Friction Piles in Fill

[Mad Mike]

I'm currently involved in some big industrial developments in South Africa where high fill embankments are being engineered over a non-compressible soil profile, predominantly weathered sandstone bedrock.

The fill materials are variably graded sands (SW-SP-SM) engineered in 12" layers to a specified 95% Mod. Proctor, total fill thickness often as much as 50 to 70 feet.

The platforms are typically left to stand for a couple of years before the construction of warehouses and associated office facilities commences upon them.

I have had it recently where proposed three-storey office blocks are underlain by relatively uniform fill thickness in excess of 50 feet. As a foundation solution, I've conservatively recommended a structural raft, my justification being that piled foundations would need to go all the way through the engineered fill to found into weathered bedrock. In severe cases, the depths to bedrock would be pushing or exceeding the practical limit of conventional CFA pile installation.

My question is this: At what stage in the lifetime of a sandy engineered fill embankment, might one be bold enough to design friction piles hanging up in the fill material? And if doing so, what might the design procedure look like? I've read the articles documenting hydro-consolidation of sandy fills, which suggest they might still be settling somewhat even 10 years after construction. Do we therefore assume negative skin friction on piles even in engineered sandy fills that have been in place for 5 to 10 years?

Any advice or references would be greatly appreciated.

Mike

 

[oldestguy]

Your compacted fills are most likely better than any natural deposit or weathered rock. No way would I use piles. If used, of course negative friction for those piles going to "rock" if that rock is compressing, which is quite unlikely after fills are placed.

 

[MTNClimber]

Hydrocompaction may not be an issue. Do you think the fill will be exposed to a lot of water? Will rainfall runoff be properly managed and diverted away from the building? Will there be a lot of hardscape around the building? If water won't be infiltrating into the fill, then I wouldn't be so concerned about hydrocompaction.

 

[Mad Mike]

Thanks guys; the final platforms will have all due drainage and will be paved; the only significant infiltration of water would have been during construction of the fills and the two year period they're left to stand before the structures are built.

Granted the fill has been well constructed and as OG notes, certainly provides suitable bearing for shallow footings in most situations; my query now relates to structures which might not be able to accommodate the normal settlement in this fill thickness, for whatever reason - my option then becomes a raft foundation or piles.

However, unless piles can be designed to rely on positive skin friction from the fill, their lengths become uneconomic (think 200 to 300mm piles, 70feet long!).

So my question again is, at what point in time would a geotechnical specialist designing piles be confident enough to assume positive skin friction in a well-engineered sandy fill material? Conventional recommendations are that, where piling through a newly constructed fill, some allowance for down-drag of the fill should be made. However there obviously comes a point in time where one could confidently rely on positive skin friction in the same fill embankment...

Any rule of thumb for estimating this turnaround time? I understand that in many / most cases, it would be influenced by what is beneath the fill, but in my case the fills are founded onto non-compressible bedrock.

If there is a chance of utilizing friction piles supported entirely in the fill, I would like to know about it!

Mike

 

[oldestguy]

I answered that before, but to this question again. "Yes"
So my question again is, at what point in time would a geotechnical specialist designing piles be confident enough to assume positive skin friction in a well-engineered sandy fill material

 

[dik]

As OG notes, yes... do you know how well the material underneath the fill has been compacted... any soil test results for the material? Are you putting significant loads on the compacted fill?

Dik

 

[Mad Mike]

Thanks OG, sorry for having you repeat yourself!

Dik; the loads are not necessarily very significant in terms of the fill bearing capacity, which I don't doubt. But for multi-storey, rigid brick structures I'm hesitant to recommend conventional shallow foundations on this 70' fill thickness, because I believe the structure will crack.

Why I believe this? I'm not exactly sure...but that much fill beneath a sensitive structure makes me nervous despite it being well engineered.

There is effectively no soil under this engineered fill - it is founded on rock.

Cheers,
Mike

 

[Okiryu]

Mike, If your settlement calculations indicate that settlements are excessive for conventional footings, I think that your recommendation for a raft or piles is defendable.

I don’t think that negative friction will be an issue since I expect that settlements already are finished and you are not adding that much load on top of the fill.

Also I am just curious how are you retaining that high fill at the boundaries of your site. MSE walls? If your building is close to retaining structures you may need to check that it is outside the active wedge otherwise you may need to account for potential displacements depending on the type of the wall.

 

[Mad Mike]

Okiryu, thanks so much for your input - my embankments are unsupported batters so retaining structures won't come into play.

I'm adopting raft foundations as my preferred choice for sensitive structures underlain by large thicknesses of fill. I don't want to recommend friction piles hanging entirely in the fill, even if it has been magnificently engineered.

All the best,
Mike