Skin friction of drilled piers 1

[lancex51]

We do drilled piers as part of our foundations somewhat regularly. When designing, we have assumed the following IBC section to mean that 1/6th of the bearing is acceptable to use as skin friction below frost:

1810.3.3.1.4: Allowable Frictional Resistance - The assumed frictional resistance developed by any uncased cast-in-place deep foundation element shall not exceed one-sixth of the bearing value of the soil material at minimum depth as set forth in Table 1806.2, up to a maximum of 500psf, unless a greater value is allowed by the building official... etc etc about geotech reports or field testing

Assuming we say to drill and cast against earth, I feel like an assumed value of 250psf (1/6th of the 1500 minimum from IBC) skin friction is a realistic number to use. Maybe this changes depending on if they set the pour up with a sonotube instead. Thoughts?

 

[oldestguy]

Sounds goofy. Let's say you apply a SF of 3.0 to the ultimate for bearing, then the side friction is 1/18th of ultimate for bearing..

 

[dik]

In these environs, typically 300 psf for skin friction with our clays.

Dik

 

[emmgjld]

I have typically used 1/5 of the 'bearing value' for the clays in Western Colorado, similar to the same criteria my father used on the Eastern Colorado Front Range. I have been fascinated for what has passed as 'bearing values' and the criteria for determination of such. GIGO!

 

[aeoliantexan]

There may be some logic behind the IBC recommendation. The ultimate bearing capacity of a shallow footing on saturated clay is approximately 6 times the cohesion. If the ultimate bearing capacity is reported as 2400 psf, using 400 psf as the ultimate adhesion would be reasonable. Then the allowable "skin friction" would be 200 psf.

 

[fattdad]

I think it has nothing to do with ultimate capacity of either the end bearing or the skin friction. It has more to do with strain compatibility.

It just takes more strain to mobilize skin friction than it does to mobilize the end bearing.

f-d

ípapß gordo ainÆt no madre flaca!

 

[BridgeSmith]

"It just takes more strain to mobilize skin friction than it does to mobilize the end bearing."

Is that only for clays? For granular soils and rock, what I've read seems to indicate the opposite, with a significant reduction in skin friction due to excessive shearing when end bearing is mobilized. Or have I had it backwards all this time? (Very possible, I deal primarily with structural design, not much geotech).

 

[LRJ]

Friction is normally mobilised before end bearing, for all soils, because the applied load is at the top of the pile; resistance is mobilised closer to the applied load.

 

[BridgeSmith]

LRJ, I would have thought that the point of load application wouldn't make much difference for axial load for a concrete shaft embedded in soil. Due to the much greater stiffness of the concrete, wouldn't it act as a rigid body? (Again, for axial load only, definitely not for lateral)

I was under the impression that the movement to mobilize side friction was very small in comparison to that required to mobilize substantial end bearing. The O'Neil and Reese Drilled Shaft Manual has a method to determine the reduction in end bearing to be used in combination with side friction, and vice versa, but we haven't been using them. We typically use the full value of either side friction or end bearing, and ignore the other, since the reductions to use them in combination are so large (we've typically gotten 90% to 95% reductions in end bearing capacity when combined with side friction).

 

[dik]

I think it has nothing to do with ultimate capacity of either the end bearing or the skin friction. It has more to do with strain compatibility.

I had been lead to believe that our bearing was about 2000 psf and from Mohr's circle shear would be 1/2 that or 1000 psf with a safety factor of 3 brings it to 333, say 300 psf.

Dik

 

[fattdad]

Hi dik,

Your post has me confused. Ultimate bearing, as described by Terzaghi is based on friction angle, cohesion and depth of burial. For cohesive soils and short term loading, we'd use about 5 times the undrained shear strength. So, if the undrained shear strength were 2,000 psf, we'd get an ultimate of 10,000 psf. Applying a safety factor of 3 would give you 3,333 psf. It'd be more if you included depth of burial.

f-d

ípapß gordo ainÆt no madre flaca!