1) Construction terminology 
In urban planning, infill is the rededication of land in an urban environment, usually open space, to new construction. Infill also applies within an urban polity to construction on any undeveloped land that is not on the urban margin. The slightly broader term `land-recycling` is sometimes used instead. Infill has been promoted as an economical.....
The stone or soil material used to fill gaps in trail or wall construction/revetment work.
Soil located behind the segmental retaining wall units and drainage fill. May be reinforced with soil reinforcement
to fill in: The old stream beds have been infilled with sediment.