Soil Classification Systems Overview

Study Notes

Soil Classification Systems Overview

Soil classification is essential for engineering purposes, focusing on particle size distribution and Atterberg limits.
Major systems include USDA, AASHTO, and USCS, each evaluating soil characteristics impacting engineering behavior.

USDA Soil Classification System

Primarily utilized in agricultural documents and soil survey reports.
Classifies soils based on textural analysis, using specific size categories:
- Coarse fragments: > 2.0 mm
- Sand: 0.05 - 2.0 mm
- Silt: 0.002 - 0.05 mm
- Clay: < 0.002 mm
Classification is determined by calculating weight percentages of sand, silt, and clay and employing a triangular diagram for visual classification.

Naming Sediments

Channery: Thin, flat fragments up to 6 inches of sandstone, limestone, or schist.
Cherty: Angular fragments < 3 inches, primarily chert.
Cobbly: Rounded fragments ranging from 3 to 10 inches.
Flaggy: Thin fragments 6 to 15 inches long of various rock types.
Gravelly: Fragments < 3 inches, lacking significant flattening.
Shaly: Flattened shale fragments < 6 inches.
Slaty: Slate fragments < 6 inches.
Stony: Rounded rock fragments > 10 inches or flat fragments > 15 inches.

AASHTO Soil Classification System

Classifies soil based on suitability for road subgrade.
Group classification (A1 - A8) and Group Index (GI 0 - 20) used; A1 & GI=0 is excellent, A8 & GI≥20 is very poor.
Classification involves using soil passing through a 3-inch sieve, following criteria to assign group classification.

Group Index (GI)

Formula: GI = (F-35)[0.2 + 0.005(Wl - 40)] + 0.01(F-15)(Ip-10), where:
- F = fines content (% passing #200 sieve)
- Wl = liquid limit
- Ip = plasticity index
For A-2-6 or A-2-7 soils, only the second term in the equation is considered.
Group classification reported followed by GI, e.g., A-4(20).

Unified Soil Classification System (USCS)

Widely adopted among geotechnical engineers for soil classification.
Initial identification of highly organic soil, characterized by dark color, organic odor, and soft consistency, results in classification as peat (Group symbol: Pt).
Soil is classified based on the gradation limit (using #3-inch, #4, and #200 sieves) and whether it is fine-grained (≥ 50 % passes #200) or coarse-grained.
Includes Atterberg limits testing for fine-grained soils; results plotted against a plasticity chart to distinguish between silts, clays, and non-plastic soils.

Description

This quiz provides an overview of various soil classification systems including USDA, AASHTO, and USCS. Focused on particle size distribution and Atterberg limits, it explores how these classifications impact engineering practices. Test your knowledge on soil types and their characteristics.