Reuse existing piles

[structuralbob]

How can one justify or reuse existing precast piles. The existing pile cap will be demoed and existing piles will remain. Can these piles be reused. If we drive new piles, is capacity affected by the existing piles if the existing pile is not used. Piles are 14" and skin friction capacity of 100 ton down and 40 ton uplift.

 

[civilperson]

A pile test will give useful vales for ultimate strenths of piles. Divide these measured values by your Factor of Safety to obtain usable values.

 

[PEinc]

Demolition of the existing pile caps could damage the tops (or more) or the existing piles, especially if the piles have dowels extending into the concrete pile caps.

 

[DRC1]

Assuming the piles are cast into the cap and being cut free, you will lose hoop reinforcing at the top of the pile and cable anchorage, allowing prestress loss, even if the cables are cast in. Driving the piles in such a manner could cause the piles to break or spall, possibly causing a harmful situation to the crew below.

 

[DRC1]

One other question: How are you planning to extract the piles? Just curious.

 

[structuralbob]

To clarify:
The existing PC piles are currently in the ground with caps. The cap would be demoed and the piles would be cut off at a new lower top/pile elevation and a new cap poured. Uplift not a problem or would be handled through epoxy anchors. The existing piles are not extracted just reused in their current position with a new cap. Currently we are planning to drive new piles among the existing ignoring any contribution of the existing piles. I am wondering how one might avoid driving new piles and take advantage of what is already there. Perhaps testing the piles and using a higher factor of safety. But what is exactly involved in testing and is it cost affective; do you test all the piles or just a few. Perhaps access to the pile would be a problem with getting to and around the existing piles. Is the capacity of the pile affected by cutting it off as one suggested or is the prestress mainly for driving and handling?

Thanks

 

[DRC1]

OH I see now.

The geotechnical capacity of the pile should not diminish from reuse. Excavating the overburden will reduce the effective stress and therefore skin friction, but if it is only a few feet on a long pile it may not make a substaial difference, otherwise the effected can be estimated analticaly. GRL has low strain methods to check pile capacity. I would suggest contacting them for field verification methods.

http://www.pile.com.

Also the orginal driving and testing data would still be valid if it is available. As all the load comes in to the pile, you would need to accomodate the fact that the hoop reinforcement in the top of the pile might be cut away. You may be able to use a lower bearing load or encase the plie with the pile cap. That would be my only concern and should be something that can be dealt with. The prestressing in the pile is only to prevent tensile stress durring driving. When the hammer's shock wave is reflected at the pile tip, the pile would be stressed in tension if not for the prestress.
You should be able to get continued use out of the piles provided they are in good condition.