This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Compressibility of Aquifers”.

1. The specific storage of a confined aquifer represents the amount of water released by which of the following?

a) Per unit volume of aquifer per unit decrease in piezometric head

b) Per unit surface area of aquifer per unit decrease in piezometric head

c) Per unit depth of aquifer per unit decrease in piezometric head

d) Specific storage does not exist for confined aquifers

View Answer

Explanation: The specific storage of an aquifer represents the amount of stored water released from a unit volume of an aquifer per unit change in piezometric head. The aquifers tend to release water when the piezometric head drops.

2. What is the dimensions of specific storage?

a) [L^{-1}]

b) [L]

c) [L^{2}]

d) [L^{3}]

View Answer

Explanation: The specific storage is essentially computed as the product of the unit weight of water and the compressibility of the aquifer. So,

Specific storage=Unit weight*Compressibility=\(\frac{N}{m^3} *\frac{m^2}{N}=m^{-1}\).

⇒Dimension=[L

^{-1}].

3. One of the assumptions made for a compressible aquifer is that the volume of an element is constrained in all directions.

a) True

b) False

View Answer

Explanation: An assumption for a compressible aquifer is that any elemental volume considered should be constrained in its lateral directions, i.e. surface area and should be allowed to undergo change in the vertical direction.

4. Which of the following are the correct assumptions with regards to a compressible aquifer?

a) Both the pores and solid grains are compressible

b) Pores are incompressible but solid grains are compressible

c) Both pores and solid grains are incompressible

d) Pores are compressible but solid grains are incompressible

View Answer

Explanation: The two assumptions for a compressible aquifers with respect to compressibility of its features are that the solid grains of the formation are incompressible, whereas the pore structure and water is compressible

5. The storage coefficient of an aquifer has units similar to which of the following quantities?

a) Specific storage

b) Permeability

c) Length

d) Unitless

View Answer

Explanation: Storage coefficient represents the amount of water released by a unit surface area of a confined aquifer per unit decrease in piezometric head. It is found by multiplying the specific storage by the thickness of the aquifer.

6. Which of the following are known as the formation constants of an aquifer?

a) Specific storage and transmissibility

b) Transmissibility and storativity

c) Storativity and permeability

d) Permeability and specific storage

View Answer

Explanation: The storage coefficient of an aquifer is also known as the storativity. It along with the transmissibility are called the formation constants of an aquifer. Together they play an important part in the unsteady flow of water through the porous medium.

7. What is the general order of specific storage values for an aquifer made of stiff clay?

a) 0.001 m^{-1}

b) 0.1 m^{-1}

c) 1 m^{-1}

d) 10 m^{-1}

View Answer

Explanation: The compressibility of stiff clay is of the order of 10

^{-7}to 10

^{-8}m

^{2}/N. The compressibility of water is of the order of 10

^{10}m

^{2}/N. Taking unit weight of water as 10000 N/m

^{3}, porosity as 0.5 and using above values,

Specific storage= γ(nβ+α)=10000((0.5*10

^{-10})+10

^{-7})=1*10

^{-3}m

^{-1}

or =10000((0.5*10

^{-10})+10

^{-8})=1*10

^{-4}m

^{-1}

Therefore, the order of specific storage is 10

^{-3}to 10

^{-4}m

^{-1}or 0.0001 to 0.001 m

^{-1}.

8. Find the value of the ratio of specific storge to storativity of a sandy aquifer of depth 9 m. Take unit weight of water as 9.81 kN/m^{3}.

a) 0.056

b) 0.083

c) 0.11

d) Data insufficient

View Answer

Explanation: Given B = 9 m. Now,

Ratio=\(\frac{Specific storage}{Storativity}=\frac{γ(nβ+α)}{γ(nβ+α).B}=\frac{1}{B}=\frac{1}{9}\)=0.111 m

^{-1}

9. Find the specific storage of an aquifer of material with compressibility 0.01 cm^{2}/N and porosity of 55%. Take compressibility of water as 4.5×10^{-6} cm^{2}/N and unit weight of water as 9810 N/m^{3}.

a) 9.81 x 10^{-3} cm^{-1}

b) 9.81 x 10^{-4} cm^{-1}

c) 9.81 x 10^{-5} cm^{-1}

d) 9.81 x 10^{-6} cm^{-1}

View Answer

Explanation: Given n = 0.55, α = 0.01 cm

^{2}/N, β = 4.5×10

^{-6}cm

^{2}/N, γ = 9810 N/m

^{3}.

Specific storage= γ(nβ+α)=9810*((0.55*4.5*10

^{-6})+0.01)

=9810*((2.475*10

^{-6})+0.01)

≅98.1 N/m

^{3}.(cm

^{2})/N=9.81*10

^{-3}m

^{-1}=9.81*10

^{-5}cm

^{-1}

10. An aquifer at a depth of 15 m below the ground is made of fractured rock of compressibility 3.5×10^{-5} cm^{2}/N and porosity 11%. If the bulk modulus of water is 2.1 GPa, what is the specific storage of the formation?

a) 3.5 x 10^{-6} m^{-1}

b) 3.5 x 10^{-5} m^{-1}

c) 3.5 x 10^{-4} m^{-1}

d) 3.5 x 10^{-3} m^{-1}

View Answer

Explanation: Given n = 0.11, α = 3.5×10

^{-5}cm

^{2}/N = 3.5×10

^{-9}m

^{2}/N, K = 2.1 GPa = 2.1×10

^{9}N/m

^{2}.

⇒Compressibility of water=β=\(\frac{1}{K}=\frac{1}{2.1*10^9}\)=4.76*10

^{-10}m

^{2}/N

Specific storage= γ(nβ+α)=9810*((0.11*4.76*10

^{-10})+(3.5*10

^{-9}))

=9810*((5.236*10

^{-11})+(3.5*10

^{-9}))=9810*3.55236*10

^{-9}

=3.485*10

^{-5}m

^{-1}≅3.5*10

^{-5}m

^{-1}

**Sanfoundry Global Education & Learning Series – Engineering Hydrology.**

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