The exposed edge of the trench helps hold the grate in position and is subject to the same loads as the grate. In addition to the effect of climate and weight of vehicles, it may be exposed to impact from items being dropped or pulled across it (e.g. skips). Once the edge fails, the grate will move and cause catastrophic failure.
Metal edges are most commonly used to withstand the abuse of traffic. Edge protection rails should be integrally cast-in or mechanically connected to the trench body. Edge rails that sit over existing standard edges are often ill-fitting and susceptible to failure.
Edge protection rails also provide some protection during installation, particularly if the wearing course of the pavement is not applied immediately. Appropriate edge protection is particularly important in asphalt situations where rolling machines can damage trench edges, leading to premature failure of the trench.