We are installing a 22' x 44' rectangular inground swimming pool (Steel walls, vinyl liner). We have just put the walls up. To level the the walls we have used patio pavers and shims under some of panels. This has created a significant gap between the the pool shelf and the walls in those areas. We will be ready to pour the concrete collar soon. Our question is: how do we keep the concrete from seeping into the pool? I.e. what should we use to block or fill those gaps to keep the concrete collar on the outside of the pool?