# Soil Mechanics Questions and Answers – Drainage and Dewatering – Vacuum and Electro-osmosis Method

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This set of Soil Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Drainage and Dewatering – Vacuum and Electro-osmosis Method”.

1. For successful dewatering in fine, non-cohesive soils, _________ method is used.
a) ditches and sumps
b) electro-osmosis method
c) well point system
d) vacuum method

Explanation: For fine grained soils that are non-cohesive like silty sand, the well point system can be extended by vacuum method. The electro-osmosis method is used for dewatering of fine cohesive soils.

2. For the use of vacuum method, the D10 should be __________
a) 1mm
b) more than 1mm
c) more than 0.5mm
d) less than 0.05mm

Explanation: For successful dewatering, the average effective particles D10 should be smaller than 0.05mm. The vacuum method is applicable for fine, non-cohesive soils like silty sand whose D10 is less than 0.05mm.

3. For the use of vacuum method, the coefficient of permeability should be __________
a) greater than 10-3cm/s
b) in the range 10-3 to 10-5cm/s
c) less than 10-5cm/s
d) independent

Explanation: For successful dewatering, the coefficient of permeability should be in the range 10-3 to 10-5cm/s where the vacuum method should be applicable. For coefficient of permeability greater than 10-3cm/s, ditches and sumps are used.
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4. The suction head ____________ capillary head in vacuum method of dewatering.
a) should be greater than
b) should be less than
c) should be equal to
d) does not depend on

Explanation: In vacuum method, it is necessary to apply a suction head in excess of the capillary head to the dewatering system. This is important so as to facilitate the creation of vacuum in the sand filling.

5. In vacuum method, the top of bore hole is sealed by __________
a) soil cement
b) bentonite
c) clay
d) soil cement, bentonite and clay

Explanation: The coarse sand is shovelled into the hole to fill upto about 0.75m to 1m from the top. The top portion of the bore hole is sealed up by tamping soil cement, bentonite or clay.

6. In vacuum method, the ground surface is subjected to unbalanced atmospheric pressure.
a) True
b) False

Explanation: Vacuum pumps are used to create a vacuum in the sand filling. When the vacuum is drawn in the well point, the ground surface is subjected to unbalanced atmospheric pressure.

7. The fine grained cohesive soils can be stabilised by ______
a) well point system
b) ditches and sumps
c) vacuum method
d) electro-osmosis method

Explanation: The vacuum method is suitable for dewatering of fine non cohesive soils, but the electro-osmosis method is suitable for fine grained cohesive soils. The ditches and sumps are used for coarse grained soils.

8. The dewatering of soil by electro-osmosis was developed by ______
a) Darcy
b) Khosla
c) Poiseuille
d) Casagrande

Explanation: The application of electro-osmosis to dewater the soil was largely developed by Casagrande(1952). Darcy gave the law of flow through the soils, i.e. permeability of the soil.

9. The electro-osmosis method of dewatering increases ______
a) consolidation
b) shear strength
c) both consolidation and shear strength
d) water at anode

Explanation: The principle of electro-osmosis is explained by electric double layer on fine grained particles. The arrangement of the electrodes is such that the natural direction of flow of water is reversed away from the excavation, thereby increasing the strength of the soil.

10. The velocity of flow towards cathode in electro-osmosis method is given by _________
a) ve=ke+ie
b) ve=ke/ie
c) ve=ke-ie
d) ve=ke+ie

Explanation: The velocity of flow towards cathode can be expressed as,
ve=ke-ie
where, ke=electro-osmosis coefficient

11. In vacuum method, the vacuum drawn on the well-point stiffened the sub-soil to carry out excavations.
a) True
b) False

Explanation: When the vacuum is drawn on the well-point, the ground surface is subjected to unbalanced atmospheric pressure consolidating the sub-soil which becomes stiff enough for carrying out excavations.

12. In electro-osmosis method, when direct current is applied to the electrodes, the soil water will _________
a) travel from positive electrode to negative electrode
b) travel from negative electrode to positive electrode
c) move towards the centre between the electrodes
d) not be affected by the current

Explanation: When direct current is applied to the electrodes, the soil water will travel from positive electrode to negative electrode. The cathode is made in the form of a well point from where seeping water can be pumped out.

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