Bus Depot Bank Challenge

A Report from the Bus Depot

Buses come and go all day at a neighborhood depot. Behind its fence, a sloping bank of soil sits above a drain. A low stone wall holds the bottom of the bank in place. After rain, workers found muddy water flowing from the bank toward the drain. On a dry, windy day, they saw loose soil blow from the bank onto a paved area. Students studied the workers’ photographs and tested model banks safely in their classroom, away from the buses.

The wall photographs showed other changes. After several wet cracks froze and thawed during winter, small stone chips lay below those cracks. Water takes up more space when it freezes. During warm months, a plant root growing inside a different crack became thicker. That crack became wider, but the broken stone stayed in place. Breaking rock into smaller pieces is called weathering. Moving loose rock or soil from one place to another is called erosion.

The class compared three model banks: bare soil, soil under mesh with small openings, and soil held by rooted grass. Each tray began with 500 grams of the same kind of soil. The trays had the same size and slope. Students slowly poured 400 milliliters of water over each tray from the same height for the same amount of time. They collected the soil carried out with the water, let it dry, and measured its mass. Their results appear in the graph.

For a separate wind test, the class prepared three fresh, dry trays with the same covers, soil amounts, and slopes. The teacher used a gentle fan inside a clear container that kept the moving soil contained. Fan distance, speed, and blowing time stayed the same. The bare tray lost 12 grams of soil, the mesh tray lost 5 grams, and the grass tray lost 2 grams.