how much sustained compression capacity for a refused pile

[newlearner]

I am a structural engineer and need to solve a problem with refused steel pipe pile.

1.This is a group of 8 (2x4) pile foundation. Concrete pedestal is located at pipe pile location. Concrete beams are provided to tie the piles together.

2. The middle pipe pile should be driven to 26m, but it hit early refusal at 13m. Geotechnical engineer abandoned the pile. Two replacement piles with smaller diameter were driven at a distance of 2.3m to the refused pile. Concrete tie beam was poured. If the capacity of refused pile was neglected, i.e. assuming the refused pile provides no vertical support, then reinforcement in the concrete beam is not enough.

3. Can we assume that the refused pile provides any support? If yes, how much(full or partial)?

Any opinion, advise and comments are welcome. Thank you very much for help.

 

[FixedEarth]

When you say "Reinforcement in the concrete beam is not enough" if the refusal pile is abondened, are you referring to the tie beam or the pile cap?

Either case, geotechnical capacity is much lower than the structural capacity. So it would be more conservative to ignore the refusal pile and count on the other piles for support. If you decide to count on it, then this abandoned pile must be connected to the pile cap and even then would probably not use more than 25% of the "normal capacity"

You can ask the project geotechnical for efficiency of the as built block of group piles. If you want to calculate it yourself, it is covered in a very nice way in the text below:

http://www.amazon.com/Design-Founda...=sr_1_7?s=books&ie=UTF8&qid=1280783643&sr=1-7

 

[Ron]

You can assume all you want that the refused pile doesn't provide any capacity; however, if it is tied to the others by a concrete beam, it will carry load; and if driven to refusal, will initially carry more load than piles not driven to refusal. Your problem my not be how little load it will carry by how much greater the load will be that it carries.

It appears that your geotech was requiring the piles to be driven to a tip depth criterion and not a driving resistance criterion. Or was it both? What was the reason for abandoning the refusal pile? Does your foundation have significant uplift? Is the geotech concerned that refusal was gained by a large,anomalous rock?

 

[newlearner]

Thank you very much for your valuable comments!

The refusal pile and two replacement piles were connected by one concrete beam (pile cap, width=1.5m), which is just a tie beam (not much reinforcement inside). Concrete pedestal is on the top of the concrete beam. It would be more conservative to ignore the capacity of refusal pile. If so, the load from pedestal will transfer to the beam, then to replacement piles. The reinforcement inside the concrete beam could not carry such load. On the other hand, it is hard to strengthen the beam since it has been cast.

At this moment I am still waiting to hear from geotechnical as to the reason for the pile refusal.

Thanks.

 

[Ron]

newlearner...please don't forget that just because you analyze something in a particular manner with certain assumptions, that it actually works that way in the installation. You have a pile that reached refusal. You have two other piles that were driven adjacent to that one. You have tied them all together with a beam. Ignoring the capacity of the refusal pile will allow you to compute the capacity of the beam, but will it be correct? Not likely, since the refusal pile will actually be taking load that you've assumed it doesn't take.

Your analysis doesn't dictate the field response. The constructed conditions dictate the field response.

 

[BigH]

What I would like to know is 1 and 2; 3 is conjecture:
1. Where did the other piles "end up"? At 13 m or were you able to drive them deeper?
2. What type of soil are you in? or bedrock? If the other piles of a group went, say to 22 m or so, then this pile obviously hit an obstruction - likely a boulder - I doubt if it would be a rock pinnacle since it would be strange (except perhaps in Laos or Kunming for tall spindly upright rock projections.
3. If you are on a boulder - it is likely that you would have a "greater" tip capacity than would be the norm for soil since the boulder would act, in a way, as an expanded base. - so the capacity might actually have been enough - but it is what it is. In most cases I would say that a pile in the group, if it becomes overstressed would pass on its load to other piles in the group and the beam would likely be sufficiently to transfer.

 

[newlearner]

Thanks, Ron and BigH for your valuable comments.

Other piles were end up at length 20 to 26m.

The refusal pile has certain capacity, but not full capacity. Otherwise it should not be abandoned. If refusal pile became overstressed, the load will pass to replacement piles by beam. During this process, will the refusal pile maintain the capacity (without unloading due to any reason)?

I agree that it is not true by ignoring the capacity of refusal pile, but it is on the conservative side to check bottom reinforcement of beam.

Thanks.

 

[BigH]

I would assume the "best" and "worse" condition would not translate significantly into the cost of the reinforcing steel - so why not choose the "worse" condition and design accordingly.

 

[Ron]

BigH..the problem is choosing which "worst" condition to design for!

Assume the three piles are in line. The middle pile is the "refusal" pile. If you design for no resistance of the "refusal" pile, the cap gets designed for simple bending between the two "good" piles. So the bottom of the pile cap gets reinforced for bending, with no consideration for reverse bending. Well, in actuality the "refusal" pile takes the first load, potentially putting the pile cap in reverse bending, with no reinforcement to accommodate that condition.

The "worst" case, would probably be to design for both bending conditions.

I agree....a few extra pieces of rebar won't break the bank and will provide good performance!