How Deep in the Water Do Bridge Pillars Go?

Bridge Pillars in Water

The idea of building a bridge over water has been around for many years and has even become iconic. You’ve probably heard of the famous Golden Gate Bridge and Sydney Harbor Bridge. Although it may seem easy to build a bridge over the body of water and have people drive, walk, or bike safely across, engineers must consider several crucial factors, such as how deep the pillars should go.

On average, bridge pillars go as deep as 80 ft (24.38 m) in the water. Generally, this height is usually lower in areas less disaster-prone. In contrast, areas that experience frequent earthquakes and tsunamis may have pillars that go as deep as 150 ft (45.72 m) underground.

Read on for a detailed rundown of the topic and insights into how engineers build stable, safe, and durable bridges.

Why Bridge Pillars Must Be Deep

So, why would engineers build pillars that go up to the height of an eight-story building underground?

Here are some of the crucial reasons for doing that:

Provide Stability

Bridge pillars need to be deep enough to provide the bridge with stability, strength, and durability.

They need to be strong enough to not sink into the mud or move along with tectonic plate movement. This is to prevent the bridge from collapsing.

Prevent Erosion

If bridge pillars aren’t deep enough, they may be eroded by the water. This would affect the structure of the bridge and its stability.

As a result, the bridge may collapse into the water, compromising the safety of the people who use it.

Minimize Wave Action

Waves also contribute to erosion and consequently affect the stability of a bridge if pillars aren’t deep enough underground.

Notably, bridges that experience constant and tumultuous wave action may need pillars with a greater height.

Consider the Motion of the Earth’s Crust

The motion of the earth’s crust is yet another factor to consider when digging bridge pillars. When there’s an earthquake or tectonic movement, underground structures such as bridge pillars move along, causing a bridge to collapse if it’s not sturdy enough.

Protect Pillar Caps

Bridge pillars are capped to protect the pillar from corrosion and to make it more aesthetically pleasing.

However, pillar caps can become loose if they aren’t built deep enough into the ground or too much weight is placed on them. This leads to movement and the possible collapse of the bridge.

Save Money (in the Long Term)

Engineers save money when they build bridge pillars deep underground.

Although it’s more expensive to build the pillars initially, you avoid spending additional money on repairs due to collapse. This saves time and effort for both engineers and city authorities.

What Does Anchoring Mean for Bridge Pillars?

Anchoring means the act of securely connecting bridge pillars to the ground. This provides additional stability and safeguards against earthquakes, wave action, erosion, or other disasters that may compromise safety.

Engineers can install pillars on loose soil by digging deep below the surface, so more of the pillar is underground. This reduces the available room for the pillar to move in case of earthquakes or other disasters.

The Golden Gate Bridge, located in San Francisco, California, was built with its bridge pillars anchoring at least 65 ft (19.81 m) underground. This technique provides extra stability against potential disasters and reduces the need for maintenance.

Similarly, engineers used this technique when designing and building the Sydney Harbor Bridge, which has seven pillars that go up to 89 m (291.99 ft) below ground.

How Engineers Test the Structural Integrity of Bridge Pillars

The structural integrity of bridge pillars is tested after they are built to ensure adequate support for the bridge.

Engineers observe the strength and integrity of pillars if they go through a series of tests, such as:

Load Tests

These test whether or not a pillar can hold a certain amount of weight. To test the load capacity of bridge pillars, engineers put weights on them and take readings that provide insight into their integrity and strength under stress.

Torsion Tests

These tests check whether or not bridge pillars are flexible enough to withstand the force applied in different ways, such as twisting. Engineers place weights on the pillars and use a torque meter to determine whether or not they can hold up under duress.

Environmental Tests

These tests consider environmental factors that may affect pillars, such as corrosion due to rain, erosion, rusting, and wave action.

They also provide an evaluation of how much weight a pillar can hold against the effects of natural disasters like earthquakes or hurricanes.

Engineers determine how much weight a pillar can hold, and depending on the factors that influence its strength, they may choose to increase the load capacity of pillars by increasing their height or digging them deeper into the ground.

Stress Test

This test provides insight into the pillars’ ability to withstand sustained stress, such as how much weight it can hold for an extended period.

It’s the most thorough test and provides answers to questions such as:

  • Whether or not a pillar can hold its weight.
  • How much force can it withstand?

The stress tests provide the backbone for what engineers consider when building bridge pillars and making decisions about their height and load capacity.

For more insights into how engineers ensure structures’ stability, a great read is Structural Integrity Cases in Mechanical and Civil Engineering (available on Amazon.com). The author explains the factors that cause structural failures, and how to monitor the durability and stability of structures, making it a worthwhile read by anyone working on a construction project.

How Do Engineers Calculate the Load Capacity of Bridge Pillars?

Engineers use a mathematical model to calculate how much weight a pillar can bear before it crumbles under duress. These calculations are based on the pressure exerted by the bridge’s weight, which is applied in different ways (such as pushing, pulling, and twisting).

The mathematical model takes into consideration the following criteria:

  • Whether or not a bridge pillar is braced against other bridge pillars to provide added stability. If it is, then it has an increased load capacity.
  • The angle at which forces are acting on bridge pillars.
  • The soil and rock strata in which bridge pillars are anchored. A pillar built deeper into the foundation of the area it stands on has increased stability and load capacity.

Engineers consider these factors when erecting bridge pillars and testing their load capacity.

How Engineers Prevent Bridge Pillars From Moving

Engineers use different techniques to secure bridge pillars and strengthen their stability against potential collapse or disaster. These include:

  • Seawall: A seawall is a concrete barrier that protects bridge pillars from water erosion.
  • Bracing: Bridge pillars are braced against one another to provide added stability.
  • Concrete reinforcement bars (rebars): This is a process in which steel bars are embedded directly into concrete to provide additional strength and prevent cracking.
  • Weight coating: A weight coating is a substance that helps bridge pillars withstand water erosion damage by protecting them against rust, corrosion, and wave action. It also increases the load capacity of pillars because it prevents materials from degrading over time.

You’d be interested in watching this 3-minute video that summarizes the process of building a bridge over water:

Final Thoughts

As a rule of thumb, bridge pillars are built deep enough to ensure stability and the safety of users. They’re often made to withstand stress, environmental factors, and normal wear and tear can be considered safe.

Engineers determine how much weight a pillar can hold, depending on factors such as corrosion due to rain, erosion, rusting, and wave action. They also evaluate how much weight a pillar can hold at maximum capacity for an extended period. This ensures that the bridge can stand for years and does not collapse without warning.

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