The last article in this series covered a brief introduction to dams and various purposes for which dams are built. This second article in the series takes a closer look at how dams are classified as well as what are different elements which come together to form a dam. We will also have a look at what are ways possible in which a dam can malfunction.
Classification of Dams:
Dams can be classified in different ways, based on their function, or the material of construction or the structural behavior or the hydraulic aspects. The info-graphic below shows various ways in which a dam can be classified.
Components of Dam:
A dam is not just a wall to restrict the water flow. A dam is a complex construction which has multiple components and processes each designed to do a specific task. Here is a list of some components or activities which are crucial to dam building.
Core: Core is generally made up of impervious material and it ensures that the dam does not leak water. To avoid any seepage, the particle size of material used for constructing core is very small.
Shell: Shell is generally made up of coarse material or pervious material. It’s primary purpose is to give strength to the core wall.
Cut off trench: Cut off trench is a trench dug and filled with impervious material like rich concrete.
Sheet pile wall: Sheet Pile Wall is constructed to stop the cross flow of water. At some sites, if trench cannot be dug because of the financial constrains, the sheet pile wall is used.
Impervious stream blanket: As the name suggests, the impervious stream blanket is made using impervious material. It is laid in the foundation of the dam to eliminate any leakages in the dam foundation.
Rip rap: The upstream face of the dam faces erosion and rip rap is a material similar to getotextiles which is used to prevent this upstream face erosion. Large stones ranging from 0.5 meters to 1 meter are used to construct this.
Horizontal drainage: Horizontal drainage is used to release the pore water pressure due to moisture or water contents present in the dam body.
Crest (or top): Dam crest is the upper most part, which separates upstream face from the down stream face. It is designed in such a way that vehicles can pass over it.
Soil turfing or Sod: Sod is used to avoid erosion due to numerous weather factors like rain, snow fall, wind etc.
Free board: This component prevents the dam from over-topping.
Service Spillway: Service spillway is the primary spillway during the floods.
Auxiliary spillway: Auxiliary spillway is secondary or complimentary spillway which operates only during large floods.
Cofferdam: A cofferdam is a temporary enclosure which either fully or partially encloses the construction area. It ensures that construction can be carried out in a dry setting.
Conduit is an enclosed channel used to convey flows through or under a dam.
Design of Dam:
Following are a few steps which are an integral part of the dam designing process:
Sub-surface Investigations: Designing and building a dam is quite complex a process. Along with the steps mentioned below, a large number of investigations, lab testings and surveys are carried out. For instance, boring are carried out in order to find the depth to bedrock. The soils and rocks under the dam are lab tested to determine their characteristics. Internal friction between the foundation materials in the laboratory. These subsurface investigations are crucial for design and construction of any dam.
Another major step involved in designing dams is finding out various forces which which will be acting on the dam structure. All the forces are found out per unit length of the dam.
Following are the forces which are taken into consideration while designing a dam. These forces are location specific and some forces might be omitted from the calculation if the factors are absent in a given geography.
Water pressure
Uplift pressure or seepage loads
Earthquake forces
Self-weight of the dam
Silt pressure
Wave pressure/ Ice pressure
Failure analysis and fulfillment of stability criteria:
After calculating various forces, various modes of failure are analysed and certain criteria for stability are derived. Following are some of the stability criteria which a design should fulfill:
No tension
No sliding
No crushing or no over-stressing
Foundation Safety
No overturning
Elementary Profile:
As per the inputs discussed above, a primary or elementary profile of the dam is created. The profile or cross section is normally designed for the maximum height of the dam, measured above the rock foundations.
Practical Profile:
The elementary profile is then further fine tuned to arrive at a practical profile. It also includes detailing about some additional components like free board etc.
Food For Thought:
Based on the above discussion, from dam safety, stability and economical perspective, what should be opted - one single large dam or multiple series of medium and small dams?
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