# Soil Mechanics Questions and Answers – Void Ratio, Porosity and Degree of Saturation

This set of Soil Mechanics Questions and Answers focuses on “Void Ratio, Porosity and Degree of Saturation”.

1. What will be the porosity of a soil sample whose void ratio is 0.35?
a) 19.5%
b) 40.65%
c) 25.9%
d) 30%

Explanation: Given,
Void ratio e=0.35
Porosity $$n = \frac{e}{1+e}$$
$$n = \frac{0.35}{1+0.35}$$
$$n = \frac{0.35}{1.35}$$
n=0.259
In percentage n=25.9%.

2. ______ is independent of the total volume of the soil mass.
a) Bulk unit weight
b) Dry unit weight
c) Submerged unit weight
d) Unit weight of solids

Explanation: The unit weight of solids is a function of the volume of solids, which does not change with water or moisture content. Whereas, bulk, dry and submerged unit weights are functions of total volume.

3. Void ratio is the ratio of _____
a) volume of voids to total volume
b) volume of voids to volume of solids
c) volume of solids to volume of voids
d) volume of soil mass to volume of voids

Explanation: The volume of voids to the volume of solids is void ratio. The volume of voids to total volume is porosity. The volume of voids includes the volume of air voids as well as the volume of water present.

4. If the porosity of a soil sample is 22%, then what will be its void ratio?
a) 0.3
b) 0.27
c) 0.28
d) 0.25

Explanation: Given,
Porosity n= 22%= 0.22
Void ratio e = n/(1-n)
e= 0.22/(1-0.22)
e= 0.28

5. For a soil sample, the ratio of saturated unit weight to dry unit weight is equal to 1.2 and specific gravity of solids G is 2.65, then its void ratio will be _____
a) 0.58
b) 0.66
c) 0.53
d) 0.94

Explanation: Given,
The ratio of saturated unit weight to dry unit weight is equal to 1.2
γsatd= 1.2
Specific gravity of solids G is 2.65
γsat = $$\frac{(G+e)}{1+e} \gamma_w \,and\, \gamma_d = \frac{G}{1+e}\gamma_w$$
→ γsatd= $$\frac{(G+e)}{1+e} \gamma_w /\frac{G}{1+e}\gamma_w$$
γsatd = (G+e) / e
→(G+e) / e = 1.2
e = 1.2*G – G
e = 1.2*2.65 – 2.65
e = 0.53.
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6. For a fully saturated soil sample, the degree of saturation is ______
a) 0
b) 0.5
c) 0.8
d) 1

Explanation: For a fully saturated soil sample, the volume of water is equal to the volume of voids and hence the degree of saturation is equal to one.

7. The percentage void ratio is defined as the ratio of _____
a) volume of air voids to total volume of the soil mass
b) volume of air voids to volume of total voids
c) volume of air voids to volume of water
d) volume of voids to total volume of the soil mass

Explanation: The percentage void ratio is defined as the ratio of the volume of air voids to the total volume of the soil mass. It is expressed as percentage. na = (Va/v) * 100.

8. If air content and degree of saturation are represented as ac and S respectively, then the relationship between them is ________
a) ac= 1/S
b) ac=1-S
c) S=1+ac
d) S=ac

Explanation: Air content is the ratio of the volume of air voids (Va) to the volume of voids (Vv). The degree of saturation (S) is the ratio of the volume of water (VW) to the volume of voids (Vv) in a given soil mass.
ac=Va/Vv
S=VW/Vv
Since Va=Vv-VW
ac=1-VW/Vv
ac=1-S.

9. The air content ratio is defined as the ratio of _____
a) volume of air voids to total volume of the soil mass
b) volume of air voids to volume of total voids
c) volume of air voids to volume of water
d) volume of voids to total volume of the soil mass

Explanation: The air content is defined as the ratio of the volume of air voids to the volume of total voids. It is related with the degree of saturation S as ac=1-S. ac= Va/Vv.

10. For a perfectly dry sample, the degree of saturation is equal to zero.
a) True
b) False

Explanation: For a perfectly dry, hence the degree of saturation is equal to zero. Sample the volume of water present is equal to zero. For a fully saturated sample, the degree of saturation is equal to one.

11. The degree of saturation is defined as the ratio of _____
a) volume of water to total volume of voids
b) volume of air voids to volume of total voids
c) volume of air voids to volume of water
d) volume of voids to total volume of the soil mass

Explanation: In a soil sample, some space is occupied by water and the rest by air. The degree of saturation is defined as the ratio of the volume of water to the total volume of voids.

12. The relationship between porosity n and void ratio e is given by _______
a) 1+n= 1/(1+e)
b) 1-n= 1/(1+e)
c) n= 1/e
d) n= 1/(1+e)

Explanation: The porosity n and void ratio e are given by,
e = Vv/Vs = n/(1+n)
n= Vv/V = e/(1+e)
n= e*(1-n)
∴ 1-n=1/(1+e).

13. The range of degree of saturation is between 0 and 0.25 for a humid soil sample.
a) True
b) False

Explanation: The range of degree of saturation is between 0 to 0.25 for humid, 0.25 to 0.5 for damp, 0.5 to 0.75 for moist and 0.75 to 1 for wet soil sample.

14. What will be the degree of saturation of a soil sample whose specific gravity G=2.7, void ratio e=0.66 and water content w=20%?
a) 85.88%
b) 90.91%
c) 81.81%
d) 92.92%

Explanation: Given,
G=2.7
e=0.66
w=20%=0.2
Since the degree of saturation is S=wG/e
S=(0.2*2.7)/0.66
S=0.8181
In percentage S=81.81%.

15. In wet soil mass, air occupies one-seventh of its volume and water occupies one- eighth of its volume. The void ratio of the soil is ______
a) 0.25
b) 0.36
c) 0.43
d) 1

Explanation: Given,
Volume of water Va= (1/7) total volume V
Volume of water VW= (1/8) total volume V
Void ratio e= Vv/VS
Vv= Va + VW
Vv=(1/7)+(1/8)
Vv=(15/56)=0.268
VS= V-Vv
VS=1-0.268=0.732
∴e= 0.268/0.732
e=0.36.

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