Impact of Abandoned Pile on Pile Group Behavior

[mferg318]

I've got a new structure at a heavy industrial site sitting on a pile supported mat foundation. We discovered that there is an existing foundation within our footprint that was used for the removal of a large piece of equipment. We assume this is a pile cap with piles but can't find any documentation or drawings to confirm. My recommended first step is to demo the cap and try to identify how many piles and their diameter. Depending on what we find, I think my approach will be to abandon the pile(s) in place and design my new foundation around it. I would have the existing pile(s) cut down far enough so that they will not bear any load from the new structure. My question is this: what effect, if any, will the abandoned (unloaded) pile(s) have on the behavior of the new pile group? My gut tells me that because they will be unloaded they wont have much impact, but I'm also worried about putting any new piles too close to them. The soils report gives a minimum c/c pile spacing of 3D, but should this also apply to the abandoned pile(s)? I'm obviously going to reach out to the geotech, but wanted to get some opinions on here as well.

 

[EngDM]

Typically we use 3Ø spacing from abandoned piles, unless the geotech allows us to go closer. The thought is that the existing piles, although being abandoned, still disturbed the surrounding soil when they were installed. Best to just stay clear, especially for a pile that's capacity is from skin friction and not end bearing.

 

[EZBuilding]

My official position on this would likely be to have the Project's Geotechnical Engineer comment on the condition and provide any required remedial action.

I believe the 3 diameter spacing is primarily related to the Geotechnical capacity of the pile - i.e. making sure you have a sufficient cone of influence around your pile that isn't overlapping. I have seen Geotechnical Engineers provide reduced capacity for 2.5 and 2 diameter spacings. I think as long as your foundation is not directly bearing on the piles - cut down to a reasonable distance below bottom of foundation - it wouldn't have a significant impact.

 

[BridgeSmith]

What type of piles are the existing? What are your new piles? Will the new piles be end bearing, friction, or driven to refusal?

 

[mferg318]

What type of piles are the existing? What are your new piles? Will the new piles be end bearing, friction, or driven to refusal?

We’re not 100% sure because we don’t have drawings and haven’t uncovered them yet, but my guess is a large diameter drilled pier or perhaps augercast.

 

[BridgeSmith]

We’re not 100% sure because we don’t have drawings and haven’t uncovered them yet, but my guess is a large diameter drilled pier or perhaps augercast.

If that turns out to be the case, I would say the only concern is hitting concrete that flowed out into a void outside the shaft. Other than that, the existing shafts will react as a pillar of really stiff soil adjacent to your new shafts.

 

[HTURKAK]

My question is this: what effect, if any, will the abandoned (unloaded) pile(s) have on the behavior of the new pile group? My gut tells me that because they will be unloaded they wont have much impact, but I'm also worried about putting any new piles too close to them. The soils report gives a minimum c/c pile spacing of 3D, but should this also apply to the abandoned pile(s)?

3D rule is for friction piles . Consider the perimeter of the pile is π*D . In order to develop full friction around the pile perimeter , the spacing would be π*D or simply 3D.
In your case , you are demolishing the top portion of the existing piles so they will not subject to any load directly from the pile cap.
You can abandon the existing piles and keep them in place. Ensure that the new piles will not clash with the existing ones.

 

[mferg318]

Thanks everyone for the responses. Demo will be happening next week so I will update with my path forward after I know exactly what I'm dealing with.

 

[jayrod12]

In similar circumstances, I've had geotech's tell me to provide a minimum of 12" clear between new and abandoned piles. That's to address the comment @BridgeSmith indicated where the concrete may have flowed outwards from the drilled shaft into a void. From a geotechnical perspective however, they said if anything it provides a stronger soil, think of stone columns for reinforcing slopes etc.

 

[mferg318]

Here is the geotech's response for any interested:

"I’m just thinking out loud. I assume you are not interested in doing pile testing on the existing pile and incorporating it into the new design. I also assume that you don’t want to tie the existing pile into the new pile cap and just ignore the additional capacity that it would provide. That might create problems with the new pile cap.

Assuming the above is correct, I agree with the proposed approach of demolishing the existing pile cap and cutting down the existing piles. As long as the existing pile will not bear any load from the new structure, they should not interfere with the new foundation design.

Are you neglecting skin friction in your calculations, and only considering end bearing? If so, I don’t have any concern with installing adjacent to the existing piles.

My only concern would be regarding constructability. We don’t know if the existing piles were installed straight, or if they are belled on the end. You also can’t guarantee that the new piles will be installed 100% straight. It would be best to not hit the existing piles while drilling your new ones. I also assume the piles will not require casing to full depth. If so, there is a higher risk of the sidewall caving into the excavation if you are close to the existing piles, say a foot or less. Some engineering judgment will be needed to determine/guess how close to the existing pile you can install the new piles. Soil type is also a factor to consider. Clayey soils will hold up better than silty or sandy soils. The further away you are from the existing pile the better, but I would recommend not installing the new pile closer than 18 to 24 inches away."

Re-using the existing piles in my new design probably isn't an option. I don't think the client will want to pay for the pile testing, and I worry about differential settlement between the new and existing piles. I am neglecting skin friction for the new piles, so I tend to agree with his thoughts. I think the biggest obstacle will be creating a layout that doesn't clash with the existing piles.