Settlement on a footing on top of a fill

Question

Settlement on a footing on top of a fill

pelelo (Geotechnical)

(OP)

12 Sep 22 22:52

Hello engineers,

I am going to compute the settlement of a footing. The situation is, that on top of the existing ground, there will be a 15 ft fill and then on top of that the proposed structure will be installed.

How would you compute the settlement:

1) Based on the load on top of the existing ground (15 ft fill) + bearing pressures from the footing itself?. Assuming a 1000 psf of bearing pressures + 15 ft x 120 pcf =~ 2800 PSF

OR

2) Based on the footing on top of the fill? (Just 1000 psf of bearing pressures).

Can anyone provide inputs?

Thanks.

dik (Structural) · Answer 1 · 2022-09-12 23:50:08

First question, how was the fill consolidated? or, was it? and what type of material was used for fill? ponder

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

pelelo (Geotechnical) · Answer 2 · 2022-09-13 00:01:03

The proposed fill will be silty gravel (GM) fill. It will be compacted in layers, 15 ft high.

TigerGuy (Geotechnical) · Answer 3 · 2022-09-13 00:05:25

A silty gravel, placed in uniform, compacted lifts, should not consolidate much if any. The fill will be supported on what material? Will you delay structural construction until the underlying material is sufficiently consolidated?

emmgjld (Geotechnical) · Answer 4 · 2022-09-13 00:10:27

There are 3 computations to perform.
1: The fill on the native ground
2: The Structure on the fill
3: The fill + structure on the native ground

dik (Structural) · Answer 5 · 2022-09-13 00:10:31

Does it need to be frost heave resistant? If so, try to eliminate the silt.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

BigH (Geotechnical) · Answer 6 · 2022-09-13 16:09:21

First off, my initial question would be what is the nature of the soil onto which you will be placing 5 m of fill?

If the soil is dense sand and gravel or even compact, the 5 m of fill will likely have little effect on the settlement of the footing as settlement of the supporting granular material will happen as the engineered fill is placed.

If, however, the fill has purchase on soft clay, that is a whole different ballgame - or even firm clay. In this case, the supporting material will settle due to the mass of the 5 m of fill. Unless there are mitigating measures taken, the settlement of the supporting soil may take many years. As a result, the structure you are placing in the engineered fill will settle over a long term based on the settlement of the fill supporting soil. Now if the supporting fill settles uniformly, this would not likely to any degree affect your structure except you might (as in Mexico City) have to step down into the structure instead of "up" wink

. If the clayey fill beneath does not settle uniformly - and this is a possibility due perhaps, to an old fill dump pile that sat for a long time over some of the area and then later removed - differential settlement may happen and this could affect your structure and cause issues.

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Settlement on a footing on top of a fill