Grading and Base Preparation in St. George, UT

Rough grading moves soil to set the overall shape: cutting high spots, filling low ones and giving the surface a fall that carries water away from the house and off the pavement. In St. George, where runoff goes matters beyond puddles. Water soaking into gypsum-bearing ground dissolves the gypsum, and the Utah Geological Survey lists street, roof and parking lot runoff and irrigation among the things that speed that up. Sizing the drains themselves is covered on driveway drainage.

Once the shape is right, the native soil (the subgrade) is compacted, then tested. A proof roll looks at the whole surface rather than a few test spots:

• Equipment: a tandem-axle dump truck, or a tri-axle with the third axle raised, loaded to at least 20 tons; a towed roller of at least 40 tons is the alternative.
• Pass criteria: a single pass, with about 1 inch of deflection at most and no pumping or cracking.
• Scope: the research applies it to the subgrade only, not to each layer of base.

Those numbers come from an Indiana DOT highway study. A driveway crew may use a lighter loaded truck, but the idea is the same: find soft spots while they are still cheap to fix.

The base course is the crushed stone that actually carries the asphalt. Angular stone mixed with sand and stone dust (often sold as crusher run) packs tightly because the pieces lock together, while rounded rock rolls under load. It goes down in layers, each compacted before the next. Base depth depends on what will drive on it; the figures for cars and heavier vehicles are on asphalt driveway paving, and an RV pad needs the heavier end.

Where the soil itself is the problem, the fix is to dig it out deeper than a normal base would need and replace it with compacted, imported material. How deep depends on what the dig finds, so ask whether the bid prices over-excavation as a unit rate (per cubic yard or per inch of depth) instead of a guessed lump sum. The soil types that call for it are listed below.

On soft or fine-grained ground, a layer of geotextile fabric can go down before the first lift of base. A Virginia transportation research report describes its main job as keeping the base aggregate and the subgrade fines from mixing, and cost guides say it stops gravel from sinking into the subsoil. It does not replace compaction or a proper base depth.