Soil three-phase system: select the correct statements about voids in saturated and dry conditions.

Introduction / Context:
The soil mass is modeled as a three-phase system: solids, water, and air. Whether a soil is dry, partially saturated, or fully saturated directly affects unit weight, compressibility, and strength. This question checks basic but essential definitions of saturation states.

Given Data / Assumptions:

• Voids = total volume not occupied by solids.
• Degree of saturation S_r varies from 0 to 100%.
• Idealized states: dry (S_r = 0), saturated (S_r = 100%).

Concept / Approach:

Partially saturated soils contain both air and water within the voids. Fully saturated soils have voids completely filled with water, while dry soils have voids completely filled with air. These are the standard textbook definitions and underpin formulas for bulk unit weight, buoyant unit weight, and relationships among e, n, and S_r.

Step-by-Step Solution:

Verification / Alternative check:

Phase diagrams and standard relationships (e.g., γ_sat, γ_dry) rely on these exact definitions.

Why Other Options Are Wrong:

Since (a), (b), and (c) are all correct, “None” is false.

Common Pitfalls:

Assuming trapped air cannot exist in saturated conditions; in practice, saturation may be slightly less than 100% unless special saturation procedures are used in lab tests.

Final Answer:

All the above