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Drilled pier foundation for a house
4

Drilled pier foundation for a house

Drilled pier foundation for a house

(OP)
I have not done this in a while and forgot what my old firm did when we detail this. The geotech recommends 10" minimum diameter with 30:1 length to diameter ratio. I think all of the piers will end up being 10" diameter. Some may end up being 12" if I have to go deeper than 25 ft. What ties do I use? #3 @ 10" o.c.? I want to specify what people typically use. Most of these piers will be in compression. Some will be in tension at the end of the counterfort. This is for foundation in expansive soil.

Cheers!

RE: Drilled pier foundation for a house

Is it common to specify and length to diameter ratio?? Should length not be based on the capacity required.

Re you stirrups - its good to get a feel for whats commonly used but you will need to size them based on the applied loads.

RE: Drilled pier foundation for a house

It's generally less costly to stick with one diameter... even if you have a lot of them. For friction piles, for the last 40+ years, I've used a single 15M (#5) bar with no ties. I guess it depends on local practice. Putting a cage in a 10" or 12" shaft can be interesting especially if you have 3" cover. If you use ties, I generally space them at 48" centres with 3@12 at the top. pipe

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

RE: Drilled pier foundation for a house

If this is really a 10" drilled pier, you will have 3" of cover. That leaves you with a 4" tie diameter. I'm not even sure if such a diameter is possible, but I would not recommend it. Also if the drilled pier is more than 10' deep, I would use a single bar in the middle. For deeper piers, it will be difficult if not impossible to accurately place, guide, and inspect that multiple vertical bars are in the correct position. If you have say 3 vertical bars and a 10' deep pier, you can probably see the bottom of the excavated hole and wiggle/adjust the bars positions into place as needed.

RE: Drilled pier foundation for a house

(OP)
Here is the soil report:

DRILLED PIERS
The design and construction criteria presented below should be observed for a straight-shaft pier foundation
system.
1. Piers should be designed for an allowable end bearing pressure of 23,500 psf and a skin friction of 2,350
psf for the portion of the pier in bedrock. Uplift due to structural loadings on the piers can be resisted by
using 75% of the allowable skin friction value plus an allowance for pier weight.
2. Piers should also be designed for a minimum dead load pressure of 5,000 psf based on pier end area only.
Application of dead load pressure is the most effective way to resist foundation movement due to swelling
soils. However, if the minimum dead load requirement cannot be achieved and the piers are spaced as far
apart as practical, the pier length should be extended beyond the minimum bedrock penetration and
minimum length to mitigate the dead load deficit. This can be accomplished by assuming one-half of the
skin friction given above acts in the direction to resist uplift caused by swelling soil and/or bedrock near
the top of the pier. The owner should be aware of an increased potential for foundation movement if the
recommended minimum dead load pressure is not met.
3. Piers should penetrate at least 5 feet into the competent bedrock with a minimum pier length of 25 feet
recommended. The requirements for minimum penetration and minimum pier length should be met.
4. All piers shall be reinforced their full length with steel rebar. The pier reinforcing shall be designed to resist
the tension resulting from the maximum uplift pressures. No less than 1% of the steel based on the pier end
area shall be used. The pier reinforcing steel shall extend into the foundation wall a sufficient distance to
fully develop the bars in tension.
5. The drilled piers should be a minimum of ten inches in diameter. The pier length to diameter ratio should
not exceed 30:1. The recommended diameter must be maintained at the top of each pier hole. We
recommend the use of cylindrical cardboard forms to maintain the diameter of the top of the pier hole and
to prevent mushrooming.
6. The drilled pier holes shall be cleaned of all loose material and filled immediately with concrete to prevent
sloughing of loose soil or infiltration of water. Based on the exploratory borings we do not anticipate
encountering water during drilling of the piers. However, if water is encountered during the drilling it
should be removed prior to placement of the concrete. If the water cannot be removed or prevented with the
use of temporary casing and/or dewatering equipment prior to placement of concrete, the tremie method should be used after the hole has been cleaned. In no case should concrete be placed in more than 3 inches
of water unless placed through an approved tremie method.
7. Concrete used in the piers should be a fluid mix with sufficient slump, so it will fill the void between
reinforcing steel and the pier hole. We recommend a concrete slump in the range of 5 to 8 inches be used.
The concrete should be designed with a minimum 28-day compressive strength of 3,000 psi.
8. A representative of the geotechnical engineer should observe pier drilling operations on a full-time basis to
assist in identification of adequate bedrock strata and monitor pier construction procedures.
9. A minimum four-inch void form shall be placed under the foundation grade beam walls between the drilled
piers.

RE: Drilled pier foundation for a house

(OP)
Thanks dik. The reason I ask is because of the clear cover. 3" clear seems a lot for 10" dia. Can they even build this cage? Should you put the ties as a spiral? I think my old firm did something similar like you mentioned. Spaced pretty far apart and have multiple on the top. The ties are only needed to hold the vertical bars really. The piers aren't really bending or taking shear.

