Test: Dams - Civil Engineering (CE) MCQ

Jawa Dam is in Jordan. It is 100 kms northeast of capital Amman. It is a gravity dam with stone wall. It is dated to 3000 BC.

Bligh’s Creep Theory:

• Bligh assumed that the percolating water follows the outline of the base of the structure which is in contact with the subsoil.
• The length of the path traversed by the percolating water is called the length of creep or creep length.
• He further assumed that the head loss per unit length of creep (i.e. H/L) which is called hydraulic gradient is constant throughout the percolating passage i.e. Loss of head is proportional to length of the creep.

Hydraulic Gradient (i) = 
The reciprocal of hydraulic gradient (i.e. L/H) is known as Bligh’s Coefficient of creep.
Additional Information
Percolation Coefficient:
It is defined as the ratio of coefficient of permeability to porosity of soil.

Where,
K = co-efficient of Permeability
η = Porosity.

Toe is the position of a dam in contact with the ground at the upstream side. Crest is top of dam. 

Retaining wall:

• A retaining wall or retaining structure is used for maintaining the ground surfaces at different elevations on either side of it.
• Whenever embankments are involved in construction, retaining walls usually necessary.

Types of retaining wall:

• Depending upon the mechanisms used to carry the earth's pressure, These are classified into the following types.
  • Gravity retaining wall.
  • Cantilever retaining wall.
  • Butters wall.

Cantilever retaining wall:

• It is the most common type of retaining wall and its height ranges up to 10-25 feet (3 to 8m).
• Counterfort retaining walls are economical for height over about 6 m.
• A cantilever retaining wall resists the earth pressure horizontal & another, by the cantilever bending action.

Additional Information
Gravity retaining wall:

• It is not used for heights of more than 3.0 m.
• In it, the resistance to the earth's pressure is generated by the weight of the structure.

The five types are storage, diversion, detention, debris and coffer dams.

Forces due to wave pressure on a gravity dam:

• Due to the generation of waves on the surface by blowing winds.

Wave height(h_w) is given by

where, 
V = Wind velocity (km/hr)
F = Fetch or straight length of water (km) 
Calculation:
Given data
F  = 30 km
V = 30 km/hr
For F < 32 km, Wave height(h_w)
 
h_w = 1.09 m = 1.1 m
The wave height generated on the surface of a reservoir is 1.1 m.

The figure shows a gravity dam. It is a massive sized dam built using concrete or stone masonry. They can hold back large volume of water.

For no tension Criteria:

Where C = 1, B = Base width of dam, H = Height of dam
No Sliding Criteria:

Where B’ = Minimum base width for no sliding criteria and S = Specific gravity of material of dam
Calculation:
For C = 0
H = b × √s

Bhakra Nangal Dam is an example of gravity dam. Idukki dam is an arch dam. Bisalpur Dam is also a type of gravity dam.

Ogee Fall:

• In this type of fall, an ogee curve (a combination of convex curve and concave curve) is provided for carrying the canal water from higher level to lower level.
• This fall is recommended when the natural ground surface suddenly changes to a steeper slope along the alignment of the canal.
• There is a heavy drawdown on the u/s side resulting in lower depth, higher velocities and consequent bed erosion.
• Kinetic energy is not well dissipated due to smooth transition.

Rapid Fall

• The rapid fall is suitable when the slope of the natural ground surface is even and long. It consists of a long sloping glacis with longitudinal slope which varies from 1 in 10 to 1 in 20. It is nowadays obsolete.
  

Stepped Fall

• Stepped fall consists of a series of vertical drops in the form of steps. This fall is suitable in places where the sloping ground is very long and requires long glacis to connect the higher bed level with lower bed level.
• This fall is practically a modification of the rapid fall. The sloping glacis is divided into a number of drops so that the flowing water may not cause any damage to the canal bed. Brick walls are provided at each of the drops.