Irrigation Engineering Multiple Choice Questions on “Rock Fill Dams – Seepage Control Through Foundations”.
1. During the maintenance of an earthen dam, the apparent seepage through the foundation of the dam is best taken care of by providing ______________________
a) chimney drain
b) rock toe
c) drain trench along the downstream toe
d) upstream impervious cut-off
Clarification: Cut-off can be provided only during construction. If seepage appears afterward, a drain with an inverted filter along the toe line is provided to collect soil-free water to check piping action.
2. The base width of a rock-fill dam in comparison to that of an earthen dam is _______________
a) much larger
b) much smaller
c) sometimes large sometimes smaller
d) almost equal
Clarification: Rock-fill dams are less flexible than earthen dams, their foundation requirements are more rigid than those of earthen dams which can be constructed on any type of foundation. Since the steeper slopes are used in rock-fill dams and the base width is quite less.
3. On moderate foundations and particularly in seismic areas, the type of dam which can preferably be considered for construction is ___________________
a) masonry gravity dam
b) earthen dam
c) rock-fill dam
d) arch dam
Clarification: Rock-fill dams provide high resistance to seismic forces due to their flexible character. They are liable to large settlements which may lead to cracking of concrete membrane.
4. According to Terzaghi, the ratio of D15 of filter to D15 of base material should be _____________
a) > 5
b) < 4
c) 4 to 5
d) < 5
Clarification: Terzaghi provided a rational approach to the design of filters. The following filter criteria should be satisfied-
D15 of filter / D85 of base materials < 4 to 5 < D15 of filter/ D15 of base materials
The filters should be such that neither the embankment nor the foundation material can penetrate and clog the filters.
5. Which of the following is not provided in order to control the possibility of sand boiling?
a) Relief wells
b) Drain trenches
c) Vertical cut-offs
Clarification: There is a possibility that the water may boil up near the toe of the dam when large scale seepage takes place through the pervious foundation overlain by a thin less pervious layer. Relief wells or drain trenches and d/s berms beyond the toe of the dam are all provided to control sand boiling.
6. The seepage forces within the filter are controlled up to permissible small magnitudes when the ratio of D15 of filter to D15 of base material is ___________________
a) between 5 to 40
b) less than 5
c) between 5 to 20
d) less than 4
Clarification: When the ratio does not exceed 4 to 5, the base material is prevented from passing through the pores of the filter. When this ratio is more than 5 and less than 40, the seepage forces within the filter are controlled up to permissible small magnitudes.