Gravity Dam Design Principles

Gravity Dam Design Principles

• A gravity dam is a structure designed so its own weight resists external forces.
• This design makes it a durable and low-maintenance structure.

Types of Loads on Gravity Dams

• Loads are classified as primary, secondary, or exceptional, based on their importance and applicability.

Primary Loads

• These are major loads impacting all dams regardless of type.
• Water load: The horizontal water pressure exerted by the water on the upstream dam face. This pressure distribution is triangular.
• Self weight load: The weight of the dam itself. This is a stabilizing force.
• Related seepage (uplift) load: Pressure exerted by water seeping through the dam and foundation.

Secondary Loads

• These loads are generally applicable, but of lesser magnitude.
• Sediment load: Forces due to sediment buildup in the reservoir.
• Wind load: Forces exerted by wind.
• Ice load: Forces due to ice buildup.
• Silt load: Forces due to silt buildup in the reservoir.
• Thermal and dam foundation interaction effect: Thermal expansion and contraction of the dam.

Exceptional Loads

• These are unpredictable and significant, typically related to earthquakes.
• Earthquake loads: Forces due to seismic activity. These are a primary concern in dam design and require testing after earthquakes.

Water Loads

• Water pressure (P) is the major external force on a dam.
• The horizontal water pressure depends on the depth of the water (hydrostatic pressure distribution). The shape is a triangle.
• Two cases exist to estimate water loads: upstream face is either vertical or partly inclined and partly vertical.

Case 1: Vertical Upstream Face

• Intensity is zero at the water surface and equal to H at the base.
• Resultant force = 1/2 *w *H^2, acting at H/3 from the base.
• w = unit weight of water, H = depth (MWL - maximum water level)

Case 2: Inclined Upstream Face

• The resulting water force can be resolved into horizontal and vertical components.
• Forces can be calculated on both upstream and downstream faces.

Self-Weight Loads

• The dam's weight is a main stabilizing force.
• The weight is equal to the dam's cross-sectional area multiplied by the unit weight of the material.
• Two cases exist : the upstream face is vertical.

Uplift (Seepage) Loads

• Water seeping through the dam's pores, foundation, and joints exerts uplift pressure.
• This pressure acts to reduce the dam's stabilizing force.
• Several cases exist to calculate the uplift pressure in dams:
• No drainage, no tailwater (triangle shape)
• No drainage, tailwater present.
• Drainage gallery present, tail water present,

Stability Requirements of Gravity Dams

• The dam must be structurally stable to withstand stresses from imposed loads.
• The foundation must be strong enough to carry the loads.
• The dam must be safe against all possible failure modes with an adequate safety factor..

Possible Failure Modes

• Overturning failure.
• Sliding failure.
• Tension failure.
• Crushing (compression) failure.

Overturning failure:

• Occurs when the resultant of all forces acts outside the base.
• A factor of safety of more than 1.25 may be acceptable but is generally not ideal.

Sliding failure:

• Occurs when the dam slides along its base.
• The factor of safety against sliding should be greater than 1..

Tension failure:

• Develops at the dam's upstream edge (heel) when the resultant force acts outside the middle third.

Crushing (compression) failure:

• Occurs when the vertical stress in the dam and foundation exceeds safe limits.
• The eccentricity of the resultant force is key to calculate this type of failure.

This quiz explores the fundamental principles of gravity dam design, including the various types of loads that affect their stability. You will learn about primary and secondary loads, such as water pressure and wind forces, and how these elements influence dam construction and maintenance. Test your knowledge on this vital topic in civil engineering.