CFA piles and high hydraulic pressures

[Mccoy]

Hi all,
I'd like to ask your opinion on the following issue :

Last year I was involved in this project of a multi-storied building on piled foundations, my reccomendations were based on large diameter bored piles, as per local practice.

Soil sequence:

-25 to 30 m homogeneous stiff clays (Su=~ 100kPA)

-Gravel layer, 5 to 10 m of thickness, N(60) 20 to 56

-Very stiff Blue clays, Su in excess of 200 kPa.

Pile base would be located a little beyond the gravel layer, or within it.

Now, the gravel layer makes up a confined aquifer with high fluid pressure, with an excess piezometric head, with respect to elevation, of 1.6 to 1.9 bars (water table would rise from 25 to 5 m of depth).

To avoid problems related to inward and upward turbulent flow, cased holes and good bentonite conditioning were suggested.

Now the contractor wants to drill CFA piles for various reasons. I have no experience with such piles, they should be pretty OK in the upper clay layer but my concern is about the gravel aquifer with excess water pressure at 25-30 m of depth, both in the drilling and cementing phases.

What's your opinion about that, did you ever come across such a situation, should I fall back to cased CFA piles, or is that not really necessary; or would CFA piles do well in that condition at all and better to rule them out. What would be the best diameter, a very large one or a moderately large one (I don't know if there are strong constraints due to structural design and loads).

In this kind of project, better to keep conservative since nobody in the team is very familiar with CFAs

 

[BigHarvey]

Mccoy

You shouldn't have a problem since the water level will still be 5 m below platform level ( I assume concrete will go up to platform level ).
I would just make sure that a colloidal agent is added to the cncrete formula like for underwater concrete ( Pozzolith hydrogel from MBT for axample ).
This being said, your deepest piles must be around 35m which is rally the limit for CFA piles. Few foudation contractors have experience of such deep piles with the CFA method. At that depth don't forget that you can't lower a reinforcement cage so if your piles need a reinforcement all the way down ( seismic design for example....), just forget it whatever the contractor claims. With the best concrete mixes I have never seen fully successfull cage installation deeper than 25 m.

 

[BigH]

McCoy - I don't have any experience with CFA piles; but the depth seems to be a bit deep notwithstanding BigHarvey's experience. The upper clay seems very good, actually - have you considered use of piled raft foundation instead of individual groups of piles? I realize it might be a bit late. But with the gravel layer - perhaps you don't really want to "break" into it . . . The water would seep up initially, likely, adjacent to your pile and soften the soil somewhat - am not sure if you are counting at all on the adhesion.

 

[Mccoy]

Mmmm, thanks guys for your insight, the contractor has already used this method to support the so called 'Fuksas nebula' in the new convention center in Rome.

Evidently he liked the benefits of not having to manage slurries and excess soil.
But I doubt the stratigraphy is the same, I'll have to ask'em for details.

BigH, what you say about depth is just about the same thing which says bigHarveY: there is a limitation which in the specific case could be significant.

The possibility of upward seepage would also concern me, as well as eventual oversize borehole or overflighting, as they call it, into the gravelbed.

The idea of shallow foundations was ruled out because of 2 to 4 m landfill, sorry I forgot to mention it previously, and basement in that area are prohibited by regulations (there is a slight risk of floods).

BigH, I thought you were travelling around in fareastern Asia, too busy eating durians and dragonfruit to link into the forum!!

 

[BigHarvey]

Maccoy

You won't have overflighting in this kind of material.You will have seepage but the colloidal agent will prevent the water to ingress the fresh concrete.
The only problem is depth for two reasons :
1)I don't know of any CFA rig that goes deeper than 35 m, so if the geology requires a pile deeper than 35 m, you just can't do it; therfore you must be sure of your SI;
2)Do you need a reinforced cage in your pile deeper tha 25 m, if the answer is positive then forget about CFA
Be careful about your design : in France at least we don't use the same soil friction parameters for rotary mud piles and CFA piles ( CFA piles would be deeper for the same capacity )

 

[Mccoy]

Thanks big Harvey,

I've talked with the structurist, he'll go and meet the construction company representatives on tuesday.

If they insist on using CFA piles, he'll require to drill one test pile outside the pile group, close to the vertical where the bearing layer is deeper. It will have also to be load-tested.

This way, operational problems will occur soon and eventual undeperformance will be visible after the load tests.

 

[oldestguy]

Every important job I have been on using them does have at least one load test and for some more than one. How else do you prove they are adequate?

 

[Mccoy]

That's a sure thing, also regulated by the new EC7-derived codes.

Only, in this case as far as I've understood it, the pile will eventually be a true 'test pile', before ever planning the piling job.

In that area so far only bored piles have been drilled and they hopefully are giving no or little surprises, so the passage to a new technique would require extra observation and testing. this in case the contractor is not discouraged by the preliminary observations. I'll let you know about the developments. Evidently, waste management costs are pretty high. They sure were in the center or Rome, less so in the province where this project is located, may be they've been carried out by this budgetary aspect.

 

[jdonville]

We install CFA piles in the Midwest US (Indiana, Kentucky mainly). Agree with BigHarvey - long cages are very hard to install in CFA piles for various reasons. Mainly here, we do CFA in sands, which provides a bleed path for the water in the grout.

J