Identifying Qs and Qp for a pile 1

[ENGR_2321]

If hypothetically I have a caisson that has:

1. a uplift force in kips , let's say of 100 k
2. and a gravity force in kips, let's say 75 k

knowing that these 2 force are NOT acting at the same time and also knowing the value of

1. the allowable soil skin friction (psf)
2. the allowable soil bearing pressure (psf)

Is it fine to say that the uplift force is the Qs (skin friction resistance) and the gravity force is the Qp (load carrying capacity of pile point) ?

If SO, can I can get the diameter of this caisson using the formula for EITHER Qs or Qp.

or am I only supposed to use one them (perhaps the highest one of the 2 forces and therefore only one the equations)?
Thanks!

 

[SlideRuleEra]

1. a uplift force in kips
2. and a gravity force in kips

knowing that these 2 force are NOT acting at the same time...

Maybe I misunderstand the question, but gravity "works" all the time. The gravity force (dead load of the caisson) will fully, or partially cancel uplift force.

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[ENGR_2321]

Sorry I updated it. Assuming the uplift is 100 kips and the gravity is 75

 

[SlideRuleEra]

ENGR 2321 - Thanks for the clarification. In that case, friction will have to resist 25 kips uplift. When the uplift force is not applied, point bearing (and perhaps friction, depending on soil conditions) will have to resist 75 kips.

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[ENGR_2321]

Okay that makes sense. so Qs= 25 and my Qp=75 . I choose the highest one of these ( so the 75) to equate it to its respective formula and solve for my diameter is that fine? I'm assuming this since if the force is higher it governs so I'm guessing that's the one I should use to find my caisson diameter

 

[SlideRuleEra]

I choose the highest one of these ( so the 75) to equate it to its respective formula and solve for my diameter is that fine?

No, hypothetical questions can be that simple but real questions have "complications".

For example, say the caisson is installed though a layer of "poor" soil (very low Q_s), then suddenly encounters "excellent" soil (very high Q_p). Drilling is stopped without significant penetration into the "excellent" soil. Calculations based on a point load of 75 kips may give a reasonable estimate of required caisson diameter. However, using that diameter, the caisson will fail miserably in uplift (25 kips) because Q_s is too low.
Note: This example is not hypothetical, these soil conditions are typical where I work.

There are other complications too:

1. We are not taking into account self-weight of the concrete caisson. This is a factor for uplift. May or may not be a factor for gravity loads... depends on the soil / ground water conditions.

2. Reasonable minimum caisson diameter for rebar cover and constructions purposes.

3. Reasonable caisson diameter to length ratio for loading conditions... say, how much lateral support does a "thick" layer of "poor" soil provide to a "long" caisson?

4. Point bearing is a relative value. For hard rock it very well may provide essentially all support. More often than not, the point bearing layer is "springy"; it can provide much support, but deflect enough that some skin friction is engaged.

5. For a very long caissons with heavy loads, actual (a very small amount) compression shorting of the concrete may need to be considered.

6. Probably some other issues, I've missed.

Designing for uplift can be very tricky.

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[ENGR_2321]

Okay I see. Thank you for your expertise