Phase relationship definition: The ratio of the volume of water present in a soil mass to the total volume of its voids is called:

Introduction / Context:
Phase relationships define how water, air, and solids occupy space in soils. Correct terminology is essential for interpreting compaction, seepage, and consolidation behavior.

Given Data / Assumptions:

• Vw = volume of water, Va = volume of air, Vv = Vw + Va = total void volume.
• V = total volume of soil; Vs = volume of solids.

Concept / Approach:
Degree of saturation S_r is defined as Vw / Vv. Porosity n is Vv / V; void ratio e is Vv / Vs; percentage of voids expresses n as a percent. Air content a_c is Va / Vv. Only S_r uses water volume over void volume as requested in the stem.

Step-by-Step Solution:

Verification / Alternative check:
At full saturation, S_r = 1.0; when the soil is dry, S_r = 0, matching intuitive end states.

Why Other Options Are Wrong:

• Porosity (Vv/V) and void ratio (Vv/Vs) have different denominators.
• Percentage of voids is n expressed in percent, not Vw/Vv.
• Air content is Va/Vv, not Vw/Vv.

Common Pitfalls:
Confusing S_r with moisture content w (a mass-based ratio, Mw/Ms).

Final Answer:
Degree of saturation