This set of Irrigation Engineering Objective Questions & Answers focuses on “Lining of Canals in Expansive Soils”.
1. Which of the following statement is incorrect about Expansive soils?
a) Expansive soils exhibit shrinkage and swelling properties
b) They have high compressibility and liquid limits with more than 50
c) Black cotton soil is the most common expansive soil in India
d) The lesser the swelling pressure, the more troublesome is the soil
Explanation: The expansive soils are clayey soils having clay mineral montmorillonite and chief property of such a soil is its swelling pressure. The soil is more troublesome and problematic if it has high swelling pressure.
2. The larger the thickness of CNS introduced over the expansive soil, the more will be the resulting swelling in soil.
Explanation: A layer of some cohesive non-swelling soil (CNS) is added over the expansive soil surface to counteract the swelling pressure. The larger the thickness of CNS, the lesser would be the resulting swelling and deformations.
3. What is the recommended value of CNS thickness to be used for lining over expansive soil with swelling pressure of 200-300 KN/m2?
a) 75 to 85 cm
b) 90 to 100 cm
c) 105 to 125 cm
d) > 125 cm
Explanation: For swelling pressure ranging from 50-150 KN/m2, the value of CNS thickness varies from 75 to 85 cm and for swelling pressure 200-300 KN/m2, the value is 90 to 100 cm. The CNS thickness is more for more expansive soil and less for less expansive soil.
4. Safety ladders are generally provided on ___________
a) side slopes
b) bed slopes
c) anywhere on the canal
d) both side slopes and bed slopes
Explanation: In large canals, it is generally provided on side slopes along the canal length at suitable intervals. The ladders are provided on both the banks in straight reaches at about 300 m staggering distance.
5. If the free swell index of soil is more than 100, the soil may be expansive.
Explanation: Free swell index is the increase in the volume of soil when subjected to submergence in water without any external constraint. If the value is less than 100, the soil is treated as non-expansive.
6. Safety ladders are provided in large irrigation canals to ______________
a) enable the fish to pass from one place to another
b) enable the cattle to cross the canal
c) enable the swimmers to get out of the canal
d) provide safe exit to avoid accidental drowning
Explanation: Safety ladders are generally provided in large canals on both the banks which consists of a number of ladder rungs at different elevations along the canal length. In case any person or cattle that may be swept away with the flowing water may catch hold one such ladder rung and can climb the higher rungs to get out of the canal to prevent themselves from drowning.
7. CNS material shall be non-swelling with a minimum of ___________
a) 10 KN/m2 of cohesion
b) 15 KN/m2 of cohesion
c) 20 KN/m2 of cohesion
d) 30 KN/m2 of cohesion
Explanation: Whenever CNS soil layer is used, it is desired to compact it to the maximum dry density of the Standard Proctor Compaction Test at OMC. When tested accordingly the minimum cohesion strength on saturated compacted soil remoulded at OMC should be 10 KN/m2.
8. Which of the following is incorrect about the identification of CNS material?
a) The CNS material can be identified by using A-line chart
b) The permissible deformation of the CNS layer is 20 mm
c) The swelling pressure should not exceed 10 KN/m2
d) The soil with liquid limit less than 30% and plastic index less than 15% is considered suitable
Explanation: The A-Line chart is used for general identification of CNS material and the only data required is liquid limit and plastic limit. The soil falling in the zone between liquid limit 30 to 50% and plastic limit 15 to 30% is considered suitable to be used as CNS material.
Sanfoundry Global Education & Learning Series – Irrigation Engineering.
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