Subgrade

Subgrade is the natural ground or compacted earth surface that sits directly beneath any structural layer above it: a slab, a strip footing, a sub-base, or a pavement. It is the foundation of the foundation. The subgrade’s bearing capacity, moisture content, and compaction state set the limit on what the structure above it will tolerate.

The layer cake under a typical residential driveway, from the bottom up:

1. Subgrade (this term): the prepared ground surface, natural or compacted fill.
2. Sub-base: an engineered crushed-rock layer over the subgrade, typically 100 to 200 mm of compacted Class 2 or Class 3 road base.
3. Base course: a thinner finer crushed-rock layer over the sub-base, where used.
4. Pavement (slab, asphalt, pavers).

Under a residential slab, the layer cake is shorter: subgrade, then any controlled fill bringing the level up, then the slab.

Compaction. The subgrade must be prepared per AS 3798:2007 (Guidelines on earthworks). Level 1 supervision by a Geotechnical Inspection and Testing Authority is required for any subgrade that ends up under a slab or footing. Typical target: 95 to 98 per cent of standard maximum dry density (MMDD) measured by sand replacement or nuclear density gauge. The geotech issues a compaction report; the builder hands it to the certifier.

Moisture matters as much as density. A nominally well-compacted subgrade laid wet will continue to consolidate after the slab is poured, taking the slab with it. The geotech sets a moisture content range (typically optimum minus 2 per cent to optimum plus 2 per cent) and rejects pours outside it.

Common builder mistakes. First, scraping topsoil and pouring on the underlying clay without checking the in-situ classification. Reactive clay subgrade pushes the site to a higher site classification under AS 2870 and changes the slab design. Second, placing fill on undisturbed subgrade without scarifying first: the fill-on-clay interface becomes a slip plane. Always scarify and re-compact the top 100 to 150 mm of subgrade before placing fill.

Subgrade vs sub-base is the most common terminology error. Subgrade is the ground; sub-base is the engineered crushed-rock layer over it. The geotech specifies both separately.

Also known as: formation level, natural ground, founding surface.

Category: Site preparation / earthworks.

Related

• Slab-on-ground construction
• Conventional reinforced slab
• Waffle pod slab
• Strip footings
• Driveways: residential

See also

• Cut and fill
• Controlled fill
• Soil report

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Last updated: 2026-05-14. Verified: 2026-05-14.