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A shallow foundation is a type of building foundation that transfers
structural load A structural load or structural action is a mechanical load (more generally a force) applied to Structural engineering#Structural elements, structural elements. A load causes stress (physics), stress, deformation (engineering), deformation, displa ...
to the earth very near to the surface, rather than to a subsurface layer or a range of depths, as does a
deep foundation A pile or piling is a vertical structural element of a deep foundation, driven or drilled deep into the ground at the building site. A deep foundation is a type of foundation (architecture), foundation that transfers building loads to the e ...
. Customarily, a shallow foundation is considered as such when the width of the entire foundation is greater than its depth. In comparison to deep foundations, shallow foundations are less technical, thus making them more economical and the most widely used for relatively light structures.


Types

Footings are always wider than the members that they support. Structural loads from a
column A column or pillar in architecture and structural engineering is a structural element that transmits, through compression, the weight of the structure above to other structural elements below. In other words, a column is a compression member ...
or wall are usually greater than 1,000 kPa, while the soil's bearing capacity is commonly less than that (typically less than 400 kPa). By possessing a larger bearing area, the foundation distributes the pressure to the soil, decreasing the bearing pressure to within allowable values. A structure is not limited to one footing. Multiple types of footings may be used in a construction project.


Strip footing

Also called a ''wall footing,'' a strip footing is a continuous strip that supports structural and non-structural load-bearing walls. Found directly under the wall, its width is commonly 2-3 times wider than the wall above it.


Isolated footing

Also called ''single-column footing'', an isolated footing is a square, rectangular, or circular slab that supports the structural members individually. Generally, each column is set on an individual footing to transmit and distribute the load of the structure to the soil underneath. Sometimes, an isolated footing can be sloped or stepped at the base to spread greater loads. This type of footing is used when the structural load is relatively low, columns are widely spaced, and the soil's bearing capacity is adequate at a shallow depth.


Combined footing

When more than one column shares the same footing, it is called a ''combined footing''. A combined footing is typically utilized when the spacing of the columns is too restricted such that if isolated footing were used, they would overlap one another. Also, when property lines make isolated footings eccentrically loaded, combined footings are preferred. When the load among the columns is equal, the combined footing may be rectangular. Conversely, when the load among the columns is unequal, the combined footing should be
trapezoidal In geometry, a trapezoid () in North American English, or trapezium () in British English, is a quadrilateral that has at least one pair of parallel sides. The parallel sides are called the ''bases'' of the trapezoid. The other two sides are ...
.


Strap footing

A strap footing connects individual columns with the use of a strap beam. The general purpose of a strap footing is alike to those of a combined footing, where the spacing is possibly limited and/or the columns are adjacent to the property lines.


Mat foundation

Also called ''raft'' foundation, a mat foundation is a single continuous slab that covers the entirety of the base of a building. Mat foundations support all the loads of the structure and transmit them to the ground evenly. Soil conditions may prevent other footings from being used. Since this type of foundation distributes the load coming from the building uniformly over a considerably large area, it is favored when individual footings are unfeasible due to the low bearing capacity of the soil.


Slab-on-grade foundation

''Slab-on-grade'' or ''floating slab'' foundations are a
structural engineering Structural engineering is a sub-discipline of civil engineering in which structural engineers are trained to design the 'bones and joints' that create the form and shape of human-made Structure#Load-bearing, structures. Structural engineers also ...
practice whereby the
reinforced concrete Reinforced concrete, also called ferroconcrete or ferro-concrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having higher tensile strength or ...
slab that is to serve as the foundation for the structure is formed from formwork set into the ground. The concrete is then poured into the formwork, leaving no space between the ground and the structure. This type of construction is most often seen in warmer climates, where ground freezing and thawing is less of a concern and where there is no need for heat ducting underneath the floor. Frost Protected Shallow Foundations (or FPSF) which are used in areas of potential frost heave, are a form of slab-on-grade foundation. Remodeling or extending such a structure may be more difficult. Over the long term, ground settling (or
subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...
) may be a problem, as a slab foundation cannot be readily jacked up to compensate; proper soil compaction prior to pour can minimize this. The slab can be decoupled from ground temperatures by insulation, with the concrete poured directly over insulation (for example, extruded polystyrene foam panels), or heating provisions (such as hydronic heating) can be built into the slab. Slab-on-grade foundations should not be used in areas with
expansive clay Expansive clay, also called expansive soil, is a clay soil prone to large volume changes (swelling and shrinking) directly related to changes in water content. Soils with a high content of expansive minerals can form deep cracks in drier seasons ...
soil. While elevated structural slabs actually perform better on expansive clays, it is generally accepted by the engineering community that slab-on-grade foundations offer the greatest cost-to-performance ratio for tract homes. Elevated structural slabs are generally only found on custom homes or homes with basements.
Copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...
piping, commonly used to carry
natural gas Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...
and
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
, reacts with concrete over a long period, slowly degrading until the pipe fails. This can lead to what is commonly referred to as slab leaks. These occur when pipes begin to leak from within the slab. Signs of a slab leak range from unexplained dampened carpet spots, to drops in water pressure and wet discoloration on exterior foundation walls. Copper pipes must be ''lagged'' (that is, '' insulated'') or run through a conduit or plumbed into the building above the slab. Electrical conduits through the slab must be water-tight, as they extend below ground level and can potentially expose wiring to
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
.


