There are 2 main kinds of Cattle Grid.
The Conventional Cattle Grid rests on has a concrete slab placed below ground level. Cast onto this as a series of concrete sleeper walls and a surround which usually includes a shelf along the sides of the pit for the ends of the steel cattle grid to rest on. The grid itself is a large fabricated grating which rests on the shelves and the sleeper walls. Its performance depends on : Precisely cast base & Properly designed and fabricated grid
The Pitless or Drop-in Unit can be installed onto either a flat concrete slab at a particular level or a bed of compacted DoT Type1 or similar, to spread the load and not crumble over time. The delivery vehicle arrives and the unit is placed into the excavation and back filled. It is ready to use.
Although there is more steel, labour and galvanising costs in a Pitless or Drop in unit this has to be set off against the cost to provide the concrete base and sleeper walls and the disruption of the track/road being closed for days.