The peak shear strength envelope of soils, rock fills, highly weathered rock and jointed rock can exhibit significant curvature over a range of stresses. In the past a power curve relation has been used to represent this non-linear envelope. An accurate and rigorous method is given which calculates the ‘best fit’ power curve relation to test data in the form of Mohr circles. The mathematical function for the power curve then represents the shear strength equation. In order to test and study the method, it is applied to shear strength results, from tests on six overconsolidated clays. The failure envelopes for the clays are significantly curved and a power curve relation provides a good estimate of the failure envelopes. Comparisons with linear failure envelopes clearly show that, over the same stress range, the power curve relation is the more realistic approximation of shear strength.

A slope stability method of analysis is given which uses the power curve relation for the shear strength of materials. It can be used for slopes in, or constructed of, soils, rock fills or highly weathered rock where a non-linear failure envelope is representative of shear strength. The method is based on Janbu's Rigorous Method of Slices and is for use in analysing failures with a failure surface of arbitrary shape.

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