Hydraulic Structure
"Hydraulic Structure"
Dams constructed out of masonry or concrete and which rely solely on its self weight for stability fall under the nomenclature of gravity dams. Masonary dams have been in use in the past quite often but after independence, the last major masonry dam structure that was built was the Nagarjunsagar Dam on river Krishna which was built during 1958-69. Normally, coursed rubble masonry was used which was bonded together by lime concrete or cement concrete. However masonry dam is no longer being designed in our country probably due to existence of alternate easily available dam construction material and need construction technology. In fact, gravity dams are now being built of mass concrete, whose design and construction aspects would be discussed in this chapter. There are other dams built out of concrete like the Arch/Multiple Arch or Buttress type.
These have however not been designed or constructed in India, except the sole one being the arch dam at Idukki on river Periyar. In India the trend for concrete dam is only of the gravity type and therefore the design other types of concrete dams have not been discussed in this course. Interested readers may know more about such dams from standard books on the subject like Engineering of Large Dams by Henry H. Thomas, Volumes I and II published by John Wiley and Sons (1976).
A slightly outdated publication, Engineering of Dams, Volumes I, II and III by W P Creager, J D Justin, and J Hinds published by John Wiley and Sons (1917) has also been long considered a classic in dam engineering, though many new technologies have do not find mention here.
It is important to note that, it is not just sufficient to design a strong dam structure, but it is equally important to check the foundation as well for structural integrity. For concrete dams, the stress developed at the junction of the base becomes quite high, which the foundation has to resist. Usually concrete gravity dams are constructed across a river by excavating away the loose overburden till firm rock is encountered which is considered as the actual foundation. Nevertheless not all rocks are of the same quality; they vary with different geological materials and the process by which they have been formed over the years. For example, the hills of the Himalayan range of the mountains are considered geologically young, as well as weaker than the massif of the Deccan plateau. The quality of foundation not only affects the design, it also guides the type of dam that would be suited at a design site. Hence, discussions on the ground foundation aspects have been introduced in this lesson as well.
It may also be realized that designing a dam based on field data (like the geometry of the river valley, the foundation allowable bearing capacity .etc) is not the only part that a water resource engineer has to do. He has to get it constructed at the design site which may easily take anywhere between 5 to 10 years or even more depending on the complexity of the work and the volume and type of the structure. It may easily be appreciated that constructing a massive structure across a flowing river is no easy task. In fact tackling of the monsoon flows during the years of construction is a difficult engineering task.
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