Safe distance for loading beyond crest of a slope
Safe distance for loading beyond crest of a slope
(OP)
Long time lurker, first time poster, so any help is appreciated... I'm looking at an existing slope/gully which is essentially marginally stable, both on inspection and when analysing (FOS = 1.0 roughly). We've been asked to provide indication of a safe 'set-back distance' from the crest of the slope for which a crane loading can be safely applied. Normally we'd simply model the loading in SlopeW, aiming for a FoS >1.3; however given that the slope is marginally stable anyway, is this a pointless exercise?
Secondly, on the same slope, we've also been asked to assess the distance from the crest of the slope which may be affected following a seismic event. Looking at nothing more than a crude (pseudo-static, applying peak horizontal PGA) approach, is it valid to suggest that the distance beyond the crest which exhibits a FoS<1.1 may be liable to deformation/movement?
Secondly, on the same slope, we've also been asked to assess the distance from the crest of the slope which may be affected following a seismic event. Looking at nothing more than a crude (pseudo-static, applying peak horizontal PGA) approach, is it valid to suggest that the distance beyond the crest which exhibits a FoS<1.1 may be liable to deformation/movement?





RE: Safe distance for loading beyond crest of a slope
The crane should be positioned sufficiently far back that it doesn't add any further load onto the soils within your theoretical failure plane. I would start by plotting the profile of a potential failure plane (I'm not sure I'd use a FOS as low as 1.1 given all the variables), then setting back your crane footing to avoid loading any soils within this plane. Then I'd set it back a bit further just to be sure.
Another consideration is how your crane footing will be affected once the theoretical failure actually occurs, with the ground level being modified for the failed soil volume. In my opinion if there is a risk of instability, it is not sufficient to place your crane foundation marginally behind the potential failure...you would need to assume the failure will actually occur and then step back progressively. You may also need to consider the construction time-frames (I'm assuming it's a temporary crane).
I'm not familiar with seismic events (living on a passive margin of South Africa), so can't help with point 2.
All the best,
Mike
RE: Safe distance for loading beyond crest of a slope
Since you are using Slope/w and its search routines are not great, you will need to run a lot of failure planes. My suggestion is run at least 50,000 (300,000 would be better) surfaces that cover the area starting near the crest and extending back several hundred feet.
Mike Lambert
RE: Safe distance for loading beyond crest of a slope
Since that is usually not a practical solution most of us, you just have to take the slope stability analysis for what it is, a check on an improper model, and consider that also.
RE: Safe distance for loading beyond crest of a slope
Also, is the crane loading, a temporary loading? If so, I think that seismic analysis may be neglected: there are few chances that the earthquake and your temporary loading to act at the same time.
RE: Safe distance for loading beyond crest of a slope
Unfortunately we don't have the resources to perform a 3D analysis - our report and analyses are caveated as such!
Just to clarify the seismic issue - that's an additional analysis, carried out as a separate analysis to the crane loading.
RE: Safe distance for loading beyond crest of a slope
But if you had a relatively uniform slope without many variations in the 3rd dimension, and the failure surface isn't evident (or is substantially granular and so not driven to repetition by a remolded clay layer).. then you'd essentially be forced to consider a slope failure starting adjacent to an outrigger -- which becomes a 2D problem again.
I'd follow Mike's method.
For part 2, I think pseudo-static and 1.1 could be reasonable, depending what the client really wants to know by defining an "affected" area. Is it the contractor wanting to figure out where they should place their laydown area, or is a developer wanting to place a permanent structure there? For the latter, you might need to sharpen the pencil a little further than pseudo-static methods.
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The name is a long story -- just call me Lo.