4.1 Soil & Aggregate Types, Classification & Properties

Why Soil and Aggregate Classification Matters

Nearly every highway cross-section rests on soil, and most of what sits above the soil — subbase, base, and often the surface course itself — is aggregate. An HCI inspector's job starts with knowing what material is in front of them, because acceptance decisions (use it, blend it, reject it, or undercut it) all trace back to classification. Level II inspectors are directly tested on identifying soil and aggregate types and judging whether a material is suitable for its intended use in the roadway section.

Basic Soil Types by Particle Size

Soils are grouped by grain size into four broad categories, from coarsest to finest:

• Gravel — particles larger than the No. 4 sieve (4.75 mm), typically rounded or angular rock fragments
• Sand — particles passing the No. 4 sieve but retained on the No. 200 sieve (0.075 mm)
• Silt — fine-grained particles smaller than sand, with little to no plasticity
• Clay — the finest particles, exhibiting plasticity (they can be molded when wet and hold that shape when dry)

Coarse-grained soils (gravel and sand) derive their strength from particle-to-particle friction and interlock. Fine-grained soils (silt and clay) derive their behavior from moisture content, mineralogy, and plasticity — which is why they are far more moisture-sensitive in the field.

Two Classification Systems

Inspectors will see two classification systems referenced on highway plans and specs:

Under AASHTO M145, soils are ranked from the best subgrade material to the worst:

• A-1 — well-graded gravel/sand, little or no fines (best)
• A-2 — gravelly/sandy soils with some silt or clay
• A-3 — fine sand, non-plastic
• A-4 / A-5 — silty soils, low to high plasticity
• A-6 / A-7 — clayey soils, moderate to high plasticity (worst)

Within groups that contain fines, a group index can be calculated from percent passing the No. 200 sieve, liquid limit, and plasticity index; a higher group index means a poorer subgrade.

Atterberg Limits and Plasticity

For fine-grained soils, plasticity is the single most important property an inspector documents, because plastic soils shrink, swell, and lose strength dramatically as moisture changes. Two lab tests define it:

• AASHTO T89 (Liquid Limit, LL) — the moisture content at which soil transitions from a plastic to a liquid state, determined with a mechanical liquid limit device (Casagrande cup)
• AASHTO T90 (Plastic Limit, PL, and Plasticity Index, PI) — the PL is the moisture content at which a rolled soil thread just begins to crumble at about 1/8-inch diameter; PI = LL − PL

A high PI signals a highly plastic, moisture-sensitive soil — a red flag for use as subgrade or embankment fill without stabilization (see Section 4.6). A PI near zero (non-plastic) is characteristic of clean sands, gravels, and silts.

Aggregate Types and Acceptance Properties

Aggregate — the sand, gravel, and crushed stone that makes up subbase, base, PCC, and HMA — is typically supplied as crushed stone, natural (bank-run) gravel, crushed gravel, manufactured sand, or, on some projects, slag or recycled concrete/asphalt aggregate. Beyond gradation (Section 4.2), inspectors verify aggregate certifications and, where required, source-sample results for:

• Soundness — resistance to freeze-thaw degradation
• LA Abrasion (AASHTO T96) — resistance to degradation from abrasion and impact in a rotating steel drum
• Deleterious materials — clay lumps, shale, organic matter, or friable particles that weaken the finished product
• Specific gravity and absorption (T84/T85) — used to convert aggregate weights to volumes for mix design and yield calculations

A material that fails classification or acceptance testing is not automatically wasted — it can often be blended with a better-graded or less plastic material, or used in a lower-demand application (e.g., embankment fill instead of base course), but the inspector's documentation of why a rejection or blend decision was made is part of the permanent project record.

Quick Field Identification

Lab results (gradation, Atterberg limits) often take a day or more to come back, but an inspector still has to make same-day calls about whether a haul truck should be turned away. Visual-manual field checks bridge that gap:

• Grab and feel — gritty, individual particles visible to the eye indicate sand; a smooth, slightly sticky feel when damp indicates silt or clay.
• Ribbon test — a moistened pat of soil is squeezed between thumb and forefinger into a ribbon; clay forms a long, cohesive ribbon (2 inches or more) that holds together, while silt breaks apart quickly.
• Dry strength test — a dried soil pat that is difficult to break by hand indicates higher clay content and plasticity; one that crumbles easily indicates silt or non-plastic fines.
• Dilatancy (shake) test — a wet pat of silt develops a shiny, "livery" surface when shaken in the palm and dulls again when squeezed, a reaction clay does not show.

None of these field checks substitutes for the lab report, but a consistent mismatch between what a field check suggests and what a source's certification claims is exactly the kind of red flag that justifies pulling an independent sample.