Soil Classification and Properties

Why Classify Soils?

Soil behavior (strength, compressibility, drainage) depends on its type. Classification gives engineers a quick framework for preliminary design decisions.

Phase Relationships

Soil is a three-phase material: solids, water, air.

Key relationships:

• Void ratio: e = V_v / V_s
• Porosity: n = V_v / V_total = e/(1+e)
• Water content: w = M_w / M_s × 100%
• Degree of saturation: S = V_w / V_v × 100%
• Dry unit weight: γ_d = G_s × γ_w / (1+e)
• G_s (specific gravity): typically 2.65–2.70 for minerals

Atterberg Limits

Define the consistency of fine-grained soils:

| Limit | Definition | Test | |-------|-----------|------| | Liquid Limit (LL) | Water content at transition to liquid behavior | Casagrande cup | | Plastic Limit (PL) | Water content at transition to plastic behavior | Thread rolling | | Shrinkage Limit (SL) | Water content below which no volume change occurs | — |

Plasticity Index: PI = LL - PL

USCS Classification

Coarse-grained soils (>50% retained on #200 sieve):

• GW, GP, GM, GC (gravels)
• SW, SP, SM, SC (sands)

Fine-grained soils (>50% passing #200 sieve):

• ML, MH (silts)
• CL, CH (clays)
• OL, OH (organic)
• Pt (peat)

Compaction

Compaction removes air voids, increasing unit weight and strength.

Standard Proctor (ASTM D698): 12,400 ft-lb/ft³ energy Modified Proctor (ASTM D1557): 56,000 ft-lb/ft³ energy

Peak dry unit weight occurs at optimum moisture content (OMC).

Field compaction spec: achieve ≥95% of Modified Proctor max dry unit weight.

Summary

Soil classification and phase relationships are prerequisites for settlement, bearing capacity, and slope stability analyses. Know the USCS chart and Atterberg limits cold.