## Effect of surface surcharge on slope stability??

## Effect of surface surcharge on slope stability??

(OP)

I need to evaluate the maximum amount of fill I can place near a 1:1 slope. I also need to evaluate various distances of the fill from the top of the slope. (for example: place 5 feet of fill 10 feet back from the top of the slope)

How should I include the surcharge forces from the fill in the slope stability analysis? I plan on using the Simplified Bishop Method (if that is an approriate method for this case).

Thank you.

How should I include the surcharge forces from the fill in the slope stability analysis? I plan on using the Simplified Bishop Method (if that is an approriate method for this case).

Thank you.

## RE: Effect of surface surcharge on slope stability??

Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

## RE: Effect of surface surcharge on slope stability??

## RE: Effect of surface surcharge on slope stability??

what about running a prior analysis on the existing slope, as you suggest, and then adding a generic distributed surcharge, of the same lenght of the design fill and simulate progressive fill elevation by increasing the value of such distributed surcharge ?

This sounds less awkward then re-punching the data for the slope profile + layers profile, unless there is some specific reason to follow your method...

## RE: Effect of surface surcharge on slope stability??

Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

## RE: Effect of surface surcharge on slope stability??

## RE: Effect of surface surcharge on slope stability??

Thanks again.

## RE: Effect of surface surcharge on slope stability??

## RE: Effect of surface surcharge on slope stability??

________

<- 20'-> / New fill

_________________/________

/

______________/___ WT (10')__________ ______

__River______/ Exist Fill (18' thick)

______________________________

Compressible Silt (26' thick)

____________________________________________________________

Stiff Silt

## RE: Effect of surface surcharge on slope stability??

I agree with all of the advice given so far. One additional point that I would make is that you should rule out the possibility of a thin weak horizontal layer or preexisting failure plane and that you may have a noncircular failure surface. We just looked at a slide that was triggered on a riverbank where about 20% of the proposed fill was placed and a very long planar failure surface was activated. It turns out that this was probably a preexisting slide plane and its geometry in relationship to the location of the new load triggered the slide.

In my opinion, prior to running the analysis, you should develop, from fundamental principles, an understanding of the likely failure modes and then use the stability analyses (and parametric studies) to confirm or disprove the postulated failure modes.

Glen

## RE: Effect of surface surcharge on slope stability??

Unless you have done a lot of stability analysis by hand, don't start now.

## RE: Effect of surface surcharge on slope stability??

But, with the exception of relatively simple models, slope stability spreadsheets can really drive you nuts, expecially so when applying the Bishop method, locating the failure circle(s) and assigning the strenght parameters along its lenght. And you are supposed to do it for tens of circles!

I too suggest a commercial software, today they're pretty much inexpensive. One advantage of spreadsheets over most commercial software though is that, by means of macros or commercial add-ins (or by approximation methods such as the one described by Mike Duncan) you may carry out a probabilistic stability analysis.

One commercial product which advertises probabilistic and reliability capabilities is Rocscience's Slide5. It comes at US $ 1495, not too bad. You come across pretty good stuff at very reasonable prices, in the region of US$ 300.

## RE: Effect of surface surcharge on slope stability??

## RE: Effect of surface surcharge on slope stability??

smallchange in the geometry is very tedious. If you are a practicing engineer, the small cost of commercial software ($300-$500 U.S.) makes hand calculations look exorbitantly expensive (considering the loss in billable time.)If you're a student, this isn't a "homework help line"...

Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

## RE: Effect of surface surcharge on slope stability??

Yes, I am an engineer and yes this is tedious. Once the spreadsheet is complete, the calculations will be revised automatically as the circular slip surface is varied with a macro. Getting there is quite a task though.

Can you suggest a software in the price range you mentioned? The couple I looked at were around $1,000. I may be able to justify to the "higher ups" the purchase of a software that is only a few hundred dollars.

## RE: Effect of surface surcharge on slope stability??

## RE: Effect of surface surcharge on slope stability??

www.dot.ca.gov/hq/esc/geotech/request.htm

## RE: Effect of surface surcharge on slope stability??

Secondly, perhaps a commercial software company (example G-Slope) will sell you an old DOS-based version for peanuts especially if they know you WILL NOT spring for the $1000 version. The DOS version is quite a good program - not as fancy, but still it does the job. You can do a "set" of analyses for a particular configuration for a simple case in less than 15 to 30 minutes.

## RE: Effect of surface surcharge on slope stability??

Personally, I think you will be wasting money applying a surcharge to the ground as presented. Existing Fill and compressible silt are terms that need to have some test data for proper engineering interpretation. Fill can be so described from visual observations of the stratigraphy at the site but has good engineering characteristics. The bank standing at 1:1 says something.

## RE: Effect of surface surcharge on slope stability??

you may have a look at this site:

www.geostru.com

this italian firm sells a slope stability software at

US$ 370.21 (today's rate-of-exchange euro-dollar)

I have used it in my latest 2 projects and it is pretty good, sure good quality/price ratio. Lots of capabilities.

## RE: Effect of surface surcharge on slope stability??

Waht is the existing fill, and what will the new fill be?

regards

## RE: Effect of surface surcharge on slope stability??

## RE: Effect of surface surcharge on slope stability??

One useful resource with the spreadsheet is "Slope Reliability Analysis Using Generalized Method of Slices." It is in the Journal of Geotech & Geoenviron. Engineering Vol 124, No 4 - April, 1998. It shows how to set up Morgenstern and Price and Spencer methods (which satisfy all conditions of equilibrium) and takes advantage of Excel's solver to do some searching for you.

## RE: Effect of surface surcharge on slope stability??

Thanks for the kudos. It was a daunting task, but I was able to develop an excel spreadsheet that completely automates the simplified bishop method. The output is practically identical to that of commercial software.

## RE: Effect of surface surcharge on slope stability??

## RE: Effect of surface surcharge on slope stability??

## RE: Effect of surface surcharge on slope stability??

## RE: Effect of surface surcharge on slope stability??

I always use the software to "zoom in" on the critical circle, then confirm the answer with hand calculations. It's quite efficient from a time use standpoint -