7. Dams

Tunnels are underground passageways that allow people, animals, and other things to follow routes they could not otherwise take. Tunnels for carrying water are called culverts. As the first video in this series showed, rock and soil generate contact pressures on any structures that they touch. To be effective, tunnels and culverts must resist these forces. Circular tunnels are popular in part because they are easy to construct. They are also strong because their walls carry the applied rock and soil loads by arch action. You may remember from our arches and chains video just how effective arch structures can be in load carrying. Another popular design is the box-shaped tunnel or culvert. Because its walls are flat, they must carry the applied loads by bending or beam action. And, as this model shows, they tend to bend inwards. Which of these two designs do you think will support a greater height of marbles? The circular one or the box shape? As these models demonstrate, circular tunnels tend to be stronger than rectangular ones that have the same size and wall thickness. However, if rectangular and circular culverts have the same cross-sectional area, the circular one will typically have a greater height. And that characteristic can give rise to other problems, such as raised road elevations. The model also nicely demonstrates why care must be taken during construction to avoid overloading, especially when there is little soil above a culvert or tunnel. It also illustrates how filling in the soil around a tunnel or culvert must be done with care. Improper backfilling can do a lot of damage, as it can apply unintended loads to the culvert, causing it to deform or even collapse. To learn more about how soil and water interact with structures, we hope you will view our videos on dams, retaining walls, and silos and tanks.