Terminology

• Plasticity: ability to deform without cracking when wet; measured by Atterberg limits
• Smectite / bentonite: swelling clay minerals
• Kaolinite: low-swell clay mineral
• Expansive soil: clay-rich soil that swells and shrinks with moisture
• Quick clay: sensitive marine clay prone to liquefaction and landslides
• CL / CH: USCS clay groups with liquid limit below 50 / 50 or more
• Loam: sand, silt, and less than 40 percent clay

Grades and types

• Particle size: finer than 2 μm (0.002 mm) under ISO 14688; sedimentologists use 4 to 5 μm, colloid chemists 1 μm
• Engineers: separate silt from clay by plasticity (Atterberg limits) rather than size

Properties

Thermal and moisture

• Kaolinite plastic limit: roughly 36 to 40 percent water; liquid limit roughly 58 to 72 percent
• Swelling: smectite clays absorb large amounts of water, expand substantially, and shrink back when dried
• Permeability: relatively impermeable

Mechanical

• Stability in excavation: dry clay is often more stable than sand

Standards

Test methods

• ASTM D2487: soil classification
• ASTM D4318: Atterberg limits and plasticity index
• ASTM D4829: expansion index
• ASTM D6913: particle-size distribution

Uses in buildings

Building elements

• Spread and Continuous Footings, Slab on Grade: expansive clay drives footing depth, over-excavation, and post-tensioned slabs
• Adobe Walls: clay is the binder in adobe block

Pros and cons

Pros

• Low permeability suits seals and liners
• Dry clay is often stable in excavations
• Raw material for adobe, cob, and brick

Cons

• Swelling clays crack foundations and damage road beds
• Retains water, so clay soils often need drainage
• Quick clay can liquefy

Sources

• Wikipedia: Clay, https://en.wikipedia.org/wiki/Clay
• Wikipedia: Unified Soil Classification System, https://en.wikipedia.org/wiki/Unified_Soil_Classification_System
• Wikimedia Commons: Clay Soil.jpg, photo by Krish Dulal (CC BY-SA 3.0), https://commons.wikimedia.org/wiki/File:Clay_Soil.jpg