Slope Stability Soil Parameters? 2

[sock76]

I am required to do a slope stability in Illinois on a surface that is currently filled with construction debris. i.e. crushed concrete, brick, soil, etc. I am going to perform the slope stability using the wedge method, however, I was wondering if anybody had any input on some conservative values for the soil parameters and properties that I could input into the program. Also, if you have any idea of material that I could reference as to why I used the values I did. Any help would be greatly appreciated.

 

[GeoPaveTraffic]

With material like you describ, the first question you must answer is about the material. Will the material behave as concrete/brick rubble or as pieces of rubble in a soil matrix. If the answer is the former, then the strength will be relatively high probably 35+ degrees. If the answer is the later, then the strength will be equal to the soil strenth for the soil between the rubble. To determine that strength you will at a minimum need some atterberg limits to estimate the strength and ideally will need some strength test results.

 

[PEinc]

You may also need to consider the long term condition. If the fill has materials which can decompose (such as wood, paper, metals), the embankment could change over time. If there are voids in the embankment due to bridging of the debris, it's possible that the soils could move and loosen due to decomposition and collapse of debris or ground water movement within the embankment. I've seen homes damaged years after being built over fill containing construction debris.

 

[sock76]

The fill material is completely mixed with soil, as far as I know. And, as I am informed, there is no wood, or garbage, etc. contained in the debris. There is steel, bricks, concrete, etc., but all of it is said to be crushed and completely homogeneous (theoretically speaking) and that there shouldn't be any bridged voids. Although, there are more than likely voids that have to exist in this soil/debris mixture. I've searched and searched, and still haven't came up with any conservative geotechnical values to use in my stability analysis. I didn't know if somebody out there has possibly encountered this situation before. I'm sure it's been done, and that's why I figured their previous experience would benefit me in this situation. All of the help is appreciated in advance.

 

[Focht3]

Hmmm,

This is one of those problems I try to avoid...

[blue]GeoPaveTraffic[/blue] and [blue]PEinc[/blue] have given good advice. From a design standpoint, I'd use very low stength parameters (c=0, [φ][≤]13[°] or so) and a "high" unit weight. Even if the lab work indicated the soils were significantly stronger.

And I'd use a higher factor of safety in an effort to reduce the apparent risk -

Of course, a better way to approach this might be to use Mike Duncan's reliability approach...



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?"

 

[PEinc]

Who's Mike Duncan? What's the Mike Duncan reliability approach?

 

[Focht3]

This is from Thread260-80582

I suggest that you check out a paper titled Factor of Safety and Reliability in Geotechnical Engineering, by J. Michael Duncan, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 127 No 8, August 2001. And be sure to read all the discussions - and Duncan's closure - of the paper. This will give you a realistic - and usable - guide to selecting appropriate factors of safety, evaluating risk, and assessing the appropriateness of the parameters that were provided to you.

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?"

 

[BigH]

You've been given some good advice from very good engineers. I would, though, like to know the following information:
1. What does the physical cross section look like? I don't know if you are making a cut into the material for say, an open excavation, or you have a current embankment situation and wish to confirm the factor of safety that you have in place presently - perhaps for code upgrades for seismic, etc.
2. Part and parcel of 1., then, is how high is the slope and at what angles do you have/or will cut to.
3. Where is the groundwater level? Is there a perched level in the slope?
4. Assuming you have an existing slope, what is the chance of flooding (if any) for short or medium term against the slope face. Construction debris is likely so permeable that any flood water may almost immediately saturate your embankment. Do you have a condition where flooding might occur increasing the water level in the embankment and then get quick drawdown of the flood-levels so that you still have "water" in the embankment.
5. Is this a 2-D (plane) problem or is there any 3-D effects that might come into play.
6. If you have a "dry" embankment sitting at a certain "angle", this might give you an idea of minimum "angle of repose" - hence, to a start, possible overall strength values.

You see, doing a stability analysis is more than just applying a set of strength parameters and running an infinite slope analysis or using the computer to generate a critical surface. There are many other factors that you must understand before you can make a judgment on the stability of the slope - and one of them, in the case of infinite-type slope assessments, is the "critical mass" of which you would be concerned - clearly you wouldn't be concerned about 500mm sheet-type failure (unless there is a reason for progressive ones).

 

[sock76]

I want to first of all thank everybody for their knowledge in this area and taking the time to post their messages. I think after careful consideration and talking it over with my boss, we decided it was probably more logical to sub this out to someone who has more expertise and experience with slope stability. We have only done simple slope stability with laboratory soil results provided. Therefore, someone with more knowledge in slope stability should probably tackle this problem. But I do want to thank everyone for their assistance. If I have any questions again, I know where I'm going to come for assistance. Thank you.