## LESSON 29. Stability Analysis of Gravity Dams: Forces and General Requirements

A gravity dam is a solid structure, generally made of concrete or masonry, constructed across a river to create a reservoir on its upstream. These dams resist the various forces acting on it by its self weight and hence coined as the gravity dam. A typical section of a solid gravity dam is shown in Figure 29.1.

Fig. 29.2.

In this module we will learn different aspects of stability and design of concrete gravity dam.

29.1 Different Forces on Gravity Dam

A gravity dam is subjected to the following forces.

29.1.1 Self weight of the Dam

Self weight of a gravity dam is main stabilizing force.

29.1.2 Water pressure

Water pressure on the upstream side (Figure 29.2) is the main destabilizing force in gravity dam. Downstream side may also have water pressure. Though downstream water pressure produces counter overturning moment, its magnitude is much smaller as compared to the upstream water pressure and therefore generally not considered in stability analysis.

Fig. 29.2.

29.1.3 Uplift water pressure

The uplift pressure is the upward pressure of water at the base of the dam as shown in Figure 29.3. It also exists within any cracks in the dam.

Fig. 29.3.

In addition to the above mentioned forces, a gravity dam may also subject to the following forces.

• Earth pressure

• Wave pressure

• Earthquake

• Force due to Wind

• Ice pressure

These forces have very little effect on the stability and therefore generally be neglected in stability analysis.

29.2 General Requirement for Stability

A gravity dam may fail in the following modes,

• Overturning

• Sliding

• Compression

• Tension

Therefore, the requirements for stability are,

• The dam should be safe against overturning.

• The dam should be safe against sliding.

• The induced stresses (either tension or compression) in the dam or in the foundation should not exceed the permissible value.