Laboratory Classification of Soils for Engineering Purposes

TxDOT Designation: Tex-142-E

• Effective Date: August 1999
• Last Reviewed: September 2014

1. Scope

• Purpose: System for classifying both disturbed and undisturbed soils.
• Based on:
  • Liquid limit
  • Plasticity index
  • Particle-size characteristics
• Comparison: Similar to ASTM Unified Soils Classification System (USCS).
• Major Soil Groups:
  • Coarse-grained soils
  • Fine-grained soils
  • Highly organic soils
• Classification: Based on visual observations and laboratory tests.
• Additional Parameters:
  • Unconfined compressive strength
  • Dry unit weight
  • Water content
• Measurement Units: Be mindful of using either system separately to avoid nonconformance.

2. Definitions

• Plasticity Chart: Differentiates plasticity and organic characteristics.
• A-Line: A sloped line on the plasticity chart.
• Clay: Fine-grained soil with plasticity and strength when dry.
• Silt: Non-plastic soil with little strength when dry.
• Organic Clay/Silt: Defined by significant changes in liquid limit after drying.
• Peat: Composed of decomposed vegetable tissue.
• Gravel and Sand: Defined based on particle sizes.
• Coefficients (Cc and Cu): Ratios used in particle-size distribution.

3. Preparing Soil for Classification

• Required Tests: Particle-size distribution and soil constants using specific Tex methods.
• Group Symbol Assignment: Based on laboratory test results.
• Criteria for Classification:
  • Coarse-Grained Soil: More than 50% retained on No. 200 sieve.
  • Fine-Grained Soil: 50% or more passes the No. 200 sieve.
  • Tables provided for detailed classification criteria based on group symbols.

Soil Classification Tables

• Gravels: Classification based on fines and coefficients.
• Sands: Similar classification criteria as gravels.
• Silts and Clays: Based on liquid limits and presence of fines.

4. Procedure

• Tests Required:
  • Liquid limit, plastic limit, and plasticity index using Tex methods.
  • Water content, particle size distribution for classification.
• Use of Figure 1: For classification with group name and symbol.

5. Consistency

• Defined by: Unconfined compressive (UC) strength.
• Table 5: Classifies soil by UC strength into categories like very soft and hard.

6. Density

• Measurement: Using SPT or TCP tests.
• Table 6: Classifies soil by density with specific values.

7. Dry Unit Weight

• Relation to Soil Type:
  • Increases with decreasing plasticity
  • Increases with more coarse-grained particles
• Table 7: Provides dry unit weights for different soil types.

This classification system helps in determining the engineering properties and suitability of soil for construction purposes. It provides a comprehensive method to categorize soil types for practical engineering applications.