Unsaturated Soil Mechanics Applied to Slope Stability

[GeoEIT]

Hoping someone can give me some direction or resources on applying Unsat. soil mechanics to slope stability problems. I have taken an Unsat. course, so i am familiar with basics, but I am having trouble finding resources to perform slope stability modelling considering Unsat.

Does anyone have experience with this kind of analysis? Is this type of analysis not used in industry due to cost/difficulty/specialized testing equipment?

Thank you for any help!

 

[emmgjld]

Try GeoSlope

 

[GeoEnvGuy]

I do not consider the benefits of matric suction which increases the effective stress to provide additional strength in slope stability.

 

[GeoEIT]

My question is specific to shallow failures in unsaturated slopes, where you might have precipitation or desiccation induced failures.

We had a problem come up awhile ago where I think this type analysis would have better captured the problem - but no one in my firm has this type of experience.

I know Slope/W has some capability. But I wonder how to derive phi^b without unsaturated triaxial testing. And also what the role of the SWCC, if any, would be in this approach.

 

[GeoEnvGuy]

This question seems to be more academic.

Shallow failures of unsaturated slopes are not typically the reason why slope stability analysis is performed for a earthworks project. The analysis to determine a suitable downstream angle for earthworks projects is justified with infinite slope analysis.

If you are considering natural slopes or mudslides the failures are caused by precipitation which causes erosion or changes the pore pressure.

 

[fattdad]

Here's my perspective: All (typical) soil strength tests are performed on saturated samples. All slopes should be weatherproof. We have no guarantee on the durability of matric suction - the actual item that give unsaturated soil mechanics greater strength than saturated soil mechanics. We see slopes fail owing to, "Rainfall." Ever wonder why? Could it be that there is a false sense of security being provided by soil tension?

Just say no!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Lomarandil]

Agreed.. there's improving understanding and research going into unsaturated slope stability, but it's not useful for design cases at this point. Still in the world of academia and maybe forensics.

If you're interested in the academic side of it, Dr. Liu at Colo. School of Mines was on the cutting edge of that last I heard.

----
just call me Lo.

 

[MTNClimber]

I’ve seen engineers use a partially saturated approach to temporary slopes. They will look at the moisture and densities of soil samples and plot them on a zero air void graph to find the depth below “groundwater” that the soil becomes unsaturated again.

Maybe this is appropriate for temporary but I’d be uncomfortable at this time to say it is ok for permanent slopes.

 

[geotechengineer1]

https://www.amazon.ca/Unsaturated-S...aturated+soil+mechanics&qid=1580967089&sr=8-1

https://www.amazon.ca/Advanced-Unsa...aturated+soil+mechanics&qid=1580967115&sr=8-8

I believe there are a bunch of Delywn Fredlun lectures floating around on ISSMGE or a similar association.

From my own limited-ish experience North American practioners don't generally use unsaturated soil mechanics in practice. For one, it is a much more complicated analyses and difficult to get the appropriate parameters (and in NA most practioners rely on correlations like SPT, atterbergs, etc, or whatever friction angle their first senior engineer told them for the local soils 40 years ago), and for another most NA slopes practitioners would deal with are governed by global stability concerns. (Tailings dams, constructed earth embankments, relatively old slopes).

I personally suspect that unsaturated soil mechanics could be quite useful in residual soils and geologically young regions where natural slopes are quite steep and rainfall-induced landslides are a dime a dozen. This is the case in New Zealand where fresh grads often do more landslip assessments in their first few years as North American geotech engineers might hope to do in a lifetime. Although even here I don't think anyone has put much thought into using unsaturated soil mechanics in practice.