Shallowfoundations

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 at
the base of the substructure
DEEP - Deep foundations transfer loads far below the
substructure. These foundations penetrate incompetent soil until a
satisfactory 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 FOUNDATION
Shallow Foundation System
i) Spread Foundation
ii) Combined foundation
iii) Mat / Raft Foundation

SPREAD FOOTING FOUNDATION
 It’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 FOOTING
This 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 Footings
 A 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 foundations
Mat-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 FOUNDATION
1- Cost (affordable)
2- Construction Procedure (simple)
3- Materials (mostly concrete)
4- Labour (does not need expertise)
DISADVANTAGES OF USING SHALLOW FOUNDATION
1- Settlement
2- Limit Capacity * Soil * Structure
3- Irregular ground surface (slope, retaining wall)
4- Foundation subjected to pull out, torsion, moment.

Shallowfoundations

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