Organic particles may include decomposed plants and animals and living plant roots. The structure of soil is determined by the arrangement of particles, which account for the pore space within the soil that may be filled with air or water. The nature of soil is determined by the parent rock from which it came, the climate that has weathered it, the vegetation it contains, the topography where it lies, and the time it has had to mature. The U.S. Department of Agriculture, which maps and collects soil data, has cataloged more than 50,000 different varieties of soil in the United States. Civil engineers specializing in soil mechanics determine whether a soil substrate is suitable to build upon, and if not, what measures need to be taken to make it safe.
Civil engineers work with structures such as buildings, highways, dams, and bridges.
Resources Article Heiniger, Paolo. The Leaning Tower of Pisa. Describes how the tower has been leaning since first built, specifies the layers of soil beneath the monument, and discusses how modern technology is being used to keep the tower standing. The author is a member of the committee working to save the tower. Provides the history of the tower and its construction.