This document provides information on different types of shallow foundations that can be used to support buildings, including strip footings, pad footings, combined footings, strap footings, and raft foundations. It also discusses considerations for foundations in expansive black cotton soil, such as using pier foundations or under-reamed pile foundations to anchor the structure below the depth of moisture movement in the soil.

2. BUILDING
CONSTRUCTION

3. FOUNDATIO
N

4. O u r
T E A M
Dhruvan Shah
160280106105
Niyati Shah
160280106109
Meshwa Shah
160280106108
Dhruv Shah
160280106106
Preet Shah
160280106110

5. • Introduction
• Types of Foundations
• Shallow Foundations
• Types of Shallow Foundation
• Strip Footing
• Pad Footing
• Grillage Foundation
• Strap Footing
• Raft Foundation
• Foundation in Black Cotton Soil
Contents…

6. FOUNDATION
• Foundation is a structural part of a building on which a building
stands which transmits and distributes its own load and imposed loads
to the soil in such a way that the load bearing capacity of the foundation
bed is not exceeded.
• The solid ground on which the foundation rests, is called foundation
bed.

7. FUNCTIONS
• Reduction of load intensity
• Even distribution of load
• Provision of level surface
• Lateral stability
• Safety against undermining
• Protection against soil
movements

8. Essential Requirements of Good Foundation
• The foundations shall be constructed to sustain the dead and
imposed loads and to transmit these to the sub-soil in such a way
that pressure on it will not cause settlement which would impair
the stability of the building or adjoining structures.
• Foundation base should be rigid so that differential settlements
are minimized, specially for the case when super-imposed loads
are not evenly distributed.
• Foundations should be taken suficient1y deep to guard the
building against damage or distress caused by swelling or
shrinkage of the sub-soil.
• Foundations should be so located that its performanced may not
be affected due to any unexpected future influence.

9. Types of Foundation
Foundation maybe broadly classified under two heads
a) Shallow Foundations
b) Deep Foundations

10. Foundation
• Rectangular raft footing.
• 2 m depth and 7000 cubic
meter reinforced concrete.
• 490 piles
• Each has 1 m diameter.
• Half under compression with
depth of 20 m.
• Other half under testation
with depth of 30m.

12. SHALLOW
FOUNDATION

13. Shallow Foundation
• Shallow foundation is a type of foundation that transfers loads to
the very near the surface. Shallow foundations typically have
depth equal to or less than its width.

14. Types of Shallow Foundations
Follow are the types of shallow foundation –
• Strip footing
• Combined footing
• Strap footing
• Mat or raft foundation

15. 1) Strip Footing
• A strip footing is the one which provides a continuous
longitudinal bearing. Thus a spread footing for a continuous wall
is call a strip footing
Types of Strip Footing are as follows
a) Simple Strip Footing
b) Stepped Footing

16. Simple Strip Footing
• When the wall carries light loads or when the safe bearing pressure is very
high, the width of the footing found from the above expression would be
very small.
• The wall directly rests on the concrete base, and no masonry offsets are
provided since spread is not required.
• As a thumb rule, the width of concrete base should not be less than twice
the width of the wall. The thickness of concrete block should at least be
equal to offset a in the case of cement concrete and 3/2 a in the case of lime
concrete base.
• An Offset is the projection of the lower step from the vertical face of the
upper step.

19. Stepped Footing
• When the wall carries heavy loads or when the safe bearing pressure of
the soil is not very high, the base width B from the equation ,will be
much greater than (T+2a) where T = width of wall.
𝐵 =
𝑊
𝑞 𝑠
, W= Total super-imposed load on the base of footing
𝑞 𝑠 = Safe bearing pressure
In that case, it is essential to provide masonry offsets, to achieve larger
spread, before the load is transferred to concrete base. The height and
width of each offset should be so proportioned that rate of spread does not
exceed the permissible value for the masonry

20. • As per National Building Code, the angle of spread of the load
from the wall base to the outer edge of the ground bearing shall
not exceed the following values
• The implication of the above recommendations is that in order to
spread the bearing width from original T (width of walI) to B
(footing width), the minimum depth required would workout as
follows:

22. • In the case of brick walls, the offset should not be greater than 5 cm; the
corresponding height of each step would work out to be 10 cm. As a thumb
rule, the width B’ of the bottom brick course should not be less than twice
the width of the wall. In the case of stone masonry, the offsets may vary
from 7 ½ cm to 10 cm(max.) corresponding to a min. height of masonry
course equal to 15 cm and 20 cm respectively.

