Shallow foundation: Difference between revisions
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Revision as of 22:42, 6 June 2007
A shallow foundation is a type of foundation which tranfers building loads to the earth very near the surface, rather than to a subsurface layer or a range of depths as does a deep foundation. Shallow foundations include spread footing foundations, mat-slab foundations, and slab-on-grade foundations
Spread footing foundation
Spread footing foundations consists of strips or pads of concrete (or other materials) which transfer the loads from walls and columns to the soil or bedrock. Embedment of spread footings is controlled by several factors, including development of lateral capacity, penetration of soft near-surface layers, and penetration through near-surface layers likely to change volume due to frost heave or shrink-swell.
These foundations are common in residential construction that includes a basement, and in many commercial structures.
Slab-on-grade foundation
Slab-on-grade foundations are slabs-on-grade used to transmit structure loads directly to the soil. Slab-on-grade foundations come in several varieties: simple slabs-on-grade, mat-slab foundations, and post-tensioned slab foundations. Simple slabs on grade, or slabs-on-grade with thickened edges, are used for lightweight buildings where the soil is not prone to movement, or where movement of the slab is acceptable for the intended use of the structure.
Mat-slab foundations
Mat-slab foundations are concrete slabs-on-grade used to distribute heavy column and wall loads across the entire building area, to lower the contact pressure compared to conventional spread footings. Mat-slab foundations can be constructed near the ground surface, or at the bottom of basements. In high-rise buildings, mat-slab foundations can be several meters thick, with extensive reinforcing to ensure relatively uniform load transfer.
Post-tensioned slab foundations
Post-tensioned slab foundations are slab-on-grade foundations reinforced with post-tensioning cables, to place the concrete in a state of compression, to minimize cracking from bending. Post-tensioned slab foundations are commonly used in areas with expansive clay soil, particularly in California and Texas, and are often cheaper to construct than conventionally-reinforced slabs.