Soil Pressure under Pile Cap! 2

[JKJohn]

When the pile cap is placed directly on the soil, does some of the load transfer to the soil? And does the code address this at all?

 

[civilperson]

Yes, and to what code do you refer? Most engineers neglect any contribution of pile cap on soil.

 

[JKJohn]

By code I mean the ACI.

 

[msucog]

i can't think of a reason not to neglect it...even if the soil is very stiff...

 

[JKJohn]

Considering it, means the load on the piles is less, so, less pile length, less money.

Anyhow, I guess my question was more, how does the ACI address this? If it does at all.

 

[DRC1]

Pile caps are designed based on the assuption that the soil will settle beneath the pile caps and the cap beam willspan between the piles. Design the cap as a beam. I don't know if ACI directly addresses this, but it is a pretty basic tennant of geoetechnical engineering.

 

[msucog]

if the soil below the cap can support the load, why use piles anyway...design it for shallow foundations and save a lot of money (probably).

 

[BigH]

The points made are helpful. I wouldn't believe that any designer, for small pile caps, would count on any support of the structure by pile cap bearing. However, saying that, there are piled-raft foundations where the interaction of the piles and raft support the structure. Google Poulous on this and you should find a few papers of his - there are quite a few others on the net.

 

[BigHarvey]

That has been done once in Italy ( in Pise I believe for a tower )

 

[kleo]

All good points above. Small pile caps are assumed to be rigid so all the load goes into the piles. If the piles deflect there may be some load sharing between the piles and cap, and in reality this would reduce the load on the piles (and increase the pile FS). This is the basic idea of a pile raft although they are designed with a pile FS near 1 to induce movement and hence much greater load sharing.

 

[oneintheeye]

yes then the ground settles and load goes back to piles until the pile stops settling then carries all the load. It may share load short term, but not long term (how long depends on the soil type). You'd be a fool to assume it does, in my opinion and I cant see it getting past any checking engineer.

 

[InDepth]

There are a few other issues to consider as well:

1) You would need to evaluate the relative stiffness of the piles to the bearing stiffness. What you will find is that the pile is stiff, but the "friction" spring stiffness of the soil may be less than the bearing stiffness.

2) Also you have to be careful with upheave caused by potential swelling of soil that may impose an upward load on the pile cap (translates to load on piles). Detailing of pile to pile cap connection!

3) Downdrag - usually due to certain soils and EQ liquefaction. Thus a load again imposed beneath the pile cap....but then how do you assess liquefaction settlement. Once again detailing.

4) A combination matt and or piles/tiedowns will require a soil structure interaction model to asses stresses within the caps due to point restraints and soil bearing.

MAKE LIFE SIMPLE - don't account for bearing of pile cap in design....but detail connections properly.