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Multistage triaxial test may not be preferable for consolidated drained test. Why?

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In multistage consolidated drained test, the soil sample is consolidated under all round pressure and then loaded by applying an axial stress. Prior to failure, loading is stopped and the specimen is consolidated under a higher confining pressure. The above steps are repeated for 3 stages to
obtain the failure envelope.

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The main problem associated with multistage consolidated drained test lies in the practical difficulty in determining the failure state of the soil sample. Judgment has to be made regarding the condition of “immediately prior to failure” on stress strain curves. It is not uncommon that wrong estimation of the failure state occurs when interpreting the stress strain curves. When there is an underestimation of deviator stress at failure, it would result in overestimation of friction angle and underestimation of cohesion. In case actual failure of soil samples occurs before visual recognition, the sample undergoes overstressing so that the deviator stress at failure in later stages is reduced. As such, this leads to overestimation of cohesion and underestimation of friction angle.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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