Soil properties describe how heavy, loose, or plastic soil is—needed before foundations and earthworks can be designed.

Scope (soil properties)

This page covers index properties and classification of soils: grain-size distribution, Atterberg limits, density and unit weight, and naming systems such as USCS (with local practice for South African projects—often aligned with SANS 6341 soil classification for civil engineering and test methods in the SANS 3001 series). Laboratory and field procedures must match the project specification.

From index data to design parameters

  1. Measure or estimate Gs, w, and e (or derive e from density) from lab or field tests.
  2. Classify the soil (e.g. gravel/sand/silt/clay) and note plasticity (LL, PL, PI) for fine-grained soils.
  3. Map classification to strength and compressibility ranges (triaxial, oedometer, CPT correlations) per your geotechnical model.
  4. Use the calculator below for a quick unit weight check; use geotechnical software for databases, logs, and spatial models.

Geotechnical Engineering — Soil Properties & Classification

Characterize soils by grain size, plasticity, density, and unit weight for design of foundations and earthworks.

Introduction

Soil classification groups soils by particle size, plasticity, and organic content so engineers can communicate behaviour and select tests. Index properties (water content, void ratio, Atterberg limits, relative density in coarse soils) support estimates of shear strength, stiffness, permeability, and settlement.

Classification systems (USCS, AASHTO, local road or rail codes) use the same basic tests but different boundary rules—always follow the governing project standard.

Soil texture triangle with sand, silt, and clay fractions
Texture triangle (sand–silt–clay) for agricultural/engineering texture classes. See licence on Wikimedia Commons
Diagram of Atterberg limits and consistency states
Atterberg limits (liquid, plastic, shrinkage) and consistency states for fine-grained soils. See licence on Wikimedia Commons

What soil characterization produces

  • Named soil classes and layer descriptions for logs and drawings.
  • Index properties (w, e, ρ, Gs, LL, PL, PI, Dr, …) for correlations and design.
  • Basis for selecting strength and compressibility parameters and groundwater assumptions.
  • Quality trail for testing (SANS 3001 or equivalent) and sampling method.

Standards (South Africa)

SANS 6341 provides soil classification for civil engineering purposes; SANS 3001 series covers geotechnical test methods (sampling, classification tests, strength, consolidation). Align reporting with the project’s geotechnical investigation report and client requirements.

Phase relationships:   γd = Gs γw / (1+e) ,   γ = γd(1+w)   (w as decimal fraction in some texts—here the calculator uses w in %).

Notation (common)

  • Gs — specific gravity of solids.
  • w — water content (% in the calculator below).
  • e — void ratio.
  • γd, γ — dry and moist bulk unit weight (kN/m³).
  • LL, PL, PI — liquid limit, plastic limit, plasticity index.

Examples in practice

  • Foundation design: classify bearing layers and select drained vs undrained parameters.
  • Earthworks: moisture–density relationship and compaction specification.
  • Pavement: subgrade classification and CBR correlation.
  • Excavation: soil type for support system and dewatering.

Calculator — unit weight from void ratio

Saturated-phase relationships: γw = 9.81 kN/m³. Water content w is entered as a percentage to match the calculator implementation.

γd = Gs γw / (1+e) ;   γ = γd(1+w/100)

Unit weight

Moist bulk unit weight γ from Gs, void ratio e, and water content w (%).

Key terms

Void ratio (e)
Volume of voids to volume of solids.
Specific gravity (Gs)
Ratio of soil particle density to water density.

Software and geotechnical data

Offices use database, logging, and analysis tools for boreholes, lab results, and ground models. Examples (official sites):

  • gINT — borehole logs and geotechnical data management.
  • PLAXIS — geotechnical finite-element analysis.
  • GeoStudio — seepage, slope stability, consolidation.
  • Autodesk Civil 3D — civil/geotechnical workflows with industry extensions.
  • Rocscience — slope and tunnel tools.

No product endorsement—select tools to match your office’s data standards, QA, and deliverable format. Index properties must come from approved testing, not from this calculator alone.

Diagram sources

Educational schematics. Files in Images/soil-properties-classification/ were downloaded from Wikimedia Commons into this repo (not copied from other topic folders). Confirm licence on each Commons file page before reuse.