Pile Cap Design 1

[someengineer]

Hello,

I am looking into the design of a pile cap and was wondering if I could get some insight into a couple questions I have.

The foundation is being used to support a large tank above grade. The foundation was first analyzed as a mat bearing on structural fill with dimensions 18m x 18m x 2.5m thick. The bearing pressure of the soil was not sufficient so we are investigating using piles underneath the mat to support the loads.

My question is - should we be assuming that the entire load of the tank and mat (same dimensions as previous) is taken up in the piles or can you consider the soil underneath the mat to support some of the load? The piles are bearing on bedrock so I imagine they are a lot stiffer than the soil.

Also, the underside of the mat is 2.5m below grade - can anyone offer some direction as to how much lateral load will be taken up in the soil adjacent to the mat? Is there a way to use the vertical subgrade modulus to determine a lateral modulus and possibly convert that into a spring constant? Can it be assumed that the adjacent soil will absorb all the lateral loads and the piles take vertical load only?

Hopefully my questions are clear.

Thanks in advance!

 

[SlideRuleEra]

...can you consider the soil underneath the mat to support some of the load?

No. Design the piles to take all the load. You don't want to rely on a combination of soil and piles for the design to work - soil settles, piles not so much.

Talk to your geotech engineer about lateral load support from the soil - probably not a good idea. A pile supported structure should be stable all by itself.

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[hokie66]

Agree with SRE about the vertical support, but as to the lateral, it depends. Building structures commonly depend on the soil, but maybe that is not the case for tanks.

 

[killi_82]

100% agree, vertical loading shall be taken by the piles.

in lateral case, the pile is more sensitive to the displacement in lateral direction, however the passive resistance of the soil is somewhat something that need to be displaced for certain displacement to get it works. passive resistance is not so easily activated as in active pressure for soil.
So in case that lateral forces is happened, I think we should ensure that the piles will not broken before the resistance of soil works.
What is your lateral loading? earthquake, wind, etc..?

 

[KootK]

Also, the underside of the mat is 2.5m below grade - can anyone offer some direction as to how much lateral load will be taken up in the soil adjacent to the mat? Is there a way to use the vertical subgrade modulus to determine a lateral modulus and possibly convert that into a spring constant? Can it be assumed that the adjacent soil will absorb all the lateral loads and the piles take vertical load only?

1) I agree with SRE that, in a perfect world, I think that all structures supported by piles for gravity loads would also be supported by piles for lateral loads. This is conceptually elegant.

2) I agree with Hokie66 that, in the world of buildings, it is common to rely on horizontal soil pressure in order to come up with an economical, competitive structure.

3) I agree with Rubudiman that piles will usually be laterally stiffer than the soil being leaned on which means that the piles will tend to attract lateral load. I know that many engineers use Ko or Ka resisting soil pressures because they require less movement.

As a compromise, I sometimes rely on earth pressures to resist lateral loads but I design the piles to be able to resist a shear load assuming that all of the shear went to the piles. My intent has been to prevent the piles from shearing apart,in brittle fashion, before the load is transferred to the resisting soil.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

Good discussion on resisting lateral loads. I have made some unstated "hypothetical" assumptions that I should clarify. Since the foundation is large (18 meters square), thick (2.5 meters), and does not work as a soil supported mat, I'm assuming the tank, including contents, is "large", "tall" and "heavy". Also, based on this size, the tank is assumed to be for an industrial project.

If the soil is too "poor" to support the mat, it "may" be too poor to rely on for significant lateral support.

On an industrial site, it is reasonable to assume that over time there will be excavations for assorted reasons - they could be immediately adjacent to existing foundations. You would not want to compromise lateral support by digging next to the mat.

If there are seismic loads, the contents of the tank (if liquid) can "slosh". This sloshing can create significant lateral loads.

That is a lot of assumptions, on my part - maybe too many. I'll defer to our colleagues, if site and lateral loading conditions justify making use of the soil for lateral support.

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[aeoliantexan]

Tanks may be grounded using several rods driven into the ground and joined by a cable laid in a trench 2 or 3 feet from the foundation. Most likely, the trench backfill receives no compaction and remains highly compressible. The presence of the trench prevents the development of passive pressure to resist lateral loads.

The same thing can happen with retaining walls and telephone cables, of course.

 

[someengineer]

Thanks for all your help.

The assumption that this is an industrial project is correct and the tank is actually wide and squat.

Based on the comments, it seems prudent to use the piles to absorb all or most of the lateral loads. The lateral loads are a result of both wind and seismic, with seismic typically governing.

Any more discussion or questions would be most appreciated.

Thanks!