Lab soil paramters compared with back analysis of slope failure

[samipoyo]

I am wondering why lab soil parameters are often larger than back-analysis of first time slope failures?

Any help on this topic would be greatly appreciated

 

[TDAA]

What lab perameters are you looking at? I assume stregth testing to get Phi and C. The question is what method was used to get them? Was it direct shear or the variety of methods to run a triax? Does the lab method generally follow the mode of failure (as in you tested consolidated / drained, but the slope had an undrained condition at the time of failure, etc). For direct shear , you are also forcing a slip plane that may not be the weakest plane in the soil structure.

Also are you looking at peak values on the lab testing or the residual values. We tend to design our slopes and walls based on the residual numbers for an added FoS.

It may also be how well you are defining the soil condition and slip plane that occured in the field. Is there a thin weak layer the slip followed? Is there a variation in the soil properties that is not visually noticable?

I am sure there are many more point to take into consideration. It is hard to tell why you see a difference without knowing your background and what methods you are using to compare. The questions above should give you some reasons why things may be different.