Iowa DOT's
policy is to be
sensitive to the environment
and foster and enhance
environmental stewardship.
IOWA DEPARTMENT OF TRANSPORTATION
Presents
ROCKS AND ROADS
Unit 4: How are highways designed?
F. Earthwork and Drainage
We have already learned that highways need to change slope gradually. Often the landscape does not allow this, and large volumes of earth must be moved to allow a safe design. Often engineers need to cut down high points of the landscape and fill in low spots. The soil cut from the high points is usually used to fill in the low spots.
In this drawing the blue shape shows an area where earth is being removed (a CUT section). The yellow shape shows an area where earth needs to be placed (a FILL section).
Engineers try to balance the amount cut with the amount needed to fill in low spots. However, since the roadway is usually built higher than the surrounding ground, there is typically the need for additional material. This material comes from areas called borrow areas.
A borrow area can be a hilltop that is cut down to the level of the surrounding ground, or it can be an area that is excavated to create a large hole that fills with water. Have you ever seen an overpass while you were traveling down the highway and noticed a pond nearby? Chances are good that the earth for the overpass came from the hole that is now a pond.
Borrow ponds can provide wonderful habitats for aquatic plants and animals, as well as provide recreation and increased land values. Sometimes the DOT buys and keeps the land in a borrow. Occasionally, people living near the project are happy to let the DOT create a borrow pond on their land. The DOT gets the earth it needs and the people get a nice new pond. Everybody wins.
When earth is placed in a "fill" section, it must be compacted so it is very strong and will not deform when traffic later passes over it. Contractors must spread the earth out in thin layers and pass heavy steel rollers over each layer to ensure good compaction. Earth placed in this manner is called an embankment.
When designing embankments, engineers need to be careful to allow water to pass across the path of the roadway. This is done by using bridges and culverts. We usually think of bridges as getting us across an obstacle, such as a river. If you were water, however, you would view a roadway embankment as an obstacle and would need to cross it. If you think about it, a bridge is kind of an overpass, but with a road and a river instead of two roads. It all depends on your perspective.
You probably know what a bridge is and what it is used for. Bridges carry a roadway over an obstacle, such as a river, ravine, or another roadway (remember the overpass). Bridges are used for water crossings if the stream or river is very large.
If the waterway is small, however, such as a small stream or drainage ditch, a culvert is often used. A culvert is a box or pipe buried in an embankment through which water can pass. They can be round, square, or rectangular, and are usually made of concrete, steel or plastic. Culverts are placed to connect low spots on the ground so water can pass from one side of the road to the other.
