You are going to have to be more specific about your retaining walls. Are you talking about a raised bond beam wall on the pool or what? Expansion joints are usually a matter of personal preference with your pool builder. I don't ever use the plastic type because sometimes they don't look too good after a year or 2 in the sun. We deep score the joints in the deck.

chickadee4, The foam is perforated near one edge, usually about a half inch depending on the width of the foam. It is easiest to use adhesive or silicone to keep the foam in place. Do not adhere above the perforation. After the pavers are in place you can remove the foam top and pour in the joint seal. Use tape along the back of the coping and pavers to make a clean edge. You can sprinkle a little sand on top of the joint seal to give it a little texture, but it is not recommended by the manufacturer. Forget about the reference to tar. Make sure joint is clean before applying seal. Deck-O-Seal a common product and easy to pour. Good luck.

i can tell by the palm tree that you're somewhere between South Carolina and Florida (or maye west coast, i.e. California) Assuming I'm correct to think that - I'd say you're going to be fine. You probably are not in a freeze-thaw area. I agree with the previous poster though - at least in my climate, I would definitely have an expansion joint Nice pool, btw!

If the original foam is deteriorating, I would remove it and replace with backer rod. The 3/4" is fine as you can work it into the joint for a tight fit. To make a consistent depth, sweep play sand into the joint. Use a jointing tool (narrow mason tool-about 1/2") to work in and create a solid base about a 1/2" deep. Then apply the Deck-O-Seal. You will use less material and prevent sags and have a consistent thickness. Be patient and careful when applying with the squeeze bottle. If you get drips on the coping or patio, it's difficult to remove.