Soil internal friction for uplift 1

[NedGan76]

Hi,

The weight of backfill helps the foundation to resist uplift. Conservatively, we could take only the part, that is contained in a prism, directly over the base area, e.g. F_bf = γ_bf*h_bf*A_f.

However, I think that due to the angle of internal friction φ, some neighboring layers would be also engaged. So, instead of prism, it is more likely to have something like inverted frustum.(Please turn the picture bellow upside-down

Is it allowed to account for the internal friction when calculating uplift and how much? What is the practice in your country?

 

[geotechguy1]

EPRI publishes some good reference material on this topic that is now freely available:

https://www.epri.com/research/products/TR-100220

https://www.epri.com/research/products/EL-6965

https://www.epri.com/research/products/EL-2870

My general experience has been that practioners and codes tend to either: Use an absurdly conservative method of calculating uplift resistance, but require only a factor of safety of 1; or alternatively use less conservative methods but with a larger factor of safety.

 

[SlideRuleEra]

NedGan76 - At what depth below the surface does the friction angle lose its' meaning for uplift? A few inches? A few feet? Does that depth change as soil moisture varies?

IMHO, that unknown depth makes relying on soil included by use of the friction angle somewhat of a guessing game.

 

[Mccoy]

I find this topic conceptually interesting. And maybe I'm missing something, since in the formulas I know, posing phi=0 means friction=0 hat is, zero uplift resistance.

Anyhow, the frictional force which develops along the potential failure surface, which is the concrete-soil interface in many simple geometries; that's as we know a function of delta (tan delta), the angle of friction at the interface, which is a function of both materials in contact. If we have cast in place concrete, then some technically sound suggestions are to take delta=phicv, the angle of friction at constant volume, which is not always a simple amount to define clearly, but which in purely, or mostly frictional soils is placed around the value of 30°.
Other suggestions are to take delta= 2/3 phi_peak.
Then there are the NAVFAC suggestions, clearly tabulated for many materials.

https://www.finesoftware.eu/help/ge...friction-factors-for-dissimilar-materials-01/

So, just correctly defining this single parameter implies some careful reasoning, not to mention the issue of the depth raised by slideruleera (my present opinion on it is that it is another design factor upon which to reason, situation by situation).

Anyway, in mostly frictional soils, if we are able to estimate the soil composition, I'm observing (from quickly tabulating usual values on a spreadsheet) that the variability of tan(delta) is not substantial.

Sands and silty sands: from 0.51 to 0.62
Gravels, depending upon angularity and uniformity: 0.62 to 0.78

That's of course the multiplying factor that transforms the horizontal component of the lithostatic force (effective earth pressure) to frictional force upon the interface (which resists the sliding).

Bottom line, most probably, the horizontal earth pressure (unfortunately a more uncertain factor) is the governing factor here.

 

[Mccoy]

NedGan76 - At what depth below the surface does the friction angle lose its' meaning for uplift? A few inches? A few feet? Does that depth change as soil moisture varies?

IMHO, that unknown depth makes relying on soil included by use of the friction angle somewhat of a guessing game.

Of course it's a guessing game, but in presence of reasonably accurate soil investigations maybe we can decrease the guessing and increase the reliability.

My obvious... guess is that if for any reason (like seasonal drying) there is no more contact, then along a determined depth interval there is no more friction. Also, surface soil has usually lesser friction, water may accumulate at the interface and so on and so forth.

My first thought is that a safe, although rough assumption would be to ignore friction for the altered thickness of soil, or perhaps for the shallower meter of thickness, where usually the effects of seasonal changes of soil moisture are assumed to be non-negligible. It would depend on site conditions though.

 

[SlideRuleEra]

...frictional force which develops along the potential failure surface, which is the concrete-soil interface...
And maybe I'm missing something...

One of us is missing something. The way I read the OP's post, the question is not about friction at the concrete-soil interface but either "the weight of backfill... directly over the base area" or increase by allowing the soil's friction angle (working "up", inverse to normal use) to enlarge the volume of soil resisting uplift (by dead weight of this soil).

In my simplified, 2-dimensional sketch either Case 1 or Case 2:

 

[Mccoy]

SRE, thanks by claryfing, by reading the OP again your interpretation is most probably correct.

What would change in my previous reasoning is that soil-to-soil friction (in the backfill) should be used as a friction multiplier, in lieu of concrete-to soil. Plus the soil weight contribution to resistance within the failure surface of course.

If the frustum method (case 2) is applied, how would you calculate the resistance contribute due to side friction? OR would you just disregard it and be happy with a greater soil weight?

 

[le99]

Curious to know what is soil-soil friction for non-clayey soil submerged in water?

 

[geotechguy1]

Referring back to my previous post:

1. Case 1, but only require FS = 1
2. Case 1, but consider the shear strength along the cylinder of soil. Typical to require a larger FS in this case
3. Case 2, considering only the weight of the wedge. Note that there are a larger number of ways in the literature to calculate the angle and shape of the wedge - eg. a straight line with an angle based on a portion of the friction angle which will vary depending on what reference you use; a lognormal or curved surface
4. Case 2, but consider the shear strength along the curved surface

Of course there are reasons you may wish to neglect some of the shear strength or alter the shape of the wedge depending on the depth, i.e. cracks in the soil etc that would result in a preferential failure path. The only way case 1 is justifiable is if you assume you have cracks extending all around and thus can neglect the shear strength

 

[Mccoy]

Geotechguy, I agree with the above, I believe, when adopting both contributions to resistance, that we may also use advantageously partial safety factors, 1 or close to it for the less uncertain soil weight and a larger one for the more uncertain friction term.

 

[Mccoy]

Curious to know what is soil-soil friction for non-clayey soil submerged in water?

If we are considering the backfill, in a loose state, then submersion in water would mean that it is likely to be at or close to its lower bound of compaction, so the use of phi[sub]cv[/sub] would be a reasonable estimate, since it is by definition the lower bound of the set of phi values for a given granular soil, with given grains properties (shape, roundness and so on).

 

[PEinc]

Here is information from the American Concrete Pipe Association on manhole flotation.

www.PeirceEngineering.com

 

[NedGan76]

Thanks, Mate! This is so simple and practical. I like to see worked examples. What is interesting is that the sliding resistance has significant contribution that is compatible to the self weight: R = 364.6 kN > W = 321 kN > B = 180.6 kN.

 

[Mccoy]

The delta values in table 1 are lower than delta=phi_cv values for the corresponding soils. I wonder if the AASHTO values are still the state of the practice.