Fill Material Types
The fill classifications as specs actually use them — with the two variance traps flagged honestly: a compaction percentage means nothing without naming the Proctor, and select-fill plasticity caps genuinely range across DOTs. The geotech report governs every row.
The classifications
| Fill type | What it is | What the spec demands |
|---|---|---|
| Structural / engineered fill | Geotech-approved, gradation-controlled material under buildings and pavement, placed in lifts with density testing | 95% standard Proctor is the classic default (heavy/critical work specs modified — always per the geotech report); lifts ~8 in loose cohesive, up to 12 in granular/vibratory |
| Select fill / select borrow | Spec-defined soil meeting gradation and plasticity limits — better than common, below crushed base | PI cap typically ~6–15 and fines limits per the project spec — DOT criteria genuinely range PI 4–13 |
| Common / general fill | Bulk fill for non-structural areas — berms, general grading | Free of organics, debris, and oversize; lighter compaction (often ~90% standard) |
| Unsuitable material | Topsoil and organics, frozen soil, debris, soft wet high-plasticity clay, oversize stone | Over-excavate and replace with approved fill (or stabilize) — the engineer sets the extent |
| Flowable fill (CLSM) | Self-leveling cement/fly-ash/sand slurry placed without compaction — the utility-trench void filler | Usually kept under ~300 psi so it stays re-excavatable (ACI 229 material) |
Import, reuse, or stabilize
The geotech report governs suitability; reuse on-site material first — cut/fill balance eliminates trucking. The money: import ≈ $15–30/cy delivered and placed; export ≈ $20–40/cy (typical, varies by market) — and marginal native soil can often be lime/cement stabilized (~$3–8/cy) cheaper than export-plus-import. That three-way comparison (haul-in vs re-use vs treat-in-place) is the highest-leverage decision in sitework estimating, and it runs on the swell/shrink factors and cut-fill balance. Moisture makes or breaks placement either way — the moisture chart covers the window and the fixes.
Common questions
What is the difference between structural fill and common fill?
Where it goes and what is proven about it. Structural (engineered) fill carries buildings and pavement: geotech-approved material, controlled lifts, compaction to a specified Proctor percentage with field density tests. Common fill raises grades where nothing settles-sensitive sits on top — clean of organics and debris, but without the gradation control or the testing regime. The price difference is the paperwork, and it is worth every penny under a slab.
What does 95% compaction actually mean?
Nothing, until the spec names the Proctor. 95% of standard Proctor (ASTM D698) and 95% of modified Proctor (D1557) are different densities — modified applies about 4.5× the compactive energy, so its 95% is materially denser. The classic default is 95% standard; heavy and critical work specs modified. Quote the spec, not the folklore, and the geotech report governs.
What counts as unsuitable material?
The spec lists are consistent: topsoil and anything organic (peat, muck, sod), frozen soil, construction debris, soft wet high-plasticity clay, and oversize stone. The remedy is over-excavation and replacement with approved fill — with the engineer setting the limits — or in-place stabilization where the numbers favor it. Unsuitable-material quantities are the classic earthwork change order; walk the site before bidding lump-sum.
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