The simplest way to store raw bulk solids is to pile them on the ground in the open air. This is satisfactory for the long-term storage of materials that do not deteriorate on exposure to the elements; for example, the seasonal stockpiling of coal at mines and power stations. For large stockpiles, permanent facilities are usually installed for distributing and reclaiming the material; traveling gantry cranes, grabs, and drag scrapers feeding belt conveyors are used. For small, temporary storage, mechanical shovels, front-end loaders, and trucks can be used. When the cost of recovery from the stockpile is large compared with the value of the stock held, storage in silos or bunkers should be considered. There is no upper limit to the amount of material that can be stored in open piles, but the design engineer should be concerned about visual impact and the possibility of water runoff contaminating local surface and ground water.