Fattdad, right, in your previous post you cited wetting and drying cycles, freezing and thawing so I should have understood sooner that you were referring specifically to the case of cuts and artifical slopes. My bad, old age approaching makes up a convenient excuse in my case.
By the way, by metastable I mean exactly what you say about changing mechanical properties when conditions change and another point of stability must be reached.
And conceptually it seems pretty much reasonable.
Also, as killswitch reasons about the more appropriate phi overall, if we measure phi-c on an intact specimen, that's undoubtedly a phi_peak, but if fissures govern the stability, we have already existing sliding planes along which phi_cv sounds more of a technically sound choice.
The sliding plane may cross thru fissures
and intact clay, so the averaged shear strenght might be some post-peak value which is not as low as the critical state. In this case phi_cv would be a cautious lower bound,.
The OP did not specify the kind of geotechnical problem I believe.
In some cases the regulations call for specific values, as in Fattdad's DOT, or in European building codes, where the sliding design of shallow foundations is required to be carried out by phi_cv.
Also phi critical state is assumed to govern when a soil-concrete interface exists, or when there is significant soil disturbance, as in drilled piles (some authors suggest phi_residual at this point).
The peak versus critical state parameters choice is probably different in different geotechnical analyses.
