SHALLOW FOUNDATIONS

Sampath Kathroju

A foundation is the lowest and supporting layer of a structure and a building component which transfers building loads to the soil.

There are two basic types of foundations:

SHALLOW - Shallow foundations transfer the load to soil atthe base of the substructure

DEEP - Deep foundations transfer loads far below thesubstructure. These foundations penetrate incompetent soil until asatisfactory bearing stratum is reached.

The objective of shallow foundation is to distribute the structural concentrated load over a wide horizontal area at a little depth rather than a range of the depths.

Shallow foundation is often selected when the soil has good bearing capacity and the structural load will not cause excessive settlement of the underlying soil layers.

shallow foundations are more simple and cost effective to construct than deep foundations because little soil is removed or disturbed.

This foundation is usually utilized in residential and light commercial buildings.

TYPES OF FOUNDATIONShallow Foundation System

i) Spread Foundationii) Combined foundation iii) Mat / Raft Foundation

SPREAD FOOTING FOUNDATIONIt’s an enlargement at the bottom of a column/

bearing wall that spreads the applied structural loads over a sufficiently large soil area.

Each column & each bearing wall has its own spread footing, so each structure may include dozens of individual footings.

Most common type of foundation used due to their low cost & ease of construction.

Most often used in small to medium size structure with moderate to good soil condition.

Spread footings may be built in different shapes & sizes to accommodate individual needs such as the following: a) Square Spread Footings / Square Footings b) Rectangular Spread Footings c) Circular Spread Footings d) Continuous Spread Footings e) Combined Footings f) Ring Spread Footings

a) Square Spread Footings / Pad Foundation - support a single centrally located column - use concrete mix 1:2:4 and reinforcement - the reinforcement in both axes are to resist/carry tension loads. b) Rectangular Spread Footings - Useful when obstructions prevent construction of a square footing with a sufficiently large base area and when large moment loads are present

c) Circular Spread Footings - are round in plan view - most frequently used as foundation for light standards, flagpoles and power transmission lines.

d)Continuous Spread Footings / Strip Foundation - Used to support bearing walls

f) Ring Spread Footings - continuous footings that have been wrapped into a circle - commonly used to support the walls above-ground circular storage tanks. - The contents of these tanks are spread evenly across the total base area and this weight is probably greater that the tank itself - Therefore the geotechnical analyses of tanks usually treat them as circular foundations with diameters equal to the diameter of the tank.

COMBINED FOOTINGThis type of footing is adopted when the space between two columns is so small that the foundation for individual columns will overlap.

Combined footings are proportioned in such a way that the centre of gravity of the loads coincides with the center of gravity of the foundation. Hence these footings have either a trapezoidal or a rectangular shape.

Cantilever or Strap FootingsA strap footing is used to connect an eccentrically loaded

column footing to an interior column.The strap is used to transmit the moment caused from an

eccentricity to the interior column footing so that a uniform soil pressure is generated beneath both footings.

The strap footing may be used instead of a rectangular or trapezoidal combined

footing if the distance between columns is large and / or the allowable soil pressure is relatively large so that the additional footing area is not needed.

Mat-slab foundationsMat-slab foundations are 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.It is a flat concrete slab, heavily reinforced with steel, which carries the downward loads of the individual columns or walls.

Mat Foundation often considered to be used when dealing with the following

conditions:

The structural loads are so high or the soil condition so poor that spread footings would be exceptionally large

The soil is very erratic & prone to excessive differential settlements. The structure continuity and flexural strength of a mat will bridge over these irregularities.

The uplift loads are larger than spread footings can accommodate. The greater weight and continuity of a mat may provide sufficient resistance.

The bottom of the structure is located below the groundwater table, so waterproofing is an important concern. Because mats are monolithic, they are much easier to waterproof. The weight of the mat also helps resist hydrostatic uplift forces from the groundwater.

USES Shallow foundations are used in many applications in highway

projects when the subsurface conditions are appropriate.

Such applications include bridge abutments on soil slopes or embankments, bridge intermediate piers, retaining walls, culverts, sign posts, noise barriers, and rest stop or maintenance building foundations.

Footings or mats may support column loads under buildings. Bridge piers are often supported on shallow foundations using various structural configurations.

ADVANTAGES OF USING SHALLOW FOUNDATION1- Cost (affordable)2- Construction Procedure (simple) 3- Materials (mostly concrete)4- Labour (does not need expertise)

DISADVANTAGES OF USING SHALLOW FOUNDATION1- Settlement2- Limit Capacity * Soil * Structure3- Irregular ground surface (slope, retaining wall)4- Foundation subjected to pull out, torsion, moment.