This Is How Underwater Constructions Are Made

Welcome to the world of civil engineering’s toughest challenge: underwater construction!

Today, we dive into the adventure of building an underwater bridge pier, where even the tiniest mistake can spell disaster.

Battling the Elements

Picture this: many laborers braving violent ocean currents, working tirelessly on an underwater bridge pier.

But hold up—did the chief construction engineer just make a colossal blunder? This error could lead to catastrophe!

The Underwater Construction Secret: Cofferdams

Contrary to what you might think, underwater construction doesn’t actually happen underwater.

Instead, engineers first build a temporary dam called a cofferdam to keep the water out. It’s like creating a dry work zone right in the middle of the ocean. But how do they do it?

  1. Guide Piles: First, workers erect guide piles using a pile driving hammer machine. These guide piles set the stage for the cofferdam.
  2. Sheet Piles: Next, they drive sheet piles into the soil with interlocking ends, minimizing water leakage. These are driven all the way to the bedrock, starting from the corners and working inward.

The Magic of Vibrations

Here’s a fun fact: the pile driving hammer machine doesn’t just push the sheet piles down; it uses vibrations!

These vibrations, thanks to counter-rotating eccentric weights, help the sheet piles penetrate the soil more effectively.

Heavy Machinery on the Scene

Enter the barge—a flat-surfaced ship carrying all the machinery and components to the construction site.

Now we’re ready for the big phase: pumping out the water from the cofferdam. But before that, engineers need to do some homework.

Geotechnical Studies: Know Your Ground

Before any water gets pumped out, engineers perform a detailed geotechnical study of the soil. They use a cone penetration test to measure soil strength, ensuring it can support the permanent structure.

The test involves a device that penetrates the soil, sending back friction and resistance values until it hits bedrock. This tells engineers how deep the sheet piles need to go.

Pumping Water: The Real Challenge

As water levels drop inside the cofferdam, leakage becomes an issue due to differential pressure.

The solution? Double-layered sheet piles filled with granular material like sand, gravel, or broken rock. This effectively resists water leakage.

The Cofferdam Collapse

But oh no! As water is pumped out, the cofferdam collapses inward. Why? When water was present on both sides, the pressure was balanced.

Without it, the inward force becomes too great, causing collapse.

The fix? A bracing frame structure with components like whales, struts, and braces to provide lateral support.

Sealing the Deal with Concrete

Once the cofferdam is stable, the next step is to tackle water seepage and reinforce the structure. This involves:

  • Clamshell Buckets: Used to remove soil from inside the cofferdam.
  • Concrete Seal Course: Concrete is poured to create a seal, preventing water leakage and providing a strong footing. The process involves a hopper bucket and a long segmented pipe to pour concrete while keeping it from mixing with seepage water.

Building the Bridge Pier

With the concrete seal course in place, workers lay out the reinforced bars to construct the bridge pier.

This structure needs to withstand both water pressure and the load of the bridge. Once the concrete is poured and gains strength (usually 14 to 28 days), the cofferdam is no longer needed and is removed.

However, the sheet piles are cut at the level of the concrete seal course to avoid compromising the structure’s strength.

The Final Result

And there you have it—a strong bridge pier ready to support a heavy bridge! This journey through underwater construction with cofferdams has been a wild ride.

Stay tuned for our next adventure, where we’ll explore other fascinating underwater construction technologies like caissons and pile foundations.