Subgrade, Embankment, and Backfill Construction Inspection
Scope
This domain covers what an inspector does on a subgrade, embankment, or backfill operation: verify materials, control lift thickness and moisture, watch compaction, and accept or reject each lift with tests.
Before compaction
Strip unsuitable material, proof roll the exposed subgrade with a loaded truck, and scarify the surface before placing the next lift so layers bond. On sidehill fills, bench the slope. Frozen, organic, or excessively wet material is rejected before it enters the fill.
During placement
Lift thickness is measured loose and checked against the compacted thickness the specification allows; a common maximum is 200–300 mm (8–12 in.) compacted. Moisture is adjusted toward optimum before rolling - discing and aerating to dry, water truck to wet - and the roller pattern must cover the full width with the required passes.
Testing and acceptance
Density tests are taken at the frequency the specification sets, at random locations, at the depth of the lift. Each result is compared to the required relative compaction and moisture window. A failing test means rework of that area, not a retest until a passing number appears.
Worked example
A 6.1 m × 91 m (20 ft × 300 ft) embankment lift = 555 m² (6,000 ft²). The specification requires one density test per 500 ft² per lift, minimum two. 6,000 ÷ 500 = 12 tests for this lift. Over five lifts, plan 60 tests and randomize locations across the lot rather than testing near the ramp where the material is easiest to compact.
Traps
Testing a lift after rain without rechecking moisture; testing only the surface of a thick lift with a backscatter gauge; missing a lift in the record; accepting backfill against a wall before it has gained strength; treating a proof-roll yield as a density result.