Bored Pile Detailing

[Zambo]

On large diameter bored piles i typically see piles overcast (i.e. the concrete placed say 1.2m above cut off level) this pushes out any concrete mixed at the top with polymer and also covers the rebar at the top of the pile protecting it during excavation. This usually works out well because a pilecap could have a similar depth. But what about when i tie into a foundation slab without a deep pile cap (note i still need starter bars for columns above)? In that case unless i overfill before piling i cannot overcast. Can anyone suggest how to finish the pile so that i can minimise the work at cut off level and when work moves from the piling work to the foundation slab work.

 

[hokie66]

Not sure why you would place an extra 1.2 metres of concrete, and then have to remove it. Never seen that done.

 

[Retrograde]

Not sure why you would place an extra 1.2 metres of concrete, and then have to remove it. Never seen that done.

If you are tremie pouring a pile under drilling fluid (polymer or bentonite) then you have to pour it up to ground level. Then when you excavate down to the cut-off level of the pile (to construct the pile cap) you end up have to break down the top of the pile. I've seen it done many times. Same applies for CFA piles.

Zambo: in your case I would overfill the pile such that you get some concrete spilling out of the top of the hole. This should flush out any polymer. You should still allow to break out the top 50-100mm to ensure you have sound concrete at the interface.

 

[Zambo]

Retrograde, so in this case I have will have the piling platform say 300mm above finished ground level (FGL). The casing will be driven to the same level as the piling platform. That gives me about 300mm to break out. I am not sure that is enough, but what you are suggesting is that I keep filling and let the extra concrete (with some polymer/bentonite contamination)spill over the casing top onto the piling platform. Then at the end of piling I scrape of 300mm which brings me to FGL. Correct?

 

[BigHarvey]

when drilling Under bentonite or polymer, the concrete coming from the bottom of the pile might be slightly contaminated with soil. This is why it is necessary to cast between 0.5 and 1 m above cut-off level. The concrete will be trimmed off at a later stage. In order to trim it off properly you can use a device called Recepieux based on expansive cement.

http://www.recepieux.com/en/the-most-accurate-recepage/

I'm not working for them but used it several times successfully. You can also trim off with a jackhammer or a hydraulc rock breaker but in all cases protect the rebars before concreting, it will make it easier when you trim off.

 

[MTNClimber]

If you are tremie pouring a pile under drilling fluid (polymer or bentonite) then you have to pour it up to ground level.

I don't believe this is entirely true. If you have concrete contaminated with drilling fluid, soil, or whatever, you can continue to tremie and remove the contaminated concrete by hand (shovels, buckets, ect.) or maybe vac-ex. until you expose fresh concrete or just let it overflow if the forms are above grade. If the pile cutoff is below the ground surface and you have a lot of drilling fluid in the hole, you can also pump the fluid out of the hole before starting to remove the contaminated concrete.

To me, cutting the pile after the fact is harder and more time consuming.

 

[Zambo]

If you have a deep pilecap and your pile cut off is significantly lower than ground level then you can still pour say 1.2m above cut off level (up to the top of the starter bars) and then fill the empty bore above with gravel. When you excavate the additional concrete/concrete bentonite mix will protect the starter bars. My problem is the opposite the cut off is closer than normal to ground level. I am still considering just over flowing the concrete at the top of the casing to push all the bentonite out.

 

[BridgeSmith]

"I am still considering just over flowing the concrete at the top of the casing to push all the bentonite out."

That's how they do it on our bridge projects. If necessary because the top of the drilled shaft is below the groundline, they excavate an overflow pit for the contaminated concrete and just let it flow off away from the shaft. Depending on the location, they either just bury it, or embed a lifting loop in the waste concrete so they can crane it out after it hardens.

 

[BigHarvey]

Agree with you MTNClimber but when cut-off level is below Platform level it's not possible to trim off fresh concrete.

 

[BridgeSmith]

There's always a way to remove the excess fluid and contaminated concrete before it hardens, by one of the methods MTNClimber suggested, or by creating low spot for it to drain off by gravity, as I suggested. Any of those methods are less labor-intensive than chipping away hardened concrete. In modern construction, where labor is expensive and time is money, I would guess that taking the time to have guys down in a hole removing hardened concrete around reinforcing would generally be the more expensive route.

Edit: Sorry BigHarvey, I should have looked at the link you provided earlier. That system looks like it would also work without a significant amount of labor involved, so it would also seem to be a viable option.