Stockpile Stability F.O.S.

[dirtandrock]

When looking at critical failure planes for large/tall stockpiles/surge piles (eg. active dynamic piles), does anyone have a reference for justifying a factors of safety below 1.3 in static analysis. A F.O.S. of 1.3-1.6 is something that would typically be defined as the analysis criteria for a permenant dam or soil embankment with some life consequences.

There is a low life safety risk to failure, some equipment and production damage is possible.

I would think that the coal industry or sulphur industry would have as much experience in this as the mining industry, but there doesn't seem to be much reference material on the subject.

Background info:
The material typically is deposited from a cantilevered conveyor height so the stockpile material would laterally spread based on a "angle of repose" (drained strength) until the F.O.S. of the pile at the outer edge is essential 1.0. So if we analyse a deep-seated failure from the crest of the pile we typically see a fairly low F.O.S.

 

[BigH]

With the stockpile being "granular" - wouldn't the minimum factor of safety be the "infinite slope" analysis? As you indicate, the material is at the angle of repose - or safety factor = 1. When you are looking at "deeper" zones, they will be, of course, with a higher factor of safety against shear. Do you have a criteria that specifies a minimum zone of concern - say one that goes 2 or 4 m into the slope - and using this as the factor of safety you are looking to have values higher? I've seen tables in a number of references that give factors of safety vs the risk. I've accepted FS of 1.3 when there is little chance of loss of life. Mines typically accept 1.1 due to the huge cost if they have to excavate flatter for deep pits. I'll try to find.

 

[cvg]

you both touched on the need for a risk based analysis. I am assuming there is no regulatory agency or perhaps insurance company dictating specific FS. The reasonable approach would then be to do a risk assessment to establish the design parameters. Cost, including loss of production can be a major influence in the amount of risk that the owner might be willing to take.

 

[dirtandrock]

No agency or insurance dictating a FOS.
I agree the infinite slope is FOS = 1
The issue is that we are getting a FOS < 1.3 for a failure which goes into the foundation. (I've specificed in SlopeW a entry-exit failure which exits in the foundations and enters in the top half of the stockpile on either side). Typically we would want a FOS > 1.3 for this type of critical surface, but given the dynamic nature of a surge pile and the low life risk, I think 1.1 is fundamentally justifiable. Just hard to prove that.

BigH: Can you give me a reference for the tables with "risk vs. FOS." or a term I should search under in google?

BigH: You say "Mines typically accept 1.1 due to the huge cost." I would agree with that statement from what I've seen in the mining industry. What I'm looking for references to this being the case, not just opinion.

Hard to justify to a client that "other mines/geotechs play it close..." when you don't have backup, right?

 

[BigH]

I will look into the various items - So the problem is with basal stability. What is the base soil - clay I would presume. What is the Su value? Do you have any room to put a counterweight berm on the outside of the pile - - or perhaps you preload foundation - using wick drains to speed up the settlement - then remove the preload - then when you put on your surge piles, you will have a greater Su to support the pile (higher FS). We built 11 m high MSE retwall on Su=20 with wickdrains in a stage loadig affair. Give me a day or two for the references.

 

[BigH]

Might be a bit of help:

http://www.wsdot.wa.gov/publications/manuals/fulltext/M46-03/Chapter7.pdf

(see page 7-4)

http://140.194.76.129/publications/eng-manuals/em1110-2-1902/entire.pdf

(see page 25 this is USACE EM 1110-2-1902)

I'll keep looking.

 

[cvg]

this is referenced by the slope stability manual (USACE EM 1110-2-1902)

ETL 1110-2-556. “Risk Based Analysis in Geotechnical Engineering for Support of Planning Studies,”
Washington, DC.

 

[dgillette]

My view of FS is that it is a tool for picking a reliable design and not a criterion. (I generally do not work with building codes or external regulators.) The required FS is (generally speaking) a function of uncertainty and consequences of failure. If the stability could be governed by undrained strength of silt or clay, the level of uncertainty is much greater than if the stability is governed by the angle of repose of the stockpiled material. If the consequences of failure are limited to "Dangit, now we have to clean up the material that sloughed from the pile," the required FS is different from when there is potential for loss of life, damage to critical facilities, long shutdown of the plant for repairs, etc.

