The construction crew starts with earth, which must be compacted so there's no loose soil. The soil must have an absorption rate of 5.24 inches of water per hour. If the soil is too hard to absorb this amount of water, sand or other particulates must be added to meet the specification.
A durable filter fabric is laid down on the soil, and the first subsurface layer is added. This layer is composed of gravel, each stone having an average diameter of 1/2 inch. The thickness of this layer must be 2 inches.
The second subsurface layer is made of coarse stone. Each stone must have a diameter of 1 1/2 to 3 inches. This is about the size of a golf ball to a baseball. The thickness of this layer can vary from 12 inches to 39 inches.
The third subsurface layer is the last layer below the asphalt. This layer must be composed of gravel, with each stone having an average diameter of 1/2 inch. The layer must be 1 inch to 2 inches thick.
The top layer is the asphalt. This isn't compacted hard, but rather left loose so water can flow through. It should have a permeability rate that's greater than 8 inches per hour. This layer can be anywhere from 2 1/2 to 4 inches thick. The final thickness is calculated out by engineers, who look at the intended use, and the amount and weight of traffic.