Bedding and shading are levels of fill packed around a subterranean utility line. Bedding refers to the fill below the pipe or conduit, shading refers to the fill above the pipe or conduit. The utility trench -- called the “work pit” in some jargon -- is not entirely backfilled with sand, because sand is more expensive than other filling materials. The bedding sits on regular soil and the shading is covered by more soil; the latter typically has some restrictions placed on it as to what it may contain.
Sand sold specifically as “bedding sand” is a small aggregate, typically collected from mines and quarries as a byproduct of the main enterprise. The sand is processed to remove impurities, such as organic matter and stones, then sold in bulk by the square yard and in smaller amounts by the pre-weighed bag load. Bedding sand is used in a wide variety of landscaping projects, most often the setting of pavers. It gets its name from having originally been used in stables to cover floors that were uncomfortable for stock to sleep on.
Alternate Sands Used for Bedding and Shading
Utilities commonly accept that sand sold as “Builder’s Sand” or “Building Sand” is acceptable for bedding and shading. Stone dust and screenings -- called “manufactured sands” -- contain a predominance of fine material, so do not allow for drainage and can sink and shift over time.
Why Use Bedding Sand
Bedding sand and the other sands used for bedding are easy to work into a flat, level plane, after which they retain that conformation with little deformation even in extreme conditions. They do not expand or contract in freeze-thaw situations unless saturated with water, and they shed water swiftly. They stay cool and therefore reduce heat transference to the utility line, and inhibit the bacterial growth that occurs in natural soil; certain bacteria can be detrimental to utility lines. Neither the bedding nor the shading sand is typically compacted, so a measure of flexibility is inherent in the installation.
Typical Shading and Bedding Requirements
The main backfill is typically soil that is free of construction debris, glass, sharp rock and rock of any shape that is larger than 10 inches in diameter. Some utilities require that if the backfill contains rocks that are larger than 8 inches in diameter the shading depth must be increased.
A typical minimum backfill for electrical service cable buried in an approved conduit is 24 inches minimum to 36 inches maximum. Of this, immediately above and below the conduit must be the bedding and shading sand; the bedding must be 4 inches deep, and the shading must be 3 inches deep. For gas service lines, the total depths are the same, with a 4-inch bedding and a 6-inch shading. No shading is required above gas lines installed inside conduit. While these advisements are common industry practice, local code must be adhered to; contact your municipal code enforcement office to learn the applicable regulations.
Utility Line Installation
It is never legal for a property owner to make changes to a utility line, and most utilities insist on laying their own lines even with massive construction projects being carried out by professional contractors. Some utility companies, however, do allow the customer to dig trenches and provide fill material for new or upgraded installations. In such cases, the utility typically informs the customer how much sand it estimates will be required, and stipulates a distance from the trench within which the sand must be delivered.
- Photo Credit Jupiterimages/Photos.com/Getty Images