ðŸ•‘ Reading time: 1 minute

Different types of forces acts on a dam structure such as water pressure, self weight, wave pressure etc.Â Determination of various forces which acts on the structure is the first step in the design of dams. These forces are considered to act per unit length of the dam.Contents:

**Forces Acting on a Dam Structure**

- Water pressure
- Uplift pressure or seepage loads
- Earthquake forces
- Self weight of the dam
- Silt pressure
- Wave pressure
- Ice pressure

**1. Water Pressure**

It is the pressure of water that acts perpendicular on the upstream face of the dam. For this, there are two cases:
A. Upstream face of the dam is vertical and there is no water on the downstream side of the dam Fig.1.
The total pressure is in horizontal direction and acts on the upstream face at a height H/3 from the bottom. The water pressure on the dam is computed according to equation 1.
Where:
w: specific weight of water. Usually it is taken as unity.
H: height up to which water is stored in m.
B. Upstream face with batter and there is no water on the downstream side as shown in Fig.2.
Here in addition to the horizontal water pressure of equation 1, there is vertical pressure of the water. It is due to the water column resting on the upstream sloping side.
The vertical pressureacts on the length â€˜bâ€™ portion of the base. This vertical pressure is is calculated as follow:
Pressure acts through the center of gravity of the water column resting on the sloping upstream face.
If there is water standing on the downstream side of the dam, water pressure will have vertical and horizontal component which can be using equation 1 except water height which is illustrated in Fig.3.
The water pressure on the downstream face actually stabilizes the dam. Hence as an additional factor of safety, it may be neglected.
**2. Uplift pressure or Seepage Loads**

When the water is stored on the upstream side of a dam there exists a head of water equal to the height up to which the water is stored.
This water enters the pores, fissures, and cracks of the foundation material under pressure. It also enters the joint between the dam and the foundation at the base and the pores of the dam itself.
This water then seeps through and tries to emerge out on the downstream end. The seeping water creates hydraulic gradient between the upstream and downstream side of the dam.
This hydraulic gradient causes vertical upward pressure. The upward pressure is known as uplift which is the second largest external pressure. Uplift reduces the effective weight of the structure and consequently the restoring force is reduced.
Therefore, it is essential to study the nature of uplift and also some methods will have to be devised to reduce the uplift pressure value.
With reference to figure 3, uplift pressure is given by
Where is the uplift pressure, B is the base width of the dam and H is the height up to which water is stored. This total uplift acts at B/3 from the heel or upstream end of the dam.
Uplift is generally reduced by constructing drainage pipes between dam and its foundation, constructing cut off walls under the upstream face, holes in the dam section, or pressure grouting the dame foundation.
**3. Earthquake Forces**

Dynamic loads created due to earthquakes must be considered in the design of all major dams located in â€˜high-riskâ€™ seismic regions, and for dams situated in close proximity to potentially active geological fault complexes.
Earthquake produces waves in every possible direction. However, it has to be resolved into vertical and horizontal components for the design purposes. The horizontal component has greater effect.
Seismic vibration influence both dam body and water in the reservoir of the dam. So, the generated dynamic loads are due to the **and**

*inertia of the dam***forces by the water in the reservoir. Â Seismic forces are estimated according to the following equations:**

*hydrodynamic***Forces due to inertia of the dam**

where:
and: horizontal and vertical seismic coefficient
: weight of the dam
where:
: soil foundation factor, and taken as 1
: importance factor
: basic seismic coefficient taken from seismic maps.
is taken as 1.5*andis taken as 0.75*.
**NOTE**: Inertia forces are considered to operate through the centroid of the dam section.