Field Test for Soil before Testing in a Laboratory

Understanding Soil Quality: A Guide for Engineers and Builders

Soil forms the very foundation of any structure, making it essential for engineers to ensure its quality before construction begins. While laboratory tests provide precise insights, a few simple field tests can help identify the basic characteristics of soil. Here's a practical guide to understanding and testing soil quality.

Step 1: Sieving the Soil

Before diving into detailed testing, soil is first sieved to separate its different particle sizes. In the U.S., standard sieve sizes include:

3 inch
1/2 inch
3/4 inch
0.186 inch (No. 4)
0.078 inch (No. 10)
0.017 inch (No. 40)
0.006 inch (No. 100)
0.003 inch (No. 200)

The classification of soil based on size is as follows:

Cobbles: > 3 inch
Coarse Gravel: Between 3 inch and 3/4 inch
Fine Gravel: Between 3/4 inch and No. 4
Coarse Sand: Between No. 4 and No. 10
Medium Sand: Between No. 10 and No. 40
Fine Sand: Between No. 40 and No. 200
Silt & Clay: < No. 200

Field Tests to Identify Soil Quality

Here are some simple, yet effective, field tests that help assess soil characteristics:

1. Soil Coarseness Test

This test identifies whether a soil is coarse-grained or fine-grained.

Remove all particles larger than 3 inches.
Weigh the remaining soil passing through a No. 200 sieve.
If over 50% of the soil is retained on the sieve, it is coarse-grained.
Coarse-grained soil is then passed through a 1/4-inch sieve:

50% retained → Gravel

<50% retained → Sand

2. Dry Strength Test

This test evaluates the soil’s plasticity.

Moisten a sample and form a 1-inch ball.
Roll it in your palm until dry, then squeeze between thumb and forefinger.

Doesn’t break: Highly plastic (clay-like)
Breaks with difficulty: Medium plasticity
Breaks easily: Low plasticity

3. Toughness Test

This test measures the soil’s consistency near its plastic limit.

Remove particles larger than No. 40.
Mold the soil into a 12.5 mm cube with a putty-like consistency.
Roll the soil into a thread ~1/8 inch in diameter, fold, and reroll until it crumbles.
Re-knead and repeat:

Tough thread & stiff lumps: High plasticity clay
Crumbles easily: Low plasticity or contains inorganic clay

4. Shaking Test

This test observes how soil reacts to mechanical stress.

Mix soil passing No. 40 sieve with water to form a 3/4-inch ball.
Place the ball between joined palms and shake horizontally.
Strike the ball gently against your other palm:

Ball becomes glossy → stiffens → cracks → crumbles
Clay: Won’t crack
Inorganic silt: Moderate reaction
Very fine sand: Reacts quickly

Conclusion

Understanding soil quality is crucial for safe and lasting construction. These simple field tests—combined with laboratory analysis—help engineers make informed decisions about foundation design and material suitability. By taking the time to analyze soil carefully, we build structures that are not just strong, but enduring.