RE: Drilled pier foundation for a house

Quote (4. All piers shall be reinforced their full length with steel rebar. The pier reinforcing shall be designed to resist
the tension resulting from the maximum uplift pressures. No less than 1% of the steel based on the pier end
area shall be used. The pier reinforcing steel shall extend into the foundation wall a sufficient distance to
fully develop the bars in tension.)


Unless you have uplift, I generally use 19'8 as a bar length... odd length. Often there is no need to have reinforcing full length. If you cut an 18m long bar into 3 you end up with close to that length.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

RE: Drilled pier foundation for a house

(OP)
But the minimum pier length is 25 ft. How do you suspend the cage 5 ft?

RE: Drilled pier foundation for a house

The 19' length extends out the top maybe a foot or so into whatever structure is over and the last 8' or so of the pile is unreinforced... Unless there is a huge moment or uplift, the last 8' doesn't need rebar. Make sure you mechanically vibrate the top 10'... I've seen concrete consolidate by over a foot for larger and longer diameter caissons, so it does do something... better soil to concrete surface. pipe

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

RE: Drilled pier foundation for a house

Dik - It appears these piers will likely see large uplift forces due to swelling soils. The piers should be fully reinforced.

EireChch - specifying length:diameter ratios are very common in Colorado, which is where OP is posting from. The larger the pier diameter, the more uplift force will be applied to the pier, resulting in a longer pile required. It’s easier to reduce it to a ratio than provide a table.

RE: Drilled pier foundation for a house

(OP)
MTNClimber, I dont think there is tension on most of the piers. The uplift you are mentioning is a pushing force from the bearing of the pier. Most will be in compression.

RE: Drilled pier foundation for a house

With uplift, they should have reinforcing full length, but it can be a single bar.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

RE: Drilled pier foundation for a house

MTN - interesting, never seen it specified that way. Thanks.

RE: Drilled pier foundation for a house

(OP)
I guess i am confused. What uplift pressure are we seeing other than the bottom of the piers? The bottom of the walls will be on form void.

RE: Drilled pier foundation for a house

It's uplift force from the skin friction on the pier. When expansive soil and/or bedrock is exposed to moisture it will swell around the pier which applies an uplift force on the pier. Piers should be drilled past zone of moisture variation into moisture-stable soil or bedrock, therefore there will be zero uplift force on the bottom or sides of the pier in the moisture-stable soil/bedrock. The geotechnical resistance from uplift is developed from the portion of the pier anchored into the moisture-stable soil or bedrock.

Here's a figure from the attached CAGE paper "Commentary on Geotechnical Pracices - Drilled Pier Design for Lightly Loaded Structures in the Denver Metropolitan Area".

RE: Drilled pier foundation for a house

Doublestud - the geotech report says minimum steel area should be 1% of the cross section. That's a single #8 bar for a 10" pier - if that's enough tension resistance. In lieu of ties at such a tiny diameter, bundled bars at the center of the larger ones may be a viable option.

RE: Drilled pier foundation for a house

(OP)
hmmm, yeah.. can I just put a #8 rebar in the center? That is exactly 1%. Also how do you feel about 4" form void below the foundation wall? Once the form void collapses, what will prevent surrounding soil not to collapse inside the void? I always wonder about this. Or maybe water will pond in the void?

RE: Drilled pier foundation for a house

I don't have to deal with expansive soils, so take these for what they're worth: if the site is properly graded, I would think water would shed away. You may have some ground water come up, but if it's a serious concern then provide drainage. As for soil collapsing, who cares? As long as your piers don't punch through the bedrock and settle, it's unlikely that those soils will be compacted enough to pose an expansion pressure risk to the underside of your grade beams. Right?

RE: Drilled pier foundation for a house

(OP)
PhamEng, I am worried about the exterior slab on grade. If the soil moves to the void, wouldnt the exterior slab settle?

RE: Drilled pier foundation for a house

Yes. It probably could. We have a lot of fill around here (swamps turned into landfills turned into developments over the course of 200 years), and I imagine there are some parallel design philosophies. Where there is a significant settlement concern, sometimes site paving like sidewalks and slabs on grade will be turned into structural, pile supported slabs. I usually see it as a repair after everyone thought the geotech was being too conservative, but I have seen it done. So if you have large slabs on grade that need tight elevation control relative to the building floor, it should have a similar support structure. Otherwise, consider using a more stable fill under the slab or turning the slab edge down so it bears at a lower elevation than the void form and prevents the soil from filling that space.

Again, I don't design in expansive soils - anyone can feel free to tell me I'm crazy.

RE: Drilled pier foundation for a house

DoubleStud - Ask the geotechnical engineer how much uplift force the piers should be designed for. I usually would tell the engineer X kips times the diameter in feet. It seems like they are missing this information in the report.

RE: Drilled pier foundation for a house

If a #8 gives you 1%, I'd use a single #8 bar... it's a lot cheaper than 4 whateverz with ties.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

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