Replacing pile foundations 2

[shahriaran]

Hi, the client wants to install a 2.5 diameter, 8 ft deep pile foundation to replace a light pole with an existing 2 ft diameter foundation. What should I put on the drawing for the contractor in terms of removal of existing foundation and putting in the new one? Is there a specific method?

 

[phamENG]

Will the light pole stay in place, or is everything new? If everything is new, I'd say "demolish pole and foundation complete." That means everything has to go. If the light pole is staying, then the foundation has to stay and you have to build around/under it and tie into it.

 

[r13]

Show two diagrams, one demolish, one final drawing.

 

[SlideRuleEra]

Include all information known (hopefully a drawing) about the existing foundation... Everything. Otherwise the Bidders will have to make an assumption on existing foundation details. No matter what the details turn out to be, your Client will be open for a claim if it turns out that you knew something but did not reveal it.

If nothing is known, say so.

http://www.SlideRuleEra.net

 

[msquared48]

Problem here... if the footing relies on lateral earth pressure according to IBC tables, then the earth has to be undisturbed. You cannot install the new footing in the same location as the earth will be disturbed. The footing must be drilled to achieve that.

Mike McCann, PE, SE (WA, HI)

 

[azcats]

Can you just demo the existing to grade or slightly below then drill a new hole adjacent in undisturbed earth?

Otherwise I agree with Mike's concern. If it has to go back in the same place, you might consider removing any loose material remaining in the 'hole' you're left with after demo and filling it with CLSM. Then you can drill your new pier thru that.

 

[BridgeSmith]

What azcats proposed is likely the most economical option if the foundation cannot be relocated, but inserting a temporary casing around the existing shaft before removing it is something you could ask the drilling contractor about.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

Sorry - I saw the word "pile" and thought we were doing something completely different.

I agree that putting it back in the same place will be nearly impossible for the reasons Mike brought up. They won't be able to get the old one out without compromising the integrity of the surrounding soils. One option would be to go with an even wider foundation, though I'm not sure how successful you'd be. If you could go to a 36" auger, that would give you 6" all around that the could disturb while extracting it.

 

[phamENG]

Another option would be to consult with a local geotechnical engineer. If they get a pole that they rip out and they have to destroy the surrounding soil, it may be good to have a procedure for back filling and compacting to get to lateral design pressures at least as good as the IBC values. That will vary based on your location and soil type, so a local geotech would be the best to talk to.

 

[shahriaran]

Thank you for all the answers... So I talked to the engineer doing the calcs, and he is suggesting to put on the notes to maintain a 95 percent compaction and not disturb the soil. I'm guessing just putting these 2 recommendations are not enough. This foundation is way over designed to accommodate for the worst soil conditions.

How about making these recommendations:

- Contractor shall remove the existing pole without disturbing the existing soil as much as possible inserting a temporary casting around the existing shaft before removal.

- Use Controlled low strength materials (CLSM) to fill in the gaps and holes after demolition.

We could potentially go with a larger diameter pole, would that be helpful? The contractor has to replace several of these all over the city and they may all be in different conditions. Is there anything else you recommend me to add?

 

[BridgeSmith]

I think you would use temporary casing or CLSM, but I don't think both would be necessary.

A question for the guys for familiar with foundations like this - Would it work to remove the existing shaft, compact backfill into the hole to completely fill it, and then drill the new hole?

Depending on the depth of the existing foundation and the lateral loads on the new shaft, you may be able to neglect the lateral resistance of soil for the depth of the existing shaft, and design the shaft for strength, stability, and deflection, using the lateral resistance of just the embedment below the depth of the existing shaft.

We always neglect the lateral resistance of the top 5' of soil due to the potential for frost heave loosening it. I include the weight of the soil in the top 5' as an overburden on the deeper soil. (Soil properties for top 5' - gamma = 120pcf, phi = 0, k = 0)

Rod Smith, P.E., The artist formerly known as HotRod10

 

[jdonville]

shaharian,

Ask the design engineer to pretend that he, personally, has to both design and perform the work. If he doesn't know exactly how to do it or is unsure if the method is practical, then he should call up a likely bidding contractor or two and discuss how to present the work for bid and if it is even possible.

I see too many times work that is only half thought out on bid plans.

 

[bridgebuster]

What is the existing foundation? Could it be retrofit to accommodate the new light?

 

[BUGGAR]

You can replace disturbed soils with lean concrete that is digable