Secondary Consolidation of Compacted Clay Fill

[mark671]

Hi. I have a proposed residential site where up to 5m of clay fill (glacial till, from various new and old stockpiles) is proposed to be compacted at 95% maximum dry density on a 300mm granular drainage layer. The houses will be piled however is long term settlement of the fill itself an issue for the roads, kerbs, patios, services etc? Thanks

 

[hokie66]

Yes, wherever differential settlement will present problems.

 

[Ron]

Is this new fill? (has there ever been this much soil over the area before)
What soil conditions are under the fill?

You have two considerations to make....one is settlement of the new fill. You can reduce that even more by requiring more compaction....why not 100% of the standard Proctor? Not that much more effort required assuming the moisture content can be controlled.

The second consideration is the soils underlying the fill. What is their settlement potential? If this is new fill, you are going to be adding somewhere around 1500 psf (72 kPa) to the top of the underlying soil.....how will this affect it? Is it sand? Silt? Clay? Overconsolidated or underconsolidated?

Will settlement of the fill add to downdrag on the piles?

A few things to consider.

 

[mark671]

The underlying strata is typical firm clay (glacial till / weathered Charmouth Mudstone), near surface soft materials will be removed and a granular drainage blanket installed, and the piles will need to be designed accordingly. The fill will be variable glacial till / arisings from adjacent developments, some of the material has been stockpiled on the area of the proposed filling for some time, some is much more recent. The stockpiles are currently approx.. 6m high which will have pre-loaded the underlying soils, but they will need to be removed and the site stripped. It's the long term settlement that is the concern. At present I've indicated settlement could be up to approx. 100mm but over what time? I note the suggestion of 100% compaction but the existing materials are significantly above optimum moisture content.

 

[BigH]

You haven't indicated any properties of the clayey till - i.e., liquid limit and plasticity index - typical Ontario silty clay to clayey silt tills are usually in the 10 to 15 range. Obviously you must have some lab data if you know the existing materials are significantly above optimum moisture content. You have not mentioned either how thick the fill will be. Based on your post, the underlying materials would be, for all intents and purposes, incompressible (I would assume that the N value of the natural till is greater than 15 to 20) so I am wondering why the residential housing is on piles. Is this for high rise apartment buildings or single/double storey residential housing? Are the piles to be installed "deep" into the underlying till/mudstone? What type of piles - driven or bored? I wouldn't mind seeing some of these details.

So the real question may just be the compressibility of the fill - under the roads if compacted to 95% standard Proctor within a couple of percent of optimum, I wouldn't see 5 m of fill causing any problems with settlement over the long term within the fill itself - so the roads, kerbs, etc should be okay. If the soil is wetter than that, you should give consideration of aerating it to reduce the moisture content. There are other toys that could be used. Make sure you put in joints at 2 to 3 m for the kerbs if done by slipform kerbing machine. As for downdrag on the piles, I would think that the fill wouldn't settle sufficient for any major downdrag forces - and if the pile tip can "give" a bit, it would be less. You can look up Fellinius to see what downdrag might have/occur.

Just some thoughts - provide some details as noted in the first paragraph and you might get some better response.

 

[mark671]

The liquid limit of the proposed 5m of clay fill typically ranges from 41-47% with one result (of 8) at 57. Plasticity indices all ranged between 24 and 27%. N values in the Till were 12-17 below the proposed embankment (upper soft layers will be removed). The housing will be piled due to the thickness of fill. The piles will be designed by a specialist contractor, the main risk for us is the settlement of the fill!

 

[L0k]

I agree with BigH, there is no significant problem for settlement in 95% Standard Proctor within the optimum water content and his wondering as well.

 

[mark671]

I think you might be right; i'm going to do a few calcs but not sure what to use for the coefficient of secondary compression (Ca) yet! I guess we need some oedometer tests on re-compacted fills?

 

[BigH]

For a glacial till, I don't think that a Ca value is of any immense importance. Lambe and Whitman quote the following: Normally consolidated clays 0.005 to 0.02; Precompressed Clays with OCR>2, less than 0.001; very plastic clays/organics (of which you do not have) 0.03 or higher