Determination of the critical slip surface in slope stability analysis

ABSTRACT: Analysis and design of the soil slopes has been an important field in geotechnical engineering for all the times. Various methods for analyzing two and three dimensional slopes have been created and developed based on different assumption and analysis methods. The factor of safety can be correctly obtained only if the critical failure surface of the slope is accurately identified. The critical failure surface for a given slope can be determined by comparing factor of safety of several trial slip surfaces. The slip surface that has the lowest factor of safety is considered to be the critical failure surface. The aim of slope stability analysis of any natural or manmade slope is to determine the failure surface that has the lowest factor of safety value. To find the minimum factor of safety, it is important to find the critical failure surface for the given slope. For that reason, different searching and optimization methods have been used in the past. However, they all carried almost the same limitation: They all had the difficulty in using them for hand calculations. In this study, effect of soil strength parameters on the failure surface and factor of safety of the slope were studied. Different slope stability analysis software programs were used and compared, and a formula was presented to calculate the length of failure arc by knowing the soil strength parameters. In this study, GEO5, SLOPE/W and FLAC/Slope software programs were used to analyze the slope stability problems and determine the critical failure surface. To investigate the validity and effectiveness of these programs, different values of shear strength parameters: cohesion (c), internal friction angle (ϕ), and soil unit weight (), were chosen and their effect on the factor of safety value were investigated. Additionally, an equation was introduced in order to locate the critical failure surface by using soils strength iv and slope geometry parameters. Finally, the obtained results from different software programs were compared and discussed. The results of the study showed that the factor of safety of the slope changes with varying cohesion c, internal friction angle ϕ, and the unit weight  of the soil. Moreover, the slip surface is affected by the dimensionless function  which is related to the cohesion, internal friction angle and the unit weight. When λ is constant, the slip surface does not change along with the change of shear strength parameters. The obtained results showed that GEO5 is more conservative slope stability analysis software, compared to SLOPE/W. It gives 5% smaller factor of safety than SLOPE/W. On the other hand, FLAC/Slope usually gives out greater value for factor of safety compared to SLOPE/W and GEO5. Keywords: Critical Failure Surface, Factor of Safety, Length of Failure Arc, Limit Equilibrium Method, Soil Slope Stability. …………………………………………………………………………………………………………………………………………………………………………………………………………