Field Identification of Soils: A Quick Guide for Engineers

 Field Identification of Soils: A Quick Guide for Engineers

Field Identification of Soils

When you're out on site, there's no laboratory—just your senses and experience. Luckily, you can identify soil types fairly accurately in the field using simple tests involving touch, sight, smell, and basic tools like acid bottles or your fingers.

Here’s how to identify soils based on their texture, appearance, and feel:

Basic Soil Types & Field Indicators

Soil Type	Texture	Field Feel (Wet)	Field Feel (Dry)	Other Clues
Gravel	Coarse	Hard, gritty	Same	Particles visible
Sand	Coarse	Gritty	Same	Doesn't stick to fingers
Silt	Fine	Smooth, slippery	Powdery, rough	Water comes up on pressure
Clay	Fine	Sticky, greasy	Very hard & powdery	Water moves very slowly

Soil Color Clues

Soil color is linked to mineral content and environment, though not directly to strength.

Color	Likely Indicator
Gray / Bluish	Unoxidized soils (often waterlogged)
White / Cream	Calcareous soils (lime-rich)
Red / Yellow	Oxidized soils (iron-rich)
Black / Dark Brown	High organic matter

Moisture Condition

Describe the water content simply as:

• Dry – no visible moisture, crumbly

• Moist – damp, darkened, but not sticky

• Wet – water present, sticky or even flowing

 Field Tests for Soil Classification

1. Feel Test

• Sand: Gritty

• Silt: Feels like fine sandpaper

• Clay: Smooth, sticky, greasy

2. Dilatancy (Shake Test)

• Place a small sample in your palm.

• Shake horizontally, then tap with your other hand.

• If water rises quickly: high silt content.

• If no water or very slow: mostly clay.

3. Consistency Test

How hard is it to dent the soil?

Consistency	What You Observe
Very Soft	Soil flows between fingers
Soft	Easily molded by fingers
Firm	Moldable with strong pressure
Stiff	Dents with finger pressure
Very Stiff	Barely dents, may crack

4. Carbonate Test

• Add a drop of dilute hydrochloric acid.

• If it bubbles or effervesces, the soil contains calcium carbonate (like marl or caliche).

5. Smell Test

• Organic soils: strong odor, especially when heated.

• Inorganic soils: subtle or no odor.

Soil Structure & Shape

Aspect	Categories
Structure	Homogeneous or Nonhomogeneous (check color & texture)
Shape	Angular, Subangular, Subrounded, Rounded, Flaky
Weathering	Fresh, Decomposed, Weathered

Packing or Relative Density (For Sands & Gravels)

Description	Observation
Very Loose	Collapses under slight disturbance
Loose	Open structure, collapses upon touch
Medium Dense	Can be indented with firm hand pressure
Dense	Hard to indent even by stomping
Very Dense	Cannot be indented even by heavy stomping

Tip for Field Logs

Always record:

• Soil type & consistency

• Moisture condition

• Color

• Carbonate or organic content (if tested)

• Any visual layers or inclusions (gravel pockets, roots, etc.)

• Depth of change in strata

Field soil identification isn’t about perfection it’s about being observant and methodical. A good field engineer or geotechnical professional learns to read the ground like a book.