slope stability analysis

Usually, i perform the analysis for both undrained and drained conditions. Especially for the retaining walls if the clay material is saturated, sliding on subsurface becomes an critical issue.

For the anchored walls, usually drained parameters become critical

But i never considered the reduction of strenght parameters due to water content.

You first need to determine if your analysis will be an undrained or drained analysis. I remember there is a chart from Duncan and Wright that can provide some reference about this. This chart relates the drainage path lenght, type of soil and time required for 99% dissipation of pore water pressures. I will look at this chart and post it here later. Then you use the correspondent shear strengt. For example, you have several drained strengths: peak strength (rare), fully softened and residual strengths. Professor Stark from University of Illinois has developed a spreadsheet which correlates fully softened and residual strengths to find contents and LL. Look at his website. There is good info there for this.

killswitch, you should apply the conditions actually occurring in the soil, in a prospected worst case scenario if that is required.
If water accumulations are believed to be likely with some significant degree of probability, then an appropriate softened value of Su (Cu) should be applied.
If using drained parameters, 8ohi and c') c is usually ignored (=zero) and phi is degraded to a phicv or phi residual by the stark procedure mentioned for example, or assuming phi residual as measured in lab tests (which in some cases may be exceedingly cautious).

All the above unfortunately makes the analyses tricky. Many judiciary procedures are started based on slopes which had to be stable by the book but turned out to be unstable. Oftentimes the cause is shear resistance degradation by water.

Shear strength degradation by water may be able to be verified with the Stark's spreadsheet. You can see how the fully softened and residual strengths will vary with changes in the liquid limits. This is only for drained analysis. Also I found the Duncan and Wright chart...

thank you all for the feedback
Are there any rules to compute PWP as a function of degree of saturation without the need of conducting a triaxial test ?