Silt

Summary

Silt is granular material between sand and clay, mostly broken quartz grains, roughly 4 to 63 microns, and floury with no plasticity when wet. It loses strength when wetted and can liquefy in earthquakes, so silty soils need geotechnical review before bearing or compaction assumptions are made.

Cols

Image

Loess: wind-deposited silt-rich soil
Siltstone: sedimentary rock made mainly of silt
Quick clay: a mix of very fine silt and clay that is a challenge for civil engineering
Fines: soil passing the No. 200 sieve (0.074 mm)
ML / MH: USCS silt groups with liquid limit below 50 / 50 or more

Composition and production

Raw materials

Mostly broken quartz grains; abundant in river deltas (Nile, Niger, Ganges) and wind-deposited in central Asia, northern China, and North America
Relatively uncommon in the tropics

Grades and types

Geologists: 4 to 63 microns (some use 2 to 63 microns)
ISSS: 0.002 to 0.02 mm; USDA: 0.002 to 0.05 mm
US civil engineers: material passing the No. 200 sieve (0.074 mm or less)
USCS ML: inorganic, liquid limit below 50, plasticity index below 4 and plotting below the A line

Properties

Physical

Texture: floury when dry, gritty between the teeth, lacks plasticity when wet
Settling: coarse silt settles in minutes; the finest silt can take days

Mechanical

Wet strength: open structure can collapse when wet; loses strength when wetted
Liquefaction: susceptible during strong earthquakes because it lacks plasticity

Standards

Test methods

ASTM D2487: soil classification (Unified Soil Classification System)
ASTM D4318: Atterberg limits and plasticity index

Uses in buildings

Building elements

Spread and Continuous Footings, Slab on Grade: weak when wet and prone to liquefaction; needs geotechnical review
Landscaping and Planting: a component of loam and planting soils

Pros and cons

Pros

Quartz grains promote good soil structure for planting
Supplies nutrients in farmland

Cons

Poor mechanical properties make construction on silty soil problematic
Liquefies in strong earthquakes
Collapses when wetted

Sources

Wikipedia: Silt, https://en.wikipedia.org/wiki/Silt
Wikipedia: Unified Soil Classification System, https://en.wikipedia.org/wiki/Unified_Soil_Classification_System
Wikimedia Commons: North Esk River silt, grass 20190425-005.jpg, photo by Gary Houston (CC0), https://commons.wikimedia.org/wiki/File:North_Esk_River_silt,_grass_20190425-005.jpg