This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Canal Lining – Requirements”.
1. What is the safe limiting velocity for cement concrete lining?
a) 1.5 m/s
b) 2.2 m/s
c) 2.7 m/s
d) 1.8 m/s
Explanation: For cement concrete lining, the safe limiting velocity is 2.7 m/s. For boulder lining, it is 1.5 m/s and for burnt clay tile lining it is approximately 1.8 m/s. This serves as a guide for selecting canal slopes and alignments.
2. The most economical type of lining is the one which shows ____________
a) minimum benefit-cost ratio
b) maximum benefit-cost ratio
c) zero benefit-cost ratio
d) benefit-cost ratio = 1
Explanation: The one which shows the maximum annual benefit-cost ratio is the most economical lining. It may have a higher cost initially but longer life than another type of lining having a lesser value of the annual benefit-cost ratio.
3. Which of the following guideline is not recommended for the choice of lining when the bed width of the canal is up to 3 m?
a) Single burnt clay tile lining or brick lining
b) PCC slab lining
c) Flexible membrane lining with adequate earth/tile cover
d) In-situ cement concrete lining in bed as well as on sides
Explanation: The choice of suitable lining for different size of a canal depends on canal slopes and alignments, size and importance of canal, the climate of the area, initial expenditure, etc. In-situ cement concrete lining in bed and on sides is generally preferred for canals when bed width is greater than 8 m.
4. Which of the following is not a cause for the hydrostatic pressure on the lining?
a) Seeping of the rainwater in the backfill
b) When the water table remains below the canal bed
c) The backfill is of low drainage
d) The backfill is of high permeability (i.e. > 3 cm/sec)
Explanation: If the subgrade is made of clear gravel or sand of good permeability (> 3 cm/sec) and the water table is not likely to go above the canal bed no drainage arrangements may be necessary. There will be no appreciable time lag in the dissipation of drawdown pore pressures in the subgrade for such case.
5. Drainage relief pockets are provided ___________
a) at isolated locations in the bed as well as sides of the lined canal
b) in the bed of the canal
c) in the sides of the canal
d) anywhere in the canal
Explanation: Drainage relief pockets are provided with pressure relief valves filled with a graded filter containing gravel, coarse sand, and fine sand. They are provided at isolated locations in the bed as well as on the sides below the lining at suitable spacing (@ 15 to 20 m intervals).
6. The permeability of lining is also governed by the depth of water in the canal and type of subgrade soil.
Explanation: The permeability of lining may decide the quantum of seepage loss from a canal. The seepage losses from a canal for a particular area will depend on the local conditions such as the values of land and water, population intensity, etc.
7. Pressure relief valves may help in ___________
a) releasing the hydrostatic pressure
b) holding the hydrostatic pressure
c) increasing the hydrostatic pressure
d) may increase or decrease the hydrostatic pressure
Explanation: Pressure relief valve is a flap valve opening upwards into the canal. It helps in releasing the hydrostatic pressure as soon as the differential head exceeds the safe pressure. These valves open out as soon as the differential pressure goes beyond 10 cm.
8. Pipe drains run _______________
a) longitudinally on the bed and transverse to the length of canal on the side slopes
b) longitudinally to the length of the canal on the side slopes and transverse to the bed
c) longitudinally to the length of the canal
d) transverse to the bed
Explanation: Provision of open-jointed pipe drains is one of the methods of drainage arrangements. It will run longitudinally on a trench excavated below the lining on the canal bed along the length of the canal and transverse to the length of the canal on the side slopes.
9. Which of the following statement is wrong about the requirement of good lining?
a) The lining should be able to withstand the differential sub-soil pressure
b) Brick lining, concrete lining or precast slab lining can be easily repaired as compared to cast-in-situ concrete lining
c) The hydraulic efficiency generally reduces with time
d) Brick tile lining may provide better abrasion resistance than cement concrete and boulder lining
Explanation: A canal may have to transport a considerable amount of sediment load which damages the lining by abrasion. Cement concrete and stone boulder linings provide better abrasion resistance as compared to the brick tile lining.
10. Which type of lining is adopted when the channels have become stable and no danger of scouring is expected?
a) Brick lining
b) Single burnt clay tile lining
c) In-situ cement concrete lining
d) Flexible membrane lining in the bed and rigid lining on the sides
Explanation: The burnt clay tile lining is to be adopted at places where aggregates for the manufacture of concrete are not available economically. The brick lining is used where seepage considerations are important. In-situ cement concrete lining is provided when the canal has a bed width greater than 8 m.
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