CIDH Pile: End Bearing/Skin Friction

[StructTaco]

I have a building design I'm working on, I'm asking the geotechnical engineer for soil design parameters and he is giving me the same end bearing numbers as he is for shallow footings:

Structural Loads
Axial Load (D+L+EQ)=100 kips
Moments (D+L+EQ)=25 ft kips
Shear (D+L+EQ)=15 kips

Preliminary Caisson/CIDH Pile size
24" diameter
15' deep

Soil Data:
Cemented Sandstone
Blow Count: 50 to 60 per foot
Density: 120 pcf
Allowable Bearing for shallow footings with 2' min. embed=3500 psf, 4500 psf with 3' min.

1. The geotech is not letting me use skin friction and end bearing at the same time, is this the standard of practice?

2. Shouldn't the end bearing value go up as the foundation gets deeper?

3. What are typical values for skin friction, is 40 psf/ft of depth in the ballpark, as in it increases with depth?

4. I know the skin friction mobilizes at a lower strain than does bearing, but are they typically combined, why or why not...?

much appreciated

 

[hokie66]

I think your geotech is being extraordinarily conservative, both for end bearing and for discounting skin friction. Interested to see what some of the geotechs here think.

 

[StructTaco]

that's what I thought too, but wanted to check with some other geotech's...

 

[mudman54]

you'll see a lot of state agencies use one or the other of skin friction or end bearing as a redundant strategy; yes, the end bearing changes with depth - try the bearing capacity factors knowing that angle of internal friction or cohesion increases with depth up to a limit. hopefully, other forum members will weigh in on this because the mobilized skin friction procedure has not been codified as far as i know.

 

[fabianjm]

1.Shallow footing commonly on end bearing because skin friction very small
2.End bearing depend on the shear strength etc of soil soit's increasing as deep of foundation increasing
3.Analytically, the skin friction value are different for shallow foundation and deep foundation
4.We can neglected the skin or end bearing capacity value if to small into account of bearing capacity

 

[FixedEarth]

I plugged your loadings with the soils data and am getting 12 ft embedment as adequate, BUT, your induced end bearing pressure is near 33 ksf. Basically, take your 100k vertical + 5 kips for weight of pier accounting for the displaced 120 pcf soil by the 150 pcf concrete & divide that by the area of a 2 ft pier.

Consider 3.5 to 4 ft diameter to bring the end bearing pressures lower.

The 40 psf/ft sounds a lot like a Active equivalent earth pressure & not passive or skin friction value. Why not ask the geotechnical for passive and then use your IBC formula?

 

[BigH]

Are you dealing with "soil" or a sandstone rock socket? To get any end bearing, the ratio of socket to diameter would have be less than about 4 (see Tomlinson's Foundation Design and Construction). Can compute rock socket shaft capacity as given in Tomlinson. I would think that your working load would all be taken in shaft capacity with no (or very little) load ever getting down to the tip.

 

[StructTaco]

The drilling results were that the soil had some cobbles for the first 10 feet, then the last 10 feet had silty sandstone with poor cementation. Blow counts were in the 50 to 70 range per foot.

I've only seen discussions of skin friction and end bearing in the FHWA pile design manual, I'm waiting for the engineer's results on what he will allow for the skin friction and/or end bearing.

33 ksf is way to big, but wouldn't 12 or 15 ksf be a reasonable allowable end bearing pressure at 15 to 20 feet deep?

 

[FixedEarth]

Yes 10 ksf end bearing pressure is reasonable for such materials. Treat it as only a side supported pier and ingnore end bearing resistance.

You will need to add strong language in the construction specs that Cobbles and difficult drilling would be encountered.

If the geotechnical engineer's parameters are super low, you may need to load test to failure and then use 50% of that values for design.