A soil has percentage air voids of the order of 30%. It has a porosity of 0.4. The air content of that soil shall be:
Try it first: pick an option, then read the explanation.
Correct answer: A. 0.75
Detailed Explanation
Given: percentage air voids na = 30% = 0.3, porosity n = 0.4.
Percentage air voids relates to porosity and air content through na = n × ac, where ac is the air content (the fraction of the void volume occupied by air, i.e., ac = 1 − S).
ac = na/n = 0.3/0.4 = 0.75
Key Concepts for Students
- na = n × ac: percentage air voids (referenced to total volume) equals porosity times air content (referenced to void volume only).
- Air content ac = 1 − S, so once ac is found here, the degree of saturation follows immediately as S = 1 − 0.75 = 0.25 (25%).
- Do not confuse percentage air voids (na, over total volume) with air content (ac, over void volume only) — they describe the same air but relative to different reference volumes, exactly analogous to how e and n both describe void volume relative to different denominators.
- This relationship (na = n × ac) is the air-side counterpart of S = Vw/Vv and n = Vv/V combined.