Subgrade preparation is the single most important step in constructing a durable pavement, slab, or building pad. If the subgrade is not properly prepared, everything built on top of it will settle, crack, or heave — regardless of how well the pavement section, slab reinforcement, or building foundation are designed. Inspectors know this, which is why subgrade inspection is one of the most scrutinized hold points during construction.

What Subgrade Preparation Involves

  1. Stripping — removal of topsoil and organic material. Topsoil contains roots, organic matter, and biological activity that decompose over time, creating voids. Typical strip depth is 6 to 12 inches, but the geotechnical report should specify the actual depth based on field observations.
  2. Over-excavation (if required) — removal of unsuitable native soil identified by the geotech. Expansive clay, peat, loose fill, or contaminated soil may need to be removed and replaced with engineered fill. Over-excavation depths range from 1 to 5 feet or more depending on conditions.
  3. Moisture conditioning — adjusting the soil's moisture content to within the range specified by the geotechnical engineer (typically plus or minus 2 percent of optimum moisture content). Soil that is too dry will not compact. Soil that is too wet compacts to a rubber-like state that rebounds when loaded. Moisture conditioning involves either wetting (spraying water and mixing) or drying (discing and aerating).
  4. Compaction — mechanically densifying the soil to the required relative compaction using rollers (sheep's foot, smooth drum, vibratory), plate compactors, or rammer compactors. The required compaction is typically 90 to 95 percent relative compaction (percent of maximum dry density from the Modified Proctor test, ASTM D1557).
  5. Proof rolling — rolling the prepared subgrade with a loaded dump truck or heavy roller to identify soft spots that passed the compaction test but deflect under load. Soft spots are over-excavated and recompacted.

Compaction Testing

Compaction is verified by in-place density testing, most commonly using a nuclear density gauge (NDG). The NDG measures the wet density and moisture content of the soil at the test location. These values are compared to the maximum dry density and optimum moisture from the laboratory Proctor test to determine the percent relative compaction and the moisture deviation from optimum.

Test frequency: The specifications or the geotechnical report typically require one test per 2,500 to 5,000 square feet of subgrade area, at a minimum of three tests per area. Tests are taken at the surface and at depth (typically 6 to 12 inches below the surface) to verify that compaction extends through the full depth of the prepared zone.

Passing criteria:

  • Relative compaction: 90% minimum for landscape and non-structural areas; 95% minimum for building pads, streets, and structural areas
  • Moisture content: within plus or minus 2% of optimum (some specifications require above optimum for expansive soils)

Proof Rolling

Proof rolling is a qualitative test that complements the quantitative compaction tests. A loaded vehicle (typically a dump truck carrying 20 to 25 tons of material) drives slowly across the prepared subgrade. The geotechnical engineer or the grading inspector walks behind the vehicle and observes the tire tracks.

  • Acceptable: The tire track is less than 1 inch deep and the subgrade does not visibly deflect or pump water.
  • Unacceptable: The tire track exceeds 1 inch, the subgrade deflects (bounces) under the wheel load, or water pumps to the surface. These areas must be marked, over-excavated to the depth of the soft material, moisture-conditioned, recompacted, and retested.
Why proof rolling matters: Compaction tests are point measurements — they verify conditions at the specific location of the test. A 5,000 SF building pad with three test locations leaves 99.9% of the pad untested. Proof rolling tests the entire surface by loading it continuously. It catches soft spots that fall between the point tests.

Expansive Soil Considerations

Expansive soils (clays with high plasticity index and expansion index) require special subgrade preparation:

  • Moisture conditioning above optimum: Expansive soils should be compacted at 2 to 4 percent above optimum moisture content. This pre-swells the clay so that subsequent wetting during the building's life does not cause additional expansion.
  • Select fill pad: For building pads on expansive soil, the geotechnical engineer may require removing the upper 2 to 4 feet of expansive soil and replacing it with non-expansive import fill. This creates a buffer that absorbs moisture variation without transmitting swelling pressures to the foundation.
  • Moisture barrier: A polyethylene vapor barrier (10-mil minimum) under the building slab prevents moisture from migrating upward from the expansive subgrade into the slab and the building interior.

What the Inspector Documents

The grading inspector (typically from the geotechnical firm) documents the following at each subgrade inspection:

  • Date, weather, and location of the inspection
  • Description of the subgrade condition (soil type, color, moisture, consistency)
  • Compaction test results (density, moisture, percent compaction, depth)
  • Proof roll results (pass/fail, location of soft spots, corrective action taken)
  • Whether the subgrade is approved for the next construction step (aggregate base placement, concrete pour, or building foundation)

This documentation becomes part of the project's compaction report, which is filed with the building department as evidence of compliance with the geotechnical recommendations and the grading permit conditions.

Common Failures

  • Compacting too dry. Soil that is significantly below optimum moisture cannot achieve the required density regardless of roller passes. The fix: add water, mix, and recompact.
  • Compacting too wet. Soil above the wet side of the compaction curve rebounds under load and appears "spongy." The fix: disc the soil to aerate and dry it, then recompact. This can add days if the weather is not cooperating.
  • Leaving soft spots. Utility trench backfill, old foundations, buried vegetation, or fill placed over soft native soil can create localized soft spots that settle under load. The fix: over-excavate, backfill with approved material, and recompact.
  • Rain damage. A subgrade that was approved at 3 PM can fail by 8 AM if it rained overnight. The surface must be retested after any wetting event. Protect the subgrade with plastic sheeting if rain is forecast and the next construction step is not ready to proceed.