At least some excavation is required for every building. Organic topsoil is subject to decomposition and to
shrinking and swelling with changes in moisture content. It is excellent for growing lawns and landscape plants but unsuitable for supporting buildings. Often, it is scraped away from the building area and stockpiled to one side for redistribution over the site after construction of the building is complete. After the topsoil has been removed, further digging is necessary to place the footings out of reach of water and wind erosion. In colder climates, foundations must be placed below the level to which the ground freezes in winter, the frost line, or they must be insulated in such a way that the soil beneath them cannot freeze.

Otherwise, a foundation can be lifted and damaged by soil that expands slightly as it freezes. Or, under certain soil and temperature conditions, upward migration of water vapor from the pores in the soil can result in the formation of  ice lenses, thick layers of frozen water crystals than can lift foundations by even larger amounts.

Excavation is required on many sites to place the footings at a depth where soil of the appropriate bearing capacity is available. Excavation is frequently undertaken so that one or more levels of basement space can be added to a building, whether for additional habitable rooms, for parking, or for mechanical equipment and storage. Where footings must be placed deep to get below the frost  line or reach competent soil, a basement is often bargain-rate space, adding little to the overall cost of the building.

In particulate soils, a variety of excavating machines can be used to loosen and lift the soil from the ground: bulldozers, shovel dozers, backhoes, bucket loaders, scrapers, trenching machines, and power shovels of every type. If the soil must be moved more than a short distance,
dump trucks come into use.

In rock, excavation is slower and many times more costly. Weak or highly fractured rock can sometimes be broken up with power shovels, tractor-mounted rippers, pneumatic hammers, or drop balls such as those used in building demolition. Blasting, in which explosives are placed  and detonated in lines of closely spaced holes drilled deep into the rock, is often necessary. In devel-oped areas where blasting is impractical, rock can be broken up with hydraulic splitters.

Excavation Support   If the site is sufficiently larger than the area to be covered by the building, the edges of the excavation can be sloped back or benched at an angle such that the soil will not slide back...

Dewatering   During construction, excavations must be kept free of standing water. Such water may come from precipitation or it may come from ground-water seepage originating from any of...

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