Shallow foundation

This article is about Shallow foundations. For other uses of the term Foundation, please see Foundation (disambiguation).

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 and vertical 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 older residential construction, most residential construction which includes a basement, and in many commercial structures.

A variation sometimes used in areas with expansive or soft soils is the waffle-slab foundation, where a waffle slab is constrcuted with the ribs bearing on the soil. This sort of foundation creates a stiff slab which resists bending, and which has a low contact area, minimizing the impact of soil movement on the foundation. This foundation can be relatively inexpensive to build, as the waffle forms are inexpensive mass-produced items.

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.

The distribution of loads in a mat slab helps reduce differential settlement due to non-uniform building loads.

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.

Post-tensioned slab foundations are designed to be able to bend without cracking in response to ground movements; they do not usually provide sufficient rigidity to prevent movement.

External links

• This Old House - Slab Foundations
• Slab-on-grade foundations for expansive clay soil