The expansion joint is that 1” space between the perimeter coping (the decorative edge of the pool) and the pool deck. It is filled with a special pool caulk that’s a flexible sealant. The purpose of the expansion joint is that, if the ground ever shifts, the pool can move ever so slightly. If there were no expansion joint, the edges of the swimming pool would crack as well as damage the coping and tiles.
A second reason the expansion joint is filled with flexible caulking is to prevent water from slipping down behind the swimming pool wall. Water could potentially rust the steel supportsholding the swimming pool walls and floor. Also, if water were to get down and around the pool and the water freezes and expands, it could crack the swimmingpool bottom.