Vertical bearing capacity

The ultimate bearing capacity of a shallow foundation depends on whether a total stress or
effective stress Effective stress is a fundamental concept in soil mechanics and geotechnical engineering that describes the portion of total stress in a soil mass that is carried by the solid soil skeleton, rather than the pore water. It is crucial for understan ...
analysis is considered. Generally, a total stress analysis is applicable for low permeability soils (
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...
) over the short-term. For high permeability soils (
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...
) or for low permeability soils over the long-term, an effective stress analysis is more appropriate.


Short-term total stress analysis

For a total stress analysis, the ultimate bearing capacity of a shallow foundation is given by the following equation: \sigma_f = s_c d_c N_c s_u + \gamma h where * s_c is the shape factor which can be conservatively taken as 1 * d_c is the depth factor which can be conservatively taken as 1 * N_c is the bearing capacity factor, which is numerically equal to 2 + \pi \approx 5.14 * s_u is the undrained shear strength of the soil * \gamma is the unit weight of the soil adjacent to the footing * h is the height from the founding plane to the ground surface


Effective stress analysis

For an
effective stress Effective stress is a fundamental concept in soil mechanics and geotechnical engineering that describes the portion of total stress in a soil mass that is carried by the solid soil skeleton, rather than the pore water. It is crucial for understan ...
analysis, the ultimate bearing capacity of a shallow foundation is given by the following equation. \sigma'_f = s_q d_q N_q \sigma'_ + s_\gamma d_\gamma N_\gamma \cdot (0.5 \gamma B - \Delta u) where * s_q and s_\gamma are analogous shape factors which can be conservatively taken as 1 * d_q and d_\gamma are analogous depth factors which can be conservatively taken as 1 * N_q is the bearing capacity factor (generally taken as K_p e^ * \sigma'_ is the in situ vertical effective stress acting at the level of the foundation base * N_\gamma is analogous to the bearing capacity factor N_q * \gamma is the unit weight of the soil adjacent to the footing * B is the width of the footing * \Delta u is the increase in pore water pressure between the founding plane and a depth of B/2 below it.


See also

* Argillipedoturbation *
Building construction Construction are processes involved in delivering buildings, infrastructure, industrial facilities, and associated activities through to the end of their life. It typically starts with planning, financing, and design that continues until the a ...
*
Construction engineering Construction engineering, also known as construction operations, is a professional subdiscipline of civil engineering that deals with the designing, planning, construction, and operations management of infrastructure such as roadways, tunnels, bri ...
*
Fiber reinforced concrete Fiber-reinforced concrete or fibre-reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers inclu ...
* Grade beam *
Precast concrete Precast concrete is a construction product produced by casting concrete in a reusable molding (process), mold or "form" which is then cured in a controlled environment, transported to the construction site and maneuvered into place; examples i ...
*
Prestressed concrete Prestressed concrete is a form of concrete used in construction. It is substantially prestressed (Compression (physics), compressed) during production, in a manner that strengthens it against tensile forces which will exist when in service. Post-t ...
*
Rebar Rebar (short for reinforcement bar or reinforcing bar), known when massed as reinforcing steel or steel reinforcement, is a tension device added to concrete to form ''reinforced concrete'' and reinforced masonry structures to strengthen and aid ...
* Steel fixer * Tie rod


References


External links


Raft or Mat Foundations
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