Pile Pull Out Capacity

[tclat]

Hi All,

I am trying to calculate the tensile capacity of a pipe pile foundation. The piles are ductile iron pipes in 5m lengths and only have a friction, bell and spigot joint.

To get tensile capacity, I have been advised that the pile needs to be fully grouted and a continuous rebar (mechanical spliced) has to be included along the length of the pile and across the joints.

I am having some difficulty in assessing the friction between the grout and the pile which to me is the primary failure mechanism. Should I be concerned about the shrinkage of the grout? Can I use standard skin friction equations?

Thanks

 

[FixedEarth]

Your primary failure in uplift will be the bond between soil and outer grout. Typically in design we use 50% of the skin friction value for uplift capacity. Ask the soils firm for values. Any interior grout, pipe pile wall thickness and continuous vertical bar will not contribute to your uplift capacity.

http://www.soilstructure.com/

 

[hokie66]

Bell and spigot pipe used as piling? Where did that idea come from?

 

[tclat]

Thanks Fixed Earth,

Our pipe piles have a frictional bell and spigot joint every 5m which to me requires the rebar so that the pile can act as one unit. Because I am in a seismic region I am recommending that the rebar is included even for piles that do not have tension. The manufacturer feels this is too conservative but it seems like the right specification to me.

I understand your comment regarding the primary failure mechanism.

 

[FixedEarth]

Attached is the analysis procedure for drilled piers subject to uplif loads. I agree, stick with the extra reinforcement.

On one recent project for ball park lights, I only had 2.4 m pier emebdment to work with due to shallow ground water. So to get large passive resistance and higher skin friction values, I used 2.4 m diameter pier and filled it with 5 MPa low strength grout. Then I drilled 1.2 m diamter pier in the center. This gave me high skin friction & higher lateral bearing pressures which was used to handle the applied shear & moment loads. Similar approach may work for you.

http://www.soilstructure.com/

 

[dhengr]

Tclat:
You say.... “ductile iron pipes in 5m lengths and only have a friction, bell and spigot joint.” Please answer Hokie66's question. Who manufactures these pipes for piles in any application, let alone in tension. What ID and OD and total length of your piles? Then you are going to grout and place a single rebar in the center of the grout. What kind of surface finish does this pipe have, has it been degreased? So, what’s the CoF grout to iron or iron to soil? How do you know you’ve even driven these straight, or that they didn’t break in the driving? Driving the bell will leave a .5" void around the pipe, btwn. the pipe and soil, won’t it?

 

[hokie66]

Sounds like a homemade solution, not one based on recognized engineering practice. Maybe some leftover sewer pipe.

 

[tclat]

Please see attached on the system that we are currently reviewing. Comments would be welcomed. I posted this in the Geotechnical section but have not got any responses as yet.

 

[tclat]

Here is another attachment.

From our review so far, the piles are driven with a special shoe which is wider that the diameter of the pile. The shoe has slots which allow for grout or concrete to be pumped into the void that is created while the pile is being driven. From what we understand, for end bearing piles, the grout on the outside of the pile is not critical unless the piles are subjected to tension which causes uplift.

On our project, the piles are about 12-15m long. The manufacturer and their construction agents recommend a 170mm dia pile with 50mm grout around the pile and fully grouted inside. Based on the soils report, they claim their recommended solution can result in ultimate capacities in excess of 1000kN or an allowable of about 700kN. We are comparing this system to conventional piling using precast concrete, H-piles or welded pipe piles.

Have done some research on Bauer and they seem to be a well established piling company.

Again, any thoughts would be great.

 

[hokie66]

OK, so this system is more developed than some of us knew, and its main market has been in Europe. But the 61 page document you linked is primarily marketing material, without a lot of design or installation information. There are several big names, e.g. Dywidag, Bauer AG, listed as partners in the implementation of the system, and they should be the best source for advice on how to use these piles in tension. I suspect that centrally grouted reinforcement would be the answer, but you should refer your questions back to the piling contractor and/or manufacturer.

 

[dhengr]

Tclat:
It’s obviously a proven piling system, that caught be by surprise. Hokie, who’da guessed? That’s quite a different bell and spigot shape and length than I first envisioned, thinking of sewer pipe cross sections and shapes, and the like. It looks like quite a bit tougher cross section through the joint. The driving shoe is also an unexpected improvement, and the fact that you get grouting inside and outside the pipe and into the soil. I wonder if you could fix a 5m rebar to the inside center of the driving shoe, with a screw coupling at the top. Install a second 5m rebar with top coupling prior to inserting the second piece of pipe piling, for its driving, etc. Then lightly prestress this rebar string before grouting, to prestress the pipe sections together. Otherwise, Hokie’s latest advice is right on the money.

 

[civeng80]

Without going to much into this
The hole is drilled, then the ductile pipe is inserted and grouted in and reinforced on the inside?
Uplift (and downward loads) resisted by the socket end of pipe together with friction but getting a bit more bearing from socket end of pipe ?
May be Ok but why not just use Franki pile system? or drill a bigger bore ?

 

[tclat]

civeng80

There is no drilling. The pile is driven with a special hammer attached to a 30 ton excavator.

hokie/dhengr/fixed earth

Thanks for the advice. I have spoken to the manufacturer but wanted to get some independent feedback and have been trying to do some verification calculations myself.

 

[hokie66]

No, civeng80. The system is driven, displacement piling...as I understand it. Think precast concrete piles, but these are lengths of ductile iron. Not sure how the external grouting comes into it, if it does.

 

[hokie66]

I looked again, and now see that the external annulus created by the enlarged driving head (and to a lesser extent, the bells) is grouted as the pile is driven. Looks like a good system.

 

[hokie66]

I looked again, and now see that the external annulus due to the enlarged driving head (and to a lesser extent, the bells) is grouted as the pile is driven. Looks like a good system.