Your soils report will dictate your engineering report. It would be best to have that in hand so you can take it to the structural engineer and he can then decide your rebar schedule. You may not even need a four bar box beam.

Thanks for the reply. In six bids, no PB has even described providing a soil report. We have a pool shell significantly out of the soil (raised), with a portion excavated in soil. My thought is that for a few hundred dollars more we can improve to 8"OC and 4 bar box beam and its money well spent. I was asking for a photo because I can't findable good depiction of one.

The only problem with just upgrading to more rebar still will not tell you weather you have enough or to much. You or your builder are not a soils or structural engineers. A soils report is cheap insurance. Especially if you have differential soils. Most builders will not get a soils report done because they were taught you do not need one for a pool. Most cities require a soils report if you were to do a house or an addition. Why should a pool be any different.

The only time we have test bores done is if we think there is a potential problem or if we dig up an old stump hole or once when we had to remediate soil around an old oil storage tank. I don't know of any builders in our area that get soils reports on jobs. Structural failures of the pool shell are very rare and usually because they built in unengineered fill or a stump hole.

I understand this is more of an issue on the west coast.