Short Answer:

Hydrostatic pressure is the pressure exerted by a fluid at rest due to the weight of the fluid above a given point. In water tanks, this pressure increases with depth and acts equally in all directions against the tank walls and floor, depending on the height of the water column.

This pressure affects tank walls by creating horizontal forces that try to push the walls outward. If not properly designed, it can lead to cracking, bulging, or even failure of the tank. Therefore, tank walls must be reinforced and thick enough to resist this increasing pressure safely.

Detailed Explanation

Hydrostatic Pressure

Hydrostatic pressure is a key factor to consider in the design of water-retaining structures like tanks, reservoirs, and swimming pools. It is defined as the pressure exerted by a fluid at equilibrium due to the gravitational force acting on it. This pressure acts perpendicular to any surface in contact with the fluid and increases linearly with depth.

In the case of tanks filled with water, hydrostatic pressure is not constant—it is zero at the surface and maximum at the base. It depends on the depth of water, the density of the fluid, and gravitational acceleration. The formula used to calculate hydrostatic pressure is:

P = ρ × g × h

Where:

P = pressure at depth h
ρ = density of water (≈1000 kg/m³)
g = acceleration due to gravity (9.81 m/s²)
h = depth from the water surface

How Hydrostatic Pressure Affects Tank Walls

Varying Pressure with Depth
Hydrostatic pressure increases linearly from the top to the bottom of the tank. This means:

The top portion of the tank wall experiences very little pressure.
The bottom portion carries the maximum pressure.
Walls need to be thicker and more reinforced at the base compared to the top.

Horizontal Outward Force on Walls

Hydrostatic pressure acts horizontally outward on the vertical tank walls.
This force tries to push the walls apart, and if the wall is weak or not reinforced, it can lead to bulging or failure.
Circular tanks naturally resist this force better due to hoop tension, but rectangular tanks need more reinforcement along the walls.

Vertical Pressure on Tank Floor

The tank base or floor experiences vertical pressure from the entire height of water.
The slab must be designed to resist uplift pressure in the case of underground tanks with a high groundwater table.

Need for Reinforcement

Walls must be designed with horizontal and vertical steel reinforcement to resist the bending moments and shear forces caused by hydrostatic pressure.
Horizontal bars resist outward pressure; vertical bars help manage wall bending.

Crack Control

Uneven or high hydrostatic pressure can lead to cracking of the tank walls, which may result in water leakage.
Crack width must be limited (usually ≤ 0.2 mm) as per IS 3370, especially in water-retaining structures.
Using water bars at joints also helps reduce leakage risk.

Effect on Structural Stability

In large tanks, the cumulative hydrostatic pressure may produce a very high force on the walls.
If not considered correctly, this can lead to instability, especially in tall or slender tanks.
The foundation and overall wall design must account for this pressure distribution.

Impact During Empty Conditions

When tanks are emptied, external earth pressure may exceed the internal pressure and can cause inward wall collapse if the tank is not properly braced or if hydrostatic forces are not balanced.

Code Guidelines

IS 3370 (Part 2) gives detailed design provisions to handle hydrostatic loads in water tanks.
Design must ensure serviceability (no leakage) and safety (no failure) under all loading conditions.

Conclusion

Hydrostatic pressure is the fluid pressure exerted by stored water that increases with depth and acts outward on tank walls. It plays a crucial role in tank wall design as it causes bending, cracking, or failure if not resisted properly. Using correct wall thickness, reinforcement detailing, and crack control measures ensures safety and water tightness in tanks.