5.1 Soil Classification & Index Properties

The Three-Phase Soil Model

Soil is a three-phase material: solids, water, and air. Geotechnical index properties describe the proportions of these phases, and on the Fundamentals of Engineering (FE) Civil exam every phase-relationship problem reduces to a phase diagram with volumes on one side and weights (or masses) on the other.

The core volumetric ratios are:

• Void ratio e = V_v / V_s (volume of voids over volume of solids). Typical sands 0.4–0.9; soft clays can exceed 1.0.
• Porosity n = V_v / V (voids over total volume). Convert with n = e/(1+e) and e = n/(1−n).
• Degree of saturation S = V_w / V_v, expressed 0–100%. S = 0 is dry; S = 100% is fully saturated.

These live in the Geotechnical section of the NCEES FE Reference Handbook; learn to find and apply them rather than memorize.

Weight–Volume Relationships

On the weight side, water content w = W_w / W_s (weight of water over weight of solids, as a decimal or %). Specific gravity of solids Gs = γ_s / γ_w, where γ_w = 62.4 lb/ft³ (9.81 kN/m³); most soils have Gs ≈ 2.65–2.75.

The single most useful identity is the saturation equation:

S·e = w·Gs

Unit weights you must distinguish:

A classic trap: mixing dry and total unit weight. Always check whether the problem gives moist or dry conditions before substituting.

Sieve Analysis & Atterberg Limits

Particle-size (sieve) analysis shakes oven-dried soil through stacked sieves; the percent passing each defines the gradation curve. Key parameters:

• D10 (effective size), D30, D60 — diameters at 10/30/60% passing.
• Coefficient of uniformity Cu = D60/D10 (well-graded sand needs Cu ≥ 6).
• Coefficient of curvature Cc = D30² / (D10·D60) (well-graded needs 1 ≤ Cc ≤ 3).
• The No. 200 sieve (0.075 mm) is the gravel/sand vs. silt/clay (fines) boundary.

Atterberg limits describe fine-grained behavior by water content at phase transitions: liquid limit (LL), plastic limit (PL), and shrinkage limit. The plasticity index PI = LL − PL measures the moisture range over which soil stays plastic. PI and LL plot on the A-line (PI = 0.73(LL − 20)) on the plasticity chart to separate clays (CL/CH) from silts (ML/MH).

USCS, AASHTO & a Worked Example

The Unified Soil Classification System (USCS) assigns a two-letter group symbol: first letter for the dominant fraction — Gravel, Sand, M silt, C clay, O organic — and a second for gradation/plasticity — Well-graded, Poorly-graded, L low, H high plasticity. Coarse soils (≤50% passing No. 200) use Cu/Cc; fine soils use the plasticity chart.

The AASHTO system (A-1 to A-7 plus group index) ranks soils for highway subgrade; lower numbers are better. A-1–A-3 are granular; A-4–A-7 are silt-clay.

Worked phase example: A sample has w = 18%, Gs = 2.70, and S = 90%. Find e and γ_d.

• From S·e = w·Gs: e = wGs/S = (0.18·2.70)/0.90 = 0.54.
• γ_d = Gs·γ_w/(1+e) = (2.70·62.4)/(1.54) = 109.4 lb/ft³.

This four-step substitution — write the identity, solve for the unknown, then compute a unit weight — is the template for nearly every FE Civil index-property question.

Relative Density & Consistency

Index tests also gauge in-place state. For coarse-grained (granular) soils the relative density Dr ranks how dense a sand is relative to its loosest and densest possible packing:

Dr = (e_max − e)/(e_max − e_min) × 100%

where e_max and e_min are the void ratios in the loosest and densest states. Dr is also expressed with dry unit weights. Loose sand (Dr < 35%) is compressible and liquefaction-prone; dense sand (Dr > 65%) is strong and stiff. This is distinct from relative compaction (a field-vs-Proctor ratio used for fills, Section 5.3) — a frequent FE Civil mix-up.

For fine-grained soils, consistency is described from the undrained shear strength s_u and the liquidity index LI = (w − PL)/PI, which locates the natural water content within the plastic range. LI near 0 means stiff (water near PL); LI near 1 means soft (water near LL). The activity A = PI/(% clay-size fraction) flags expansive clays such as montmorillonite. These index numbers feed directly into the strength and settlement work of the next section, so treat classification as the gateway, not an isolated topic.

** A USCS or AASHTO symbol immediately implies engineering behavior: a GW (well-graded gravel) drains freely, compacts well, and makes excellent fill or pavement base; a CH (fat clay) is nearly impervious, shrinks and swells with moisture, and is poor structural fill. Because the FE Civil exam is open to the Handbook, the skill being tested is reading the gradation curve and plasticity chart correctly and mapping the result to expected permeability, frost susceptibility, and compaction quality — not memorizing the chart. 81 kN/m³ consistently through a phase calculation.