Subgrade Preparation and Verification

Subgrade preparation requires: removing all unsuitable material (organic soil, topsoil, material with PI > 30 per spec); shaping the surface to within ±0.05 ft of design grade at a 25-ft grid; proof rolling with a loaded tandem truck to identify soft spots; compacting to 95% modified Proctor (ASTM D1557) or as specified; and verifying density at required test frequency before placing base course or paving.

Why Subgrade Preparation Is Critical

Subgrade is the in-situ or compacted soil layer that supports the pavement or floor slab. Poor subgrade preparation is the leading cause of premature pavement failure. When a road or slab cracks, the first question from the engineer is always: what was the compaction on the subgrade? If the answer is “unknown,” the contractor is exposed. If the answer is “documented, passed, and here are the test results,” the contractor has a defense.

Step 1: Remove Unsuitable Material

Before any compaction work, identify and remove: organic soil with visible roots or organic matter, clay with PI above the specification limit (often PI > 30), saturated soils that deflect under foot traffic, and material that proof rolling shows > 1 inch rutting. Over-excavate, confirm with the geotech, bring in select fill, and compact in lifts.

Step 2: Shape to Grade (±0.05 ft)

Rough grade and fine-shape the subgrade to within ±0.05 ft of design before proof rolling and compaction testing. Check at a 25-ft grid with a rotating laser and rod or a level and rod. Flag any out-of-tolerance areas for re-shaping before the proof roll.

Step 3: Proof Roll

A loaded tandem truck (40,000–50,000 lbs) drives slowly across the subgrade while an engineer or inspector observes. Soft spots show as rutting (> 1/2 to 1 inch, per spec), pumping (soil moves under the truck), or heaving adjacent to the tire path. Mark all failing areas. Do not pave over a failed proof roll — excavate and replace the soft material.

Step 4: Compact and Test

Compact to the specified density (typically 95% modified Proctor per ASTM D1557 for road subgrade). Test density with a nuclear gauge at the required frequency — typically one test per 5,000 sq ft. Document every test with station, offset, field density, moisture, and % compaction vs. required. Log in Gradelog.

Compaction Requirements by Project Type

Final Grade Verification Before Paving

After compaction testing passes, take a final grade check at 25-ft intervals before the paving crew sets up. This is the last chance to catch out-of-tolerance grade without disrupting paving operations. Document the final grade check in Gradelog.

See also: Grading Tolerances and Compaction Percent Calculator.