Back-up rings have no intended sealing function. Instead, as their name indicates, they are protective and supporting elements made from extrusion-resistant materials which generally have a rectangular cross-section. They are installed in a groove together with an elastomer sealing element, preferably with a corresponding O-ring in static applications.
Comment: assessing the future of Britain’s road network
It seems to me, as I drive around, that a significant proportion of potholes are not actually potholes at all, but sunken grids and manhole covers....