About raft foundation.

1. RAFT FOUNDATION

2. INTRODUCTION
 A raft or mat foundation is a thick
reinforced concrete slab supporting
arrangement o columns or walls in a
row and transmitting the load into the
soil.
 It is used to support storage tanks,
industrial equipments, chimney’s and
various structures.

3. NECESSITY
 The spread footings cover over 50% of the
foundation area because of large column load.
 The soil is soft with low bearing capacity.
 When the expenses of deep foundation is higher
than the raft foundation.
 Walls of the structure are so close that individual
footing will overlap.

4. Working Principle of Raft Foundation
Raft foundation transmits the total load from the
building to the entire ground floor area. Stress
distribution mechanism of raft foundation is very
simple.Total weight of the structure and self-
weight of the mat is calculated and is divided by the
total area of the foundation it is covering to
calculate the stress on the soil.
As in case of raft foundation the contact area of the
foundation with soil is much more than any other
type of foundation, so the load is distributed over a
larger area and thus the stress on soil is lesser and
the possibility of shear failure of soil is also reduced.

5. TYPES OF MAT/RAFT FOUNDATIONS
1. Flat Plate Raft
2. PlateThickened Under Columns
3. Two-Way Beam and Slab
4. Plate With Pedestals
5. Piled Raft
6. Rigid Frame Mat

6. 1. FLAT PLATE RAFT
This is the simplest form of raft foundation. This type of mat
is used when the columns and walls are uniformly spaced
at small intervals and the subjected loads are relatively
small. Reinforcement is placed in both directions and more
reinforcement is required at the column locations and load-
bearing walls. The thickness of this types of raft foundation
is generally restricted within 300mm for economic reason.
A thicker slab would not be economical.

8. 2.PLATE THICKENED UNDER COLUMNS
When the columns and load bearing walls
are subjected to heavier loads, the slab is
thickened under the columns and walls and
extra reinforcement is provided to resist
against diagonal shear and negative
reinforcement.

10. 3. TWO-WAY BEAM AND SLAB
In this type of raft, beams are cast
monolithically with the raft slab connecting
the columns and walls. This type of raft is
suitable when the columns are placed at a
larger distance and the loads on the columns
are variable.

12. 4. PLATES WITH PEDESTALS
The function of this mat is same as
that of flat plate thickened under
columns. In this mat pedestals are
provided at the base of the columns.

14. 5. PILED RAFT
This type of raft foundation is
supported on piles. A piled raft is used
when the soil at a shallow depth is
highly compressible and the water
table is high. Piles under raft help in
reducing settlement and provides
resistance against buoyancy.

16. 6. RIGID FRAME MAT
In this type of raft, the foundation walls act as a deep
beam. Rigid frame mat is referred when columns
carry extremely heavy loads and the connecting
beams exceeds 90cm depth. Here two concrete slabs
are placed, one on top of another and connected with
foundation walls in both directions and thus forms a
cellular raft foundation. This type of raft is very rigid
and is economical when the required slab thickness is
very