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5.1 Introduction **

Circular failure is generally observed in slope of soil, mine dump, weak rock and highly jointed rock mass. It is very important to identify the position of most critical circle in analysis of such failure. Although, field observations may provide valuable clues about the mode of failure (rotational, translational, compound, etc.) and possible position of the slip surface, the centre of the most critical circle can only be found by trial and error. Various slip circles may be analysed and the one yielding the minimum factor of safety can eventually be obtained.

Circular failure of slope can be studied for short and long terms depending on condition and site specific requirements. Short term refers to stability of slope during and immediately after construction. In these cases, there is a little opportunity for drainage to occur. Therefore, the analysis should be carried out in terms of total stress using the undrained strength parameters. However, dissipation of pore water pressure can occur in the long term. Therefore, an effective stress analysis using the drained strength parameters, c’, ф’ is carried out under this conditions. The following information is required for the assessment of the stability of a slope against circular failure (Hunt,1986):

· Location, orientation, and shape of a potential or existing failure,

· Distribution of the materials within and beneath the slope,

· Types of material and their representative shear strength parameters,

· Drainage conditions: drained or undrained,

· Distribution of piezometric levels along the potential failure surface and

· Slope geometry to its full height.

For a total stress analysis, the shear strength parameters, friction
ф_{u}=0 and undrained shear c_{u}
are considered. In contrast to this, the effective strength parameters c' and ф'
are used in conjunction with specified value of pore pressure while making an
effective stress analysis. The shear strength of the soil is normally given by
the Mohr-Coulomb failure criterion:

s=c_{u} (for total stress analyses) ф

s=c'+σ'tanф' (for effective stress analysis)