Settlment characteristics of deep engineered fill 4

[smcc23]

Can anyone assist me with information of the settlement of engineered fill. the fill is a minimum of 4 m thick and has been well compacted throughout to become stiff to very stiff in consistency.
S McCormack

 

[jdonville]

smcc23,

Settlements will largely depend on

a) the magnitude of the applied pressure
b) the size, shape and type of the foundation
c) the preconsolidation pressure of the soil layer(s)
d) the thickness of the soil layer(s)
e) the initial void ratio of the layer(s)
f) the shear strength of the soil layer(s)

You will need to determine/estimate the void ratio, and preconsolidation pressure. You can estimate these from determinations of the unconfined compressive strength/undrained shear strength, in-situ moisture content and Atterberg limits.

You will then need to compute the settlements using these parameters. Most geotechnical textbooks can show you how to do this. I prefer to discretize the soil profile into layers corresponding to the soil sampling interval and sum the individual results, after computing the change in stress due to the applied load for each layer.

Be careful that the applied stress does not extend below the zone of engineered fill into a relatively weak soil.

If you have any more specific information, please share it and we can give more particular advice.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[BigH]

jdonvillle - I think, perhaps, he is talking about self-consolidation of the placed fill??? You are quite right about the factors if we are speaking of the embankment's foundation settlement. For the self-consolidation of the fill, one would need to know, too, the loading rates applied, the likely OCR due to the compaction effort, etc. It's an interesting question. I don't have any "finger-tip" references on this. If I find out, I'll advise.

 

[smcc23]

Thankyou for these responses. very helpfull for someone being out of the geotechnical testing field for 10 years.

I have requyested atterberg tests and moisture contents and the consistency was assessed using pocket penotrometer and dynamic cone penetrometer to 5 m. Below the controlled fill is a stiff natural clay layer which in turn is underlain by shale bedrock at approx 8m. The structure proposed to be constructed is relatively light requiring an allow. bearing pressure of 150 kPA which should be adequate.

How would i estimate void ratio and preconsolidated pressure

 

[BigH]

First off, in most cases, given the stratigraphy you have indicated, one wouldn't be worried too much about settlement or about bearing capacity - other than the fill layer itself. For OCR, you would probably have to do a consolidation test - as the material has undergone large compaction pressures, you can't assume that it is normally consolidated. You might use a test pit to obtain a block sample (best) and do a consolidation test to get idea of OCR. One point - I am aware of a case history where a large clayey silt fill was used and it caused problems with a housing development. The lifts were too large and the compaction was only in the upper half with little in the lower half of a lift. This was quite evident in strength testing that was carried out. The uncompacted sublayers were the big culprit.

 

[GeoPaveTraffic]

BigH has provided very good information, particularly about ensuring that the fill was properly placed. Your risk is much much greater if placement of the fill was not monitored on a regular basis during construction.

Assuming that the fill was monitored during placement and compacted to at least 95 percent standard or 90 percent modified Proctor, I would not be the least bit concerned about the self weight settlement of 4 meters of fill.

If you don't know what the compactive effort was, then you have a big problem and need to do borings and/or test pits to evaluate the fill.

 

[fndn]

Generally in this case we need to determine two things. Is the new fill as compacted as we think?-we can check this by taking split spoon or tube samples connected to drill rig and measuring the unconfined compressive strength. The other item is to see if the stiff base soils have completed their consolidation due to the new fill placement. This we may check by taking undisturbed samples and performing a consolidation test. The only catch is if those base clays are overconsolidated, it is difficult to compare the preconsolidation pressure from the test with the new effective stress and make any conclusions. So that leaves us with judgement issues. We could place few settlement plates and start construction once the readings have levelled off. Also you may consider reducing the bearing pressure by 33% to help out on the settlement numbers. What BigH is referring to will work, but the pits will have to extend few meters into the base soils, consolidation strains should ideally be less than 4 or 5% and time between end of fill construction and begining of proposed building should generally be several months. One last point is if your building construction will take place over several months this can be factored into the settlement calculations and may allow you to start building sooner.

 

[BigH]

fndn - with a stiff clay that he has, 4m of fill is only about 70kPa which is almost assuredly in the recompression range of a stiff clay - so the fill's effect on consolidation of the stiff underlying clay is really mute. I agree with GeoPaveTraffic in this. I was implying that a test pit to obtain a sample of the fill only might be in order - especially if one is "worried" about settlement of the fill. Footings of his light construction would not load the stiff natural clay either. Point of doing a pentest through the fill to confirm that there is a uniformity of compaction might be a good idea.
The salient point to be studied, in my view, is whether or not the "stiff to very stiff fill" is in any way problematic. I guess that where I have/do practice, we don't number crunch as much as others might for the situation as described - our experience and judgment is important. (I know, all, go ahead and hit the bulls-eye on this last statement!)