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Full Stack Settlement

Full Stack Settlement

Full Stack Settlement


I am involved on a project which consist of development of a landfill.

The idea is to store Ash up to a height of about 60 feet from grade (surcharge ~ 7200 psf). The length of the area is 1000 feet by 900 feet.

Somehow client is concern about any potential settlements at the base of the landfill. Soils are SP and SC in the top 20 ft, N from 5 to 20. We did 1 boring 100 feet deep and N values were on average 30 down to 60 feet. Then found a 5 foot layer of SP, with N = 2 and again, SP with N between 10 and 20, down to 100 ft. Rock was not found.

Having a width of 1000 feet, I think that 100 feet deep boring will not provide all information needed for settlement analyses of this landfill area (load ~ 7200 psf). It is not economical to drill 1000 ft borings either.

In that case, how would you perform a settlement analyses?

By using schmertmann, having a L/B ~ 1, means that my analysis will end at 2 B, which is something out of proportion.

At the same time, I think all these methods for stress distribution are for footings and small structures, I don't think they are effective for LARge structures such as a 1000 ft landfill.

Please le me know what you think.

RE: Full Stack Settlement

We think in terms of modulus for each layer, but. . .

Modulus does vary by confinement. Hyperbolic parameters can be considered to show that soil modulus really goes high at these various depths!

Also, the seat of settlement is typically taken to the depth where the delta sigma H is less than 10 percent of the in-situ (i.e., pre-placement) stress. A delta sigma v of 7200 psf would then extend to the depth where the confinement is 72,000 psf and that's 600 ft. Then again, you'll have some attenuation from the original load.

good luck in this? I'm not completely sure how I'd handle this problem, 'cause I don't know where the project is located. Glacial terrain? Coastal Plain? Depth to bedrock?


ípapß gordo ainÆt no madre flaca!

RE: Full Stack Settlement


Per geologic publications, the rock is about 150 feet from the ground surface.

Yes, I know this is not a typical settlement analyses since the area is so large. Job location is in Florida.

I know that the recognized settlement analyses do not consider such large areas.

RE: Full Stack Settlement

Just playing devil's advocate, but hear me out. Say the allowable bearing capacity is 3000 psf and 1" of settlement is expected (pretty typical). You're at 7200 psf, so say your settlement doubles to 2". First, how would he/she notice a settlement of 2" on a pile 60' high? Second, why would he/she care?......its a landfill. Seems like an academic exercise.

RE: Full Stack Settlement

Quote (pelelo)

Somehow client is concern about any potential settlements at the base of the landfill.

What is the source of the ash? Maybe municipal waste... or coal?
Since the project is in Florida, how far below the surface of the existing ground to the water table?

Keeping the bottom of an ash disposal area above the water table is important for environmental reasons. That is probably why the settlement prediction is needed.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Full Stack Settlement

This isn't all that hard.

Drill to rock so you know where it, not just from maps.

Ignore settlement in the rock.

Calculate the settlements based on standard settlement theory based on your materials. Looks like there is no clay to speak of, therefore no consolidation settlement to worry about. All elastic. Can't get much easier.

If you want to get fancy, use something like Settle3D or just do it by hand.

Mike Lambert

RE: Full Stack Settlement

Don't worry about "seat of settlement" for such a wide area. There ain't any significant spread. Luckily the SP will hardly settle due to small range from loose to dense. You might run a trial "settlement" on firm samples of SC to estimate deflection under different pressures and go with that. It likely would be on the generous side.

RE: Full Stack Settlement

Rather than settlements (you are not adding any structure on top of the fill, correct?), I would be more worried about the slope stability at the edges of the fill. 60 feet is quite high and if you have structures around you should also look at slope stability.

Also, I am not in the US but I heard that in some areas of Florida there is karst geology. Any concerns about cavities/sinkholes?

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