Bridge Unit Study - Lesson 4: Cofferdam Model

My son built a cofferdam.

Have you ever wondered how construction engineers build structures underwater? Well they don't. They remove the water from the area they want to build with a structure called a cofferdam or a caisson. One method of building a cofferdam is to drive large metal structures into the soil in rings.  Then the water inside the ring is pumped out so the construction can be completed.

Cofferdams were used as many as 2000 years ago by the Romans who developed a unique type of concrete. Roman concrete incorporated ash material from nearby Mt. Vesuvius which made it very similar to modern concrete, but most importantly, the ash made the concrete waterproof. Since the Romans weren't building with steel, their cofferdams were constructed by driving two rings of wooden posts into the soil. The space between the rings was then filled with a watertight material such as clay to prevent water coming into the inner ring. They also did not have pumps, so the water was removed with buckets. Nonetheless, this method for removing water from an area allowed them to build support structures for bridges in the water.

While learning about cofferdams, we watches the video Engineering an Empire Rome as it was a very educational video which linked our bridge study to history.

The book Bridges: Amazing Structures to Design Build and Test is a fantastic resource for learning everything about bridges. It has been the foundation for our bridge study, and is packed with both information as well as activity ideas to help solidify concepts.

My son followed the instructions in the book for building his cofferdam. First he put sand into the bottom of a bowl and added water covering the sand. Next he put craft sticks into the sand in the water in a ring. The Romans would have used tree trunks for this step.

Then he put tape around the craft sticks to hold them in a circle and added a second ring of craft sticks.

The Romans filled the gap between the rings with waterproof material such as clay. Plastic wrap worked well for my son.

Finally, he used a turkey baster to remove the water from inside the cofferdam.

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