Thanks for the reply.

I don't think it's a settling problem, at least at this point. The
pool was built in 1989, and the crack was there in 2000 when I bought
the place. I tried to fix it by removing all tile in the area and
patching the crack with polysulfide caulk, which can be used in
submersible situations. I thought that polysulfide, which has great
elasticicty, would endure any slight movement. But, apparently, the
caulk has become delaminated from the concrete, rather than failing
through gross movement.

I cannot comment on the shell itself, except to say it was built by one
of the areas premier pool builders and is fine everyhwere but this one
location.

I was hoping that driving a bunch of rebar vertically through the deck
and into the shell wall would stabilize any stresses and movement that
might be occuring through uneven expansion. If that's a lame idea,
then I might try another simple repair, using polyurethane caulk this
time, which also can be applied for submersible use and which has one
of the strongest adhesions of any material, although its elasticity is
slightly less than polysulfide.

See your point on the Torque-Locks.

Thanks, again.