Construction of a new drilled shaft next to an abandoned shaft

[jmmoncada]

Hi,

I am currently working on foundation design project for a Transmission line (500kV) and the owner is proposing to construct a new concrete drilled shaft (6' in diameter) about 10ft away center-to-center from an abandoned shaft (6'in diameter). The shafts will not be connected by with a concrete cap. The abandoned shaft was fully installed (i.e. rebar cage, anchor bolts and concrete were installed). However, due to the project tolerance requirements the owner wants to abandoned the foundation completely and relocate it. My initial thoughts are that placing the new foundation 10ft from the abandoned would not hinder its capacity (uplift/compression & lateral) since the abandoned shaft will not be loaded and any grouping effects will not apply in this case. Also, I think that the abandoned shaft would act as a soil strengthening mechanism and provide additional capacity for the new foundation. I would like to know if anyone has input on this or if any one has experience with a similar situation.

Thank you

 

[PEinc]

IMHO, ignore the existing, abandoned shaft. I would not count on it helping to increase the capacity of the new shaft.

http://www.PeirceEngineering.com

 

[miningman]

So if I read this correctly , the pillar between the two shafts is only 4 feet?? Dont do it. Redistribution of the insitu stresses will likely cause this pillar to fail. For two 6 foot diameter shafts , the pillar should be at least 12 feet.

 

[jmmoncada]

Thank you miningman for your response. To clarify, What the owner is proposing is to drill and cast a new shaft foundation next to an existing/abandoned shaft foundation (with no loads) with about 4ft of clear distance between them. They are not proposing any kind of pillar between the foundations. It would just be the native soil. Is this what you are referring to?

 

[jmmoncada]

Thank you PEinc for your response. I will not be counting on the abandoned foundation for capacity of the new foundation. I was thinking that the abandoned foundation would provide soil strengthening similar to a rock or grout pillar.

 

[PEinc]

I would not count on any soil strengthening.

http://www.PeirceEngineering.com

 

[dauwerda]

I agree with you assessment. That is, you would not need to reduce the capacity of your new pier due to the close proximity of the abandoned pier. However, depending on soil type, the new foundation may require temporary casing when they drill it to preclude the soil sloughing that might occur due to the abandoned shaft.