24. 2) Pad Footing
• A spread footing for a single column is either
known as the isolated footing or a Pad Footing
• Follow are the types of pad footing
a) Simple Pad Footing
b) Stepped pad footing
c) Footings for reinforced concrete columns

25. Simple Pad Footing
• Equation 1 is used to determine the base area A of a simple pad footing
• A=
𝑃
𝑞 𝑠
….(1) where P= total load transmitted by column, 𝑞 𝑠=safe bearing
pressure
• If P is small, or 𝑞 𝑠 is large, A will also be small. In that case, the footing may
consist of simple concrete block projecting out from the column face on all
sides. The shape of the footing is generally kept the same as that for the
column (i.e. trapezoidal square or circular)
• The value of offset a may vary from 10 to 20cm. As a thumb rule, the base
dimensions of the concrete base should not be less than twice the
appropriate lateral dimension of the column in that direction. The thickness
of concrete block should at least be equal to a in the case of cement concrete
and 3/2 a in the case of lime concrete base.

27. Stepped Pad Footing
• If the column load is more, or if the safe bearing pressure of the
soil is less, the base area found by Eq (1) will be large (much
greater than b + 2a).
• In that case, it is necessary to provide masonry offsets, to achieve
larger spread, before the load is transferred to the concrete base.
• The height and width of each offset should be so proportioned
that rate of spread does the stepped footing

29. Footings for reinforced concrete columns

30. Grillage Foundation
• Grillage foundation is used when heavy structural loads from
columns, piers or stanchions are required to be transferred to a
soil of low bearing capacity.
• Grillage foundation is often found to be lighter and more
economical. This avoids deep excavation and provides necessary
area at the base to reduce the intensity of pressure within
safe bearing capacity of soil
• Depending upon the material used in construction of grillage
foundation can be broadly divided in the following two
categories.
(a) Steel grillage foundation
(b) Timber grillage foundation

31. Steel Grillage Foundation
• Steel grillage foundation consists of steel beams also known as grillage
beams which are provided in single or double tiers. In case of double tier
grillage foundation, the top tier is laid at right angles to the bottom one.
• The beams are suitably spaced so as to provide facility for the placing and
compacting of concrete between them. A minimum clearance of 8 cm is
considered most suitable. In any case, the distance between the flanges of the
beams should not be more than one and half to two times the flange width
with a maximum of 30 cm.
• In order to protect the beams against corrosion, a minimum cover of 10 cm
is kept on the outer sides of the external beams as well as above the upper
flange of the top tier, Cover of concrete under the lower beam should not be
less than 15 cm.

33. Timber Grillage Foundation
• Timber grillage foundation is provided for heavily loaded timber
column or masonry wall.
• The foundation uses timber planks and timber beams in the place
of steel joists. This foundation is specially useful in water logged
areas where the bearing power of the soil is very low, and where
the steel beams may get corroded due to subsoil water
• No concrete is embedded between the timber joists. However, the
bottom concrete (provided in steel grillage foundation is replaced
by timber platform constructed of timber planks.

35. Combined Footing
Whenever two or more columns in a straight line are carried on a
single spread footing, it is called a combined footing.
Combined footings are provided only when it is absolutely
necessary, as
• When two columns are close together, causing overlap of adjacent
isolated footings
• Where soil bearing capacity is low, causing overlap of adjacent
isolated footings
• Proximity of building line or existing building or sewer, adjacent
to a building column.

36. • The combined footing may be
rectangular, trapezoidal or Tee-
shaped in plan.
• The geometric proportions and
shape are so fixed that the
centroid of the footing area
coincides with the resultant of
the column loads. This results
in uniform pressure below the
entire area of footing.

37. Rectangular combined footing
• Rectangular footing is provided when one of the projections of the footing
is restricted or the width of the footing is restricted.
• Longitudinally, the footing acts as an upward loaded beam spanning
between columns and cantilevering beyond.
• The footing is also subjected to transverse bending and this bending is
spread over a transverse strip near the column.

39. Combined Trapezoidal Footing
• The combined footing of R.C.C may be either provided without
longitudinal beam or it may have longitudinal beam

40. Strap Footing
• A strap footing comprises of two or more footings of individual
columns, connected by a beam, called a strap
• When a column is near or right next to a property limit, a square
or rectangular footing concentrically located under the column
would extend into the adjoining property, which may not be
permissible.
• if the distance between this column and the adjoining column is
large, the combined trapezoidal footing will be quite narrow, with
high bending moments. In that case, strap footing may be
provided.
• The function of the strap beam is to transfer the load of heavily
loaded outer column to the inner one. In doing so , the strap beam
is subjected to bending moment and shear force and it should be
suitably designed to withstand these.

42. Raft Foundation
• A raft foundation, also called a mat foundation, is essentially a
continuous slab resting on the soil that extends over the entire
footprint of the building, thereby supporting the building and
transferring its weight to the ground.
• A raft foundation is often used when the soil is weak, as it
distributes the weight of the building over the entire area of the
building, and not over smaller zones (like individual footings) or
at individual points (like pile foundations). This reduces
the stress on the soil.
• A raft foundation is also very good for basements. Foundations
are created by excavating soil in order to find strong, compact,
undisturbed natural soil that is at least a few feet below ground
level. This soil is much stronger than the loose soil at the surface.

