Drilled Shaft with Both Permanent and Temporary Casing 1

[STrctPono]

I have a job currently being bid on. New concrete pier in the ocean. 2'-6" drilled shafts (yes, you heard that right, not driven piles). Our shafts are 72ft long with 28ft long permanent steel casings... coral at 69ft deep. Temporary casing is optional if Contractor wants.

RFI came in during bidding. RFI asks, "can Contractor oversize the permanent casing to allow extraction of the 3'-0" ID temporary casing". I discussed it with Geotech and told him that I do not want the shaft oversized as this affects structure behavior and affects the precast superstructure elements. He responded back to the RFI that shafts cannot be oversized and that this is a means and methods issue. Seems a bit of a cop-out answer, but, oh well.

That means the Contractor was thinking that they could extract the temporary casing on the inside of the permanent casing? Forgive my ignorance, but is that how you typically withdraw a temporary casing when it is below a permanent casing? Geotech seems to think the more logical way is to advance the permanent casing down to the depth required to satisfy constructability and then withdraw permanent casing back to the final elevation during concreting. Does this seem more logical.

I was hoping to get your guys' input on the matter.

Thanks.

 

[STrctPono]

Bridge and Pier design philosophy is such that the structure is detailed to force the ductile fuse/hinge to develop in the pile whereas for Buildings, the hinge occurs in the beam. The stirrup reinforcing in the cap stops several inches from the face of the drilled shaft since it physically would not fit to run it through the joint. See attached image. Therefore, there are joint shear requirements that must be met and those are resisted by the spiral reinforcing above the cap (in the joint).

Furthermmore, since the highest strains in the vertical reinforcing steel occur at the pile to cap connection and we are expecting a moderate level of inelastic strain to occur in the reinforcement (i.e. the plastic hinge length extends above the top of pile into the cap) then we have further ductility requirements to satisfy, and the best way to provide ductility is by confining the concrete in the joint with spirals or closed hoops. Welded seismic hoops are the best but were not needed in this application since the levels of inelastic strain are not that high.

Concrete pier joints have historically performed rather poorly in seismic events so I try to pay close attention to the joint detailing.