Missing Pile for the Pile Cap 2

[jrmbac]

Good Afternoon!

The Fly Ash silo is reinforced concrete. It is supported by pile cap and driven piles. However, during the driving of the piles, one was not driven. Basing from actual site conditions, they cannot drive the missing pile. Can anyone suggest a remedial measure for this?

Thank you

 

[ztengguy]

Can the rest of the piles be checked for the load, and if they can take the load, the pile cap be enlarged as needed to span between?

 

[SteelPE]

How do you miss a pile? I would agree with ztengguy, check the existing pile configuration to see if it is overloaded. If it is, then drive additional piles as needed to reduce the overload in the pile group. Keep in mind that section 1810.3.1.3 of IBC allows an overstress in deep foundation elements of 110% for mislocated piles, however, I'm not sure this would apply in your instance as your piles was missed.

The only other option would be to ask the geotech if the overloaded piles are acceptable in their current state. If the overload is small the geotech may find this acceptable.

 

[SlideRuleEra]

How many piling are supposed to be in the fly ash silo pile cap? What is the pile spacing? How thick is the pile cap? What are the diameter and height of the silo? Is the project in a high wind or seismic area? If this is for a typical electric utility's fly ash silo, there may be many dozen, closely spaced piling with a thick pile cap. This information is needed to address a meaningful solution to the problem.

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[jrmbac]

Please see attached for the configuration of the piles and the location of the missing pile. The thickness of the pile cap is 2000mm. The structure is located in a high seismic area and there is a possibility for liquefaction. The silo's diameter is 12.6m and its height is 25.0m.

I have checked the capacity of the piles, and those piles near the missing pile fails. If I will only check for axial capacity then I would not have any problem with this.

I have tried tying up the three piles near the missing pile. I linked the piles by means of a tie beam within the pile cap to create a frame action and thus, reducing the moment transferred to piles. But still, the piles cannot sustain the load.

 

[SlideRuleEra]

jrmbac - I see, you do have a problem. At least the missing pile is near the center of the silo so that it is primarily carrying live load from the fly ash, not the silo wall's dead load or overturning moment during wind or seismic events. Pile cap is thick enough (as it should be) to make it sufficiently rigid to uniformly distribute loads more or less equally to all piling, even if they are not in exactly the correct locations.

First thing, take a look at the fly ash load assumptions. Are they so conservative that the true loading may actually be lower than assumed. For example, does the fly ash unit weight assume fluidizing air is operating in the silo or is the fly ash tightly packed? Is the silo assumed to be 100% filled, or does the maximum fly ash level allow empty space for fly ash volume expansion when fluidizing air is operating?

I'll think about this problem overnight and get back with you tomorrow.

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[dhengr]

Jrmbac:
Fill a path wide enough, around and over the existing piles, to get a crane and leads into position to drive the missing pile. Compact the fill as best you can and put mats on the fill to run the crane on. Then excavate the fill when you are done driving the pile. Since you can make the gravity load work without that pile, leave a void in the pile cap/mat for that pile. Pour the pile cap, with the void. Then bring the crane in on the pile cap, drive that pile, and with some attention to the void detail and rebar fill the void with conc. to pick up the lateral loads. Maybe adjust the thickness and shape of the pile cap/mat in this region also.

 

[racookpe1978]

Can he go inside, drill through the current concrete with a rig on the current "floor" deep enough to inject grout and fill underneath? Its not a pile (being held rigid by the resistance of the soil and muck and mud around the pile walls) but would not the injected concrete below "span" the gap "good enough"? I'm imaging a drilled hole 2-3 diameter by 20-50 deep filled with injected concrete.

Alt: Increase the floor strength (from wall to wall) to allow the load to be "bridged" across the missing pile?

 

[jrmbac]

According to the owner, he won't be renting the rig anymore. The driving of the piles for all the structures in the power plant has been finished. Leave it as it is. The option to drive the missing pile is out, and we are preparing for the remedial measure. Thank you for the inputs.

 

[Guest262653]

Is a micropile solution feasible? The piling rig is much smaller and cheaper and some 3 or 4 micropiles could potentially have the same capacity as the missing pile.

 

[SlideRuleEra]

jrmbac - I don't believe that the missing pile will NOT cause a problem, if this foundation was designed with normal conservative assumptions use for driven piling.

Pile caps for this type heavily loaded structure are designed for minimal deflection (made thick, to increase the cap's moment of inertia). The benefit is that the pile cap strength (section modulus) is much more than theoretically needed. Minimal deflection is important. The loads are so heavy and concentrated that an adequate number of piling will not "fit" directly under the load. The piling have to be properly spaced to perform as intended. Many piling will be some distance from the load. By controlling deflection, the piling that are away from the applied load still contribute almost all of their load carrying capacity and the piling directly under the applied load are not overloaded.

This pile cap (as designed, without modification) should easily bridge the missed pile location and distribute the excess load more or less equally on nearby pile.

You did not mention the fly ash load assumptions, but a fly ash silo in never intended to operate completely filled. If it is totally full (been there, done that) removing the fly ash is an out-of-the-ordinary task. Chances are this silo will never see the maximum load it was likely designed for.

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[PMR06]

Is the central region of the pile cap open? If so, maybe you can explore reinforcing and filling that area in to help redistribute load from the missing pile to other piles in that inner region?

 

[jrmbac]

avscorreia - any types of driving is out of the question, be it a micropile or not.

PMR06 - yes, it is open. Thinking of the cost implication of your proposal and its corresponding adjustment to the project schedule, i it also out of the question. But if its only the solution, then we will let the project manager explore this.

SlideRuleEra - I will try to fill up and reinforce the center and as PMR06 said, maybe it can help redistribute load from the missing pile to the other piles.

Anyway, for the load assumptions, the packing density is 12 kN/m3. The calculated top of stored materials is 13.7m.

 

[racookpe1978]

When you reinforce that center section with a new layer of reinforced concrete, slope it so the "high point" is over the missing piling, and the equivalent beam strength is maximum at that point, but the variable load is "pushed" away from the missing spot. After all, you don't need concrete elsewhere, just over the missing pile underneath to current flat floor.

 

[jrmbac]

just a question:

I have modeled the pile cap in SAFE. From the pile cap, i put in 101 fixed supports as the pile head. The reactions from the result will be then transferred to the pile. Based the SAFE report, is the moment transferred directly to the pile or the pile cap first?

 

[SlideRuleEra]

jrmbac - I have to agree that the load assumptions for packing density and depth of fly ash inside the silo are reasonable. Live load reduction is not advisable.

If you proceed with foundation structural additions to compensate for the missing pile, but do not drive additional piling, won't you be adding dead load to the remaining piling?

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