There was speculation here that when casing is left behind in drilled shaft after concreting, concrete may shrink and loose contact with casing and coefficient of friction between steel casing and concrete becomes important in transfer of structural load to the sides of the hole. The transfer of load from structure (e.g. tower leg) may be like this:
1) From leg to concrete pier 2) Then from pier to steel casing 3) And Finally from steel casing to concrete
This may be more important for the case of uplift force.
My understanding is that concrete pier and steel casing will act as single unit and structural load is transferred to the sides of hole directly.
I would appreciate if somebody comments on transfer of load to sides of hole for the above case.