Soil Classification Methods

Overview

This lecture outlines the laboratory methods for classifying soils for engineering purposes using liquid limit, plasticity index, and particle-size characteristics under the TxDOT Tex-142-E procedure.

Soil Classification System

• Soils are classified based on liquid limit (LL), plasticity index (PI), and particle-size distribution.
• The main groups are coarse-grained soils, fine-grained soils, and highly organic soils.
• Classification involves visual observations and laboratory tests to assign group symbols and names.
• Supporting properties include unconfined compressive strength, dry unit weight, and water content.

Key Definitions & Measurement

• Plasticity chart differentiates plasticity and organic content using LL and PI.
• Coarse-grained: >50% retained on No. 200 sieve; Fine-grained: ≥50% passes No. 200 sieve.
• Use test methods: Tex-110-E, Tex-104-E, Tex-105-E, Tex-106-E for measurements.

Group Symbol Assignment Criteria

• Gravels and sands classified based on %fines, grading (well or poorly), and type of fines (clay, silt, organic).
• Fine-grained soils identified by PI and position relative to the "A-line" on the plasticity chart.
• Organic soils assessed by LL reduction after oven drying.
• Soil group names modified by predominant sand or gravel content.

Laboratory Procedures

• Determine LL, PL, PI via Tex-104-E, Tex-105-E, Tex-106-E.
• Measure water content using Tex-103-E.
• Determine % passing No. 200 sieve per Tex-110-E (fine) or Tex-401-A (coarse).
• Perform decantation test (Tex-406-A) for retained material grading.
• Assign soil classification and group symbol using results and Figure 1 (plasticity chart).

Soil Consistency & Density

• Consistency for clay/silt is defined by unconfined compressive strength (UC): very soft to hard.
• Soil density (for granular materials) is based on SPT N-value or TCP blows: very loose to very dense.

Dry Unit Weight

• Dry unit weight increases as plasticity decreases or coarse particles increase.
• Organic soils have higher water content and lower dry unit weight.
• Clays retain more water and show lower dry unit weight compared to sands or gravels.

Key Terms & Definitions

• Liquid Limit (LL) — Water content where soil changes from plastic to liquid state.
• Plasticity Index (PI) — Difference between liquid and plastic limits; measures soil plasticity.
• A-Line — Line on plasticity chart: PI = 0.73(LL-20).
• Coefficient of Curvature (Cc) — (D30)²/(D10·D60), describes particle-size distribution.
• Coefficient of Uniformity (Cu) — D60/D10, measures range of particle sizes.
• Unconfined Compressive Strength (UC) — Strength of clay/silt when unconfined.

Action Items / Next Steps

• Review test methods Tex-103-E, Tex-104-E, Tex-105-E, Tex-106-E, Tex-110-E, Tex-401-A, and Tex-406-A.
• Complete soil classification lab by applying procedure and filling out group names using laboratory data.