Gravel has a specific meaning in both Civil Engineering and Geology. In both cases it is related to the diameter of the grains which the soil is composed of. Gravels are composed of grains of sizes...
Don't know if it'll work for gravel, but in hauling asphalt, we have a couple different means. At every asphalt plant, there'll be a dispenser, which dispenses a soap and water mix to keep the beds...