Fill is earthy material which is used to fill in a depression or hole in the ground or create mounds or otherwise artificially change the grade or elevation of real property. It is generally sourced from cuttings, borrow pits or from other excavations on-site. In some instances, where fill is not available on-site, it may need to be imported from elsewhere.
Fill is usually subsoil (soil from beneath the top soil) which has little soil organic matter or biological activity. Fill should be as free of organic matter as possible since organic matter will decompose creating pockets of empty space within the fill which could result in settling. Uneven or excessive settling of the fill can result in damage to any structures built on the fill. Fill is generally a natural material that has been cut/dug out of the ground with little to no processing, and is required to provide sufficient strength to hold form when compacted or placed.
In roadworks, fill is most commonly used to build up the road embankment or shoulders, or to replace material that is unsuitable to construct upon. Fill used for embankment construction is most prevalent when filling valleys/gullies in undulating terrain, and where roads are to be 'built up' on flood plains and bridge approaches. Types of fill include:
Select fill
Select fill is superior quality material, used principally as capping, selected material, structural material and/or verge material and which is specified with a minimum California Bearing Ratio (CBR) value. Select fill materials are commonly specified for use in the construction of:
- the earthworks layer immediately beneath the pavement layers
- earthworks capping and verge layers
- spill-through abutment fills and behind abutment areas of bridge approach embankments
- reinforced soil embankments
- fill placed adjacent to retaining walls
- pipe and culvert backfill.
The maximum stone size for select fill used for roadwork's is 75mm. Select fill is required to be compacted; and when compacted, should be non-permeable (i.e water should not flow through it).
General fill
General fill is a medium quality material which does not meet the requirements of select fill, which is usually specified with a minimum CBR value. General fill materials are usually used to:
- Provide a stable platform during construction so that select fill and pavement layers can be satisfactorily placed.
- Provide an elevated platform for a pavement such that adequate clearance is provided between the groundwater table or inundation level and the bottom of the pavement, and to protect the pavement from moisture infiltration.
- Provide a more economical earthworks design than when using higher quality material for the whole of the fill embankment.
Non-structural fill
Non-structural fill is a lesser quality material which does not meet the requirements of select fill or general fill material, and which may be used in non-structural zones of embankments.
Rock fill
Rock fill is a permeable material comprised of larger rock and rock fragments which may be used as drainage layers in areas subject to inundation or groundwater inflows or where general and non-structural fills are unsuitable, commonly due to the low strength of in situ soils.
Permeable fill
Permeable fill material is a self-draining material, typically sand or aggregate that is generally used to provide a capillary break within a layered earthworks/pavement profile or to provide a drainage medium to intercept and drain groundwater inflow.