Build a Cofferdam. There are many bridges around the world with different
functions and purposes. However, when building over the water, bridge
construction ...
Build a Cofferdam There are many bridges around the world with different functions and purposes. However, when building over the water, bridge construction can undergo special obstacles. To span such a space, many bridges have support piers that descend into the water, but how do engineers create these piers in the first place? Whether made of wood, steel, or concrete, making piers that reach bedrock, while dealing with rushing currents, can present a huge challenge. In this activity, students will learn how engineers use cofferdams to create bridge foundations under water, and create a model cofferdam in the classroom. Materials
Deep pan or tray Sand Gravel Water 2 liter bottle 20 oz bottle Pipette Scissors Ruler Small bowl
Directions 1) Have a group of 2 to 3 students work as a team, starting by filling a pan halfway with sand. 2) With a ruler, the group should measure and make a mark 1 inch above the sand. 3) Pour the water up to the mark measuring the water level. 4) Have an adult help each group cut the top and bottom of their 2 liter and 20 oz bottles. Each bottle should look like a 5 inch long open cylinder after cutting.
5) Each group should insert the cut 20 oz bottle into the sand and water until it is flush with the bottom of the pan. 6) Insert the cut 2 liter bottle into the pan so that the smaller bottle is in the center of it, and also flush with the bottom of the pan. 7) Carefully pour gravel in the outer ring created between the large and small bottle, being careful to leave the inner center ring untouched. 8) Using their pipettes, have the students slowly remove the water from the center ring and discard it in a small bowl.
Background Information Cofferdams are temporary enclosed spaces, typically built in a large body of water. By removing the water at the center of the cofferdam, builders create a dry space to work under the water and construct things like piers for bridges. In many past civilizations, such as ancient Rome, cofferdams were made out of two rings of wooden stakes, and water was removed manually by workers, one bucket at a time. In order to create a water tight space, cofferdams consisted of both an outer and inner ring. The outer ring was filled in with another substance, such as sand or clay, to block out the water from the center ring. After the water was removed from the center ring, the mud and gravel were removed until the builders reached bedrock. At this point, they had reached a stable, dry portion of the ground beneath the water where they could pour waterproof concrete to form the foundation and pillars for the piers. Today, cofferdams are often made of steel boxes, and motor‐powered pumps remove the water. In the above activity, the 2 liter and 20 ounce bottles represent the inner and outer ring of the cofferdam. The gravel acts like clay to displace the water in the outer ring and contribute to forming a water tight space around the inner ring. Just as the ancient Romans, students use pipettes to manually remove water from the inner ring until they reach the bottom of the sandy pan. However, since there is no bedrock represented in the pan, nor a true water tight seal between the inner and outer rings, students will probably not be able to remove all of the water from the inner ring. This is not a problem! The ancient Romans had similar setbacks when creating their cofferdams. Students should still be able to get the water level in the inner ring significantly lower than the water outside of their cofferdam. The important thing is for them to understand how engineers used structures like cofferdams to make building under water possible.