Home >> Science & Nature >> Science

How Does Gravel Compact Surfaces?

Many roads around the country have gravel surfaces rather than concrete or asphalt. Typically, gravel roads are used for lesser traveled areas, such as in rural regions. However, gravel roads exhibit effective roadway surface compaction, depending on specific mixtures of sand, stones, clay and silt. Gravel roads require specific designs for a long-term roadway surface.

Gravel Composition
- Gravel consists of three distinctly sized particles. Clay and silt, referred to as fines, offer an extremely small particle size, mainly less than 1 mm (millimeter) in diameter. Silt by itself does not compact well due to its tiny granular particle size. Clay offers moisture to the gravel mixture for adhesion between particles. Sand particles, typically much larger than silt, cannot be more than 0.25 inch. Similar to silt, sand has little compaction properties because of its granular state. Stones within gravel are the largest particles, commonly larger than 0.25 inch.
Gravel Compaction
- Although each particle type may not compact well individually, the three particle sizes mixed together form a compact gravel roadway surface. The larger stones, mixed with the smaller particles, aggregate. Spaces between the stones fill with sand, silt, and clay, forming a cohesive layer. As a result, the surface beneath the gravel becomes compacted from the layer's weight and cohesiveness.
Gravel Roadway Design
- Gravel roadways can have a gravel layer of 8 inches or more. However, laying a flat gravel surface is not recommended, states the University of Wisconsin. Rainfall cannot drain adequately from a flat surface. In addition, tire marks create small ditches that erode the gravel surface, especially if the gravel is laid as a flat surface. To counteract this, gravel roadways must have a crown, meaning the middle of the road is higher than either road shoulder. During a rainstorm, water will run from the middle of the road to the shoulder, effectively draining the water away from the gravel surface and preserving the stone and sandy layer.
Maintenance
- Depending on the roadway's traffic, it is necessary to periodically reshape the gravel surface. Motor graders or bulldozers can refine the gravel layer by digging into the top few inches with their blades, filling and cutting the gravel's layer until the proper level surface is achieved. However, the gravel should be moist, such as after a rainfall, for an appropriate gravel reshaping. Moisture hinders dust from leaving the gravel mixture, providing more particles for a compact final surface.

Science