Drilled Pier Design- High P.I. Clays

[PatSablatura]

How would a person design the tension steel in a belled(underreamed) concrete pier to resist the uplift forces of the surrounding hi plasticity clays?

Figuring the bell diameter is easy in regards to bearing capacity of the soil as given.

I've got a tiltwall panel and metal roof frame loading the pier with about 60,000 lbs. and the geotech report recommends the following...

24 foot belled piers,

Uplift Force= 105*D where D= Shaft diameter.

(that equation estimates the magnitude of the potential uplift force)... I suppose this is ''skin friction'' forces acting upwards around the perimeter of the shaft.

Reiforcing to resist tensile forces?

Thanks, Pat.

 

[phuduhudu]

Once you know the actual tension (including ultimate load factors) you need to resist you can divide that by the yield stress of reinforcing bar (multiplied by material factors) to get the required cross sectional area of steel. For your tension you can reduce it by the minimum applied load to your pier since that would resist the uplift forces.

I am curious to know what tiltwall panels are - I saw you mentioned them in another thread too.
Carl Bauer

 

[dik]

The nominal reinforcing in a pile shaft often provides sufficient area for nominal uplift forces. The maximum factored resistant force of the reinforcing steel is equal to: the material resistance factor * yield strength * area of steel. Some of the bars can extend full length, and others can be cut back accordingly, or can make all bars full length. The area of steel can be designed for strength as a minimum and steel strain should be considered also. The section should be checked for maximum acceptable 'crackwidth'.

For friction piles and caissons in compression only, I usually use 1/2% of reinforcing as nominal and this usually extends the top 20'. Ties, except for the top 4' are usually circular ones at 36" +/- on centre. Check with your local plan examiner or building code.

I would assume that the skin friction was effective and uniform from about about 8' below the top of the caisson to the base. The reason for neglecting the top 8' is the possible dessication at the top and the high shrinkage accompanying clays with a high PI. Since you have a geoteckkie on board, you should confirm this with him.

To ensure a tight soil-concrete interface, I would vibrate the top of the pile. This will consolidate the concrete, and improve bonding to the steel and soil.

If it is possible that water can enter any cracking of the caisson, then consideration of galvanized bars can be made. Often with highly plastic clays, they retain the moisture and there is no free water for corrosion.

Also, some clays have a high sulphate content; check with your geotekkie.

 

[PatSablatura]

Yeah, I figured, the skin friction uplift less the compressive load of the walls= amount of steel required...

x-sectional steel area * yield strength of rebar less load factors.

Just a wondering if there was some mystical magical equation to ease my mind.?? or rule of thumb??

Gracias,

Pat.

 

[dik]

Not that I'm aware of, other than the general reinforcing outline posted.

In Winnipeg, Manitoba, with the exception of a few areas of the city, it was common to use 300 psf for skin friction. Clay was a highly plastic, sulphate rich material that, within inches almost, was 40' deep except for one location (varved glacial deposit). For uplift I would normally consider half that value (felt more comfortable with things sinking than lifting, I guess). Again, it was common to neglect the upper portion of the pile/caisson due to dessication.

 

[JAE]

Pat:
I spent most of the 80's designing drilled pier footings in Texas where expansive clays were very prevalent. Generally, we would get an expression, similar to your formula based on diameter, from the geotechnical engineer who would come up with it based on the degree of swell potential in the soils across the full length of anticipated pier depth. I seem to remember seeing F = 250 x D a lot.

We would do exactly what dik indicated, calculating our F and multiplying by a load factor (perhaps 1.7) and dividing that result by a phi factor of 0.9 and the ultimate tensile strength (60 ksi) to get a minimum required area of steel.

We would also use as a minimum value 3/4% of the cross sectional area of the pier. Ties were 3/8" hooping at 12" pitch (1/2" hooping for 36" shafts and larger). The vertical reinforcing would extend the full length of the pier. Laps were allowed but only in the top half of the pier but no closer than 5 feet from the pier cut-off elevation.