Soil Tests: Soil Classification & Properties

Question 1

What are the primary soil classifications based on grain size?

Answer 1

Boulders > 300 mm
Rocks > 76 mm
Gravel 76 mm – 4.75 mm
Sand 4.75 mm – 0.074 mm
Silt 0.074 mm – 0.002 mm
Clay < 0.002 mm

Question 2

What is the Unified Soil Classification System (USCS)?

Answer 2

A standardized system that classifies soils based on grain size distribution, Atterberg limits, and organic content.

Question 3

What is the Plasticity Index (PI)?

Answer 3

PI = Liquid Limit (LL) - Plastic Limit (PL); determines the plasticity and cohesion of fine-grained soils.

Question 4

What is the difference between clay and silt in soil classification?

Answer 4

Clay has smaller particles (< 0.002 mm) and high plasticity, while silt is slightly larger (0.074 – 0.002 mm) with lower plasticity.

Question 5

How is the organic content of soil determined?

Answer 5

By burning a soil sample at 550°C and measuring mass loss.

Question 6

What are the different soil colors and their chemical significance?

Answer 6

Black → High organic content
Red/Brown → Iron oxidation
Yellow → Sulfates
Green → Glauconite
White → Lime, gypsum