43. • A raft foundation is also very good for basements. Foundations
are created by excavating soil in order to find strong, compact,
undisturbed natural soil that is at least a few feet below ground
level. This soil is much stronger than the loose soil at the surface.
• A true raft or mat is a flat concrete slab with uniform thickness
throughout the area, This is adopted only when the column
spacing is small and column loads are also relatively small.

44. • If the column loads are heavy, the slab under the columns is
thickened, as shown in Figure

45. • If the column spacing is large, and/or the column loads are heavy,
thickened bands may be provided along the column lines in both
the directions. These bands are cal1ed main and secondary beams.
If the loads are extremely heavy, two way grid structure made of
cellular construction may used

46. • Where basements are to be provided, the basement walls may be
used a ribs or deep beams

47. Foundation for Black Cotton Soil
• Black cotton soils and other expansive soils have typical
characteristics of shrinkage and swelling due to moisture
movement through them.
• During rainy season, moisture penetrates into these soils, due to
which they swell.
• During summer season, moisture moves out of the soil and
consequently, the soil shrinks. Shrinkage cracks are formed on the
ground surface. These shrinkage cracks sometimes also known as
tension cracks, may be 10 to 15 cm wide on the ground surface
and may be ½ to 2 m deep

49. • In designing footings on these soils, the following points should be kept in mind:
1. The safe bearing capacity should be properly determined,
2. The foundation should be taken atleast 50 cm lower than the depth of moisture
movement. This depth should also be much more than depth of tension cracks.
3. The foundation should be laid on non-shrinkable non- expansive soil.
4. Where the depth of clay layer is large, the foundation or footing should be prevented
from coming in contact with the soil.
5. Where the soil is highly expansive, it is very essential to have minimum contact
between the soil and the footing.
6. Where the bearing capacity of soil is poor, or soil is very soft, the bed of the
foundation trench should be made firm or hard by ramming mooram and ballast into
it.
7. Moisture should not penetrate the foundation during rainy season

50. • Types of Foundations in Black Cotton Soils are :
a) Strip or Pad Foundation
b) Pier Foundation
c) Under-reamed pile foundation

51. 1) Strip or Pad Foundation
• For medium loads, strip foundation (for walls) and pad
foundation (for columns) may be provided, along with special
design features
• when the soil, though expansive, have little swelling pressure. A
60 cm thick layer of cohesion less sand is placed below the
foundation concrete, and is compacted. Sand is also filled around
the footing. When the soil swells, the sand grains would yield by
moving up, thus relieving the swelling pressure.
• When the soil shrinks, the sand layer would expand, but there will
be no discontinuity in the soil support. Sand fill should also be
used below flooring. a

53. • Where the swelling pressures are relatively high. The alternate
layers of mooram (or ballast) and sand act as a spring which can
compress or expand along with the sub-soil movements.
• It will, thus absorb all the movements, thus keeping the footing
free from these effects. If the soil is soft and has poor bearing
capacity, a 30 cm thick layer of ballast and mooram should first be
rammed into the soil.
• Over the top of it, a min. of 30 cm thick layer of coarse grained
sand may be placed. In all the three cases, the foundation concrete
may be done in rigid cement concrete, and if possible, it may
contain nominal reinforcement

55. • For soils of high swelling pressure, and having high shrinkage properties.
After compacting the base of the trench, 25 to 30 cm wide strips of concrete,
25 to 30 cm thick, may first be laid and compacted.
• After the strip concrete is cured, the space between the two is filled with
sand. The space between the two strips of concrete(i.e. width of sand fill)
may be kept equal to width of the bottom course of masonry.
• On the top of this, the foundation concrete layer, preferably of reinforced
concrete is laid. The sides of the masonry footings is filled with sand as
usual.
• In addition to this, 80 mm dia. pipes spaced at 1.5 to 2 m etc. are placed
through masonry and concrete bed, so as to reach the bottom sand fill a
shown, and sand is filled in the pipe.

57. 2) Pier Foundation
• Piers are dug at regular interval and filled with cement
concrete. The piers may rest on good bearing strata.
• These piers are be connected by concrete or masonry arch,
over which the wall may constructed.
• The arches are constructed with a gap above the ground
level. This gap would permit free vertical movement of soil
during swelling and shrinkage operations.

59. Under-reamed pile foundation
• An under-reamed pile is a pile of shallow depth (1 to 6 m) having
one bulb a its lower end.
• If this bulb is taken or provided at a level lower than the critical
depth of moisture movement in expansive soils, the foundation
will be anchored to the ground and it would not move with the
movement (i.e. swelling and shrinkage) of the soil.
• The piles are connected by a rigid capping beam, suitably
reinforced, over which the wall is constructed. The capping beam
is kept 8 to 12 cm above the ground level, so as to provide air gap
to accommodate the soil movements without adversely affecting
the super-structure.