Geotechnical design and analysis Subject:
Slope Stability analysis
I am analyzing the temporary and long term stability of a slope which is comprised of alternating layers of saturated clayey and sandy materials using limit equilibrium methods (Slope/W). I am thinking that the saturated clays should be modeled with undrained strengths and using total stresses, and the sands with drained strengths and effective stresses. In some cases, the sands would be below the clay, so that the groundwater level used for calculating effective stresses in the sand would be a level above the overlying clayey unit. Is it appropriate to analyze the slope with this method?
Also, if temporary dewatering occurred so that the sand layer was not saturated, but the clay still retained some moisture but may not be completely saturated, would it still be appropriate to use undrained strengths and total stresses for the clay?
Also, if a permanent slope has a 6” thick concrete lining, do you think it is appropriate to model this lining in the analysis by using the compressive strength of the concrete?
I suggest you start with evaluating the nature of your two load cases: short and long term.... whether these two conditions represent either a drained or undrained condition for your slope. For normally consolidated soils, the undrained (i.e. short term or "immediate") condition will typically control the slope stability, particularly if a rapid drawdown condition can develop (ie. dewatering). For overconsolidated clays, the opposite is true: the long-term (i.e. effective stress) condition will generally control the slope design.
It seemed to me that you are not fully understanding the difference between: 1. total and effective stress 2. undrained and drained shear strengths and how they should be used in design. I suggest you do some quick review before assessing your slope's stability.