slope stability vs bearing capacity on a slope

[NS4U]

this is a pretty basic question as i am trying to understand the fundamental differences between slope stability and ultimate bearing capacity. I do understand the whole 2pi vs 1pi thing for bearing capacity vs slope stability.

if i have a shallow foundation at the top of a slope (assume homogenous soils), what is the difference between the ultimate bearing capacity, using say Myerhoff's equation (modified to account for the slope), vs the capacity given by a slope stability analysis?

Maybe i am picturing this too simplicitcally, but I imagine the failure plane for slope stability should look pretty close to the failure surface that the ult. bearing capacity formulas are based on.

I ask because I am getting some signficantly different results when I calc the bearing capacity by hand compared to when I do a slope stability analysis in slopeW. I would have expected them to be reasonably close to each other.

thanks for the help

 

[GeoPaveTraffic]

Generally, if you determined the ultimate bearing capacity and then applyied that load in a slope stability model you should get a factor of safety close to unitity. This would be true with a "footing" on a slope or one on flat ground. Some difference is to be expected due to differences in models and methods, but the results should be reasonably close. I would expect a slope stability factor of safety of 1.0 +/- 0.1 or so.

If you aren't getting results that close, could be a calculation error or a modeling error.

If you post more info, someone may be able to help out more.

Good luck.

Mike Lambert

 

[fattdad]

I consider this question when evaluating the soil conditions for segmental retaining walls. Where in an SRW analysis is "bearing capacity" even relavent? I'd say not at all. The loads from the reinforced zone that act on the original soil do impart some "bearing pressure," but using the rational equations to determine safety factor for bearing capacity seems wrong to me.

Bearing capacity is basically bound by the ultimate shear strength of the soil. So is slope stability. It seems to me the question of bearing capacity for the reinforced zone acting on teh native soil is properly addressed by the global stability evaluation(s).

Maybe I'm just agreeing with you or feel your confusion. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[GeoPaveTraffic]

fattdad,

From a gross movement prespective I agree that there is no reason to check bearing capacity for a segmental block wall. However, from a long term settlement prespecitve (at least with clay foundation soils), checking bearing capacity gives a somewhat implict settlement check. Also, with some wall systems the load under the facing material can be a fair amount higher than the average bearing pressure in the reinforced zone and settlement of the face can look really bad.

Mike Lambert

 

[Mccoy]

I don't know if you already considered this aspect, but I would expect a difference in the results since we are contemplating two different failure models. The one usually considered in bearing capacity analysis (phi-c soils) is based usually on a single wedge and lateral logarithmic spyrals going all the way out to the sides of the foundation. There are some solutions in the literature on cases where the foundation lays near a slope and one of the logarithmic spyrals is truncated by the topographic surface. This entails a reduction of the frictional lenght/area contributing to the stability hence lesser bearing capacity.
If one of such solutions is used, I would expect a closer result to the slope/W analysis, but I would be really surprised if the results were very similar, since the algorithms and the theory behind them are significantly different.
There might be a closer affinity if an undrained analysis is carried out, since the surface could be a circle in both cases, according to the failure surface shape chosen.

Bottom line: I would use the slope/W result since the geometry of the problem is specific to your case hence more exact.