain the best possible basis for evaluation, always to consider the option of performing probabilistic analysis as a supplement to deterministic.


A variety of tools are available for analyzing rock slope stability, and in theory, high accuracy calculation is possible for practically any situation. The steps that precede calculation are crucial for the analysis result, and will be briefly commented on. Main emphasis in this paper will be placed, however, on discussing the calculation principles and the interpretation of calculation results, on which there has been a considerable development over the last few years as consequence, particularly, of the introduction of new professional standards and guidelines such as Eurocode 7 (CEN, 1994), and the increased use of probabilistic methods. To illustrate the alternative analysis principles, aspects of stability analyses carried out for a proposed spillway channel at the Paunglaung hydropower project in Burma (Nilsen, 1984) will be used as example.

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