With active piles of granular material, you probably won't really be able to control the slopes unless you require constant shaping to something flatter than angle of repose. I like BigH's idea of preconsolidation of the foundation (assuming it's that kind of material).

 

[dirtandrock]

Foundation is a clay till to some depth sitting on fractured weathered bedrock. Part of the issue that is giving the low F.O.S. for the critical surface through the base is the parameters being used are very conservative. I don't view this as my goal with this review though - as the values are "reasonable" given the limited data available.

My first step is to under take a probability analysis using slopeW's built in variation tools for the material properties. And finding the true probability of having a F.O.S. below 1.0.

While I think this probabilistic approach will likely give me a solution I'm still looking for reference with respect to a F.O.S of 1.1-1.2 being acceptable in mining applications. I've heard this from many people as being "industry standard" but never seen it in writing anywhere.

dgillette: my view of F.O.S. is similar to yours. It's a number. A guideline of more research or effort required if you don't meet some minimum criteria guidelines. Without finding a guideline specific to mining saying a F.O.S. of 1.1 is acceptable this means a much more time consuming analysis for a situation that is undoubtabely actually ok..

BigH: The reference you provided still refer to a F.O.S. of 1.25 - 1.5 for typical applications. WSDOT on pg. 7-4 provided 1.25 as the lowest and US Army is only allowing 1.1-1.3 for rapid drawdown on pg. 25, otherwise >1.3 is still required. This is typical for dam or embankment applications.

Thanks for your guys input so far.

 

[BigH]

I've never heard of too many problems with foundations on glacial clayey till. We have this all the time in Ontario - and typically we use 350 to 400kPa for allowable bearing pressures for spread foundations. Qall ~ 8 to 10 N . I'm quite surprised that you would have such low safety factors in stiff clayey till. Use of Su is a short term condition - have you determined the "long term" safety factor under drained conditions? Still looking . . . .

 

[BigH]

http://www.fhwa.dot.gov/bridge/checklist.pdf

- see document page 16. Embankment stability 1.25 for side slope and 1.3 for end slope of bridge approach embankments.

 

[BigH]

Referencing:
1. Silvia et al, "Probability and Risk of Slope Failure", Proc. ASCE Journal of Geotechnical and Geoenvironmental Engineering, Vol 134, No. 12;
2. Wyllie and Mah Rock Slope Engineering, Civil and Mining 4th Edition (based on Hoek and Bray's 3rd Edition), Spon Press, 2004.

Attached I have plotted the risks for selected engineering projects (Mine Pit Slopes and Foundations) onto Fig.1 of Silvia, et. al. This shows the range of FoS from Type I to Type IV projects defined as:
Type I: Facitilities designed built and operated with state of the practice engineering (high failure consequences)
Type II: Facilities designs built and operated using standard engineering practice (most ordinary facilities)
Type III: Facilities without site-specific design and substandard construction or operation (temp facilities low failure consequences)
Type IV: Facilities with little or no engineering.

Obviously those structures with better engineering (i.e., investigations, selection of soil properties, etc) can "go" with smaller FoS than a similar structure with little or no engineering. As shown, for Type II, for mine pit slopes, the FoS would be in the order of 1 to 1.2 whereas Type III would require 1.2 to 1.4 FoS.

This might help you in your arguments of risk and FoS. I noted another paper in the ASCE Geo Journal by Athanasiou-Grivas (1979-V105, GT9) that putatively has Probability of Failure vs FoS - but I do not have access to that paper at present - it is not "on-line" yet and my hard copy is in storage.

 

[dirtandrock]

Thank you for your valuable help and direction. Very useful. I hope I haven't taken too much of your time.

The link at engineering.com doesn't seem to work though.... I am not a member of that site, is that the issue? Or maybe the naming of the file is an issue? I couldn't see any requirement to sign up for the website to download. Think I've got files from there before.


I agree on the foundation design and using > 400 kPa for a description of clayey till and a footing of moderate width.

The FOS of <1.3 is more for a long-term stability analysis I suppose, short term should consider an Su, as you stated.

The surge pile is in the order of 40-60 m tall, so the stresses are fairly high at the toe, even with a conical stress distribution (which I haven't assumed in a planar slope analysis.) About 20 m outside the edge of the pile is something which is important financially, although not a life safety issue. I think a FOS of 1.2-1.3 is quite common, in my experience, for this type of situation.