Phase system in soil mechanics: A partially saturated soil mass is classified under which idealized phase model?

Introduction / Context:
Soil mechanics models a soil mass as a combination of solids and voids filled with fluids (water and/or air). Correctly identifying the phase system is essential for computing index properties such as degree of saturation, void ratio, and unit weights used in geotechnical design.

Given Data / Assumptions:

• Soil is partially saturated (both water and air are present in voids).
• Solids form the skeleton (minerals and organics).
• Standard geotechnical definitions apply.

Concept / Approach:
A partially saturated soil contains three constituents: solids, water, and air. Therefore, it is a three-phase system. A fully dry soil would be two-phase (solids + air), while a fully saturated soil is also two-phase (solids + water) because air is absent. A “four-phase” concept does not apply in classical soil mechanics phase diagrams.

Step-by-Step Solution:

Verification / Alternative check:
Phase diagram constructions in textbooks show separate blocks for Vs, Vw, and Va for partially saturated states; removing one fluid collapses the system to two phases.

Why Other Options Are Wrong:

• One-phase: would imply a homogeneous single medium, contrary to reality.
• Two-phase: valid only for fully dry (solids + air) or fully saturated (solids + water) soils.
• Four-phase: non-standard in introductory soil mechanics.

Common Pitfalls:
Confusing two-phase saturated soils with three-phase because pores are “full”; misdefining degree of saturation and air voids.

Final Answer:
Three-phase soil