Dirtwork term
Sub-grade
also called: subgrade
The natural soil surface that supports everything you build on top of it, pad, road base, slab.
Sub-grade is the prepared native soil that everything else sits on. For a residential slab, the sub-grade is the dirt the slab is poured on. For a road or driveway, it's the dirt the base rock is spread over. For a building pad, it's the cut-and-filled surface the structural fill is placed on.
Sub-grade prep is most of the dirtwork on any new build. It includes stripping topsoil, cutting to the right elevation, removing soft spots, compacting to the right density, and grading to drain. Bad sub-grade is the root cause of foundation cracking, road rutting, and standing water on a finished pad.
In Houston Black clay, sub-grade prep usually means scarifying and recompacting the top lift to a controlled moisture content, then sometimes installing a moisture barrier or geotextile before the structural fill goes in.
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Sub-grade comes up on excavation jobs all over East Texas.
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When you need it
When sub-grade comes up on a job
This is the layer you check before a driveway, slab, or pad covers it. If the dirt underneath is soft or wet, that weakness telegraphs up through whatever sits on top, so the sub-grade gets tested and corrected while it is still exposed.
Related terms
Other words that come up alongside this one
Pad
Prepared, compacted area of ground sized and elevated for a structure to be built on top of it.
Compaction
Mechanical pressing of soil to reduce air voids, increase density, and provide a stable surface.
Cut and Fill
Moving dirt from high areas (cut) to low areas (fill) to reach a planned grade, balanced so net hauling is minimized.
Select Fill
Imported soil chosen for known compaction and stability properties, used where native soil isn't suitable.
Scarify
Mechanically loosening the top few inches of compacted or dried soil to break it up before re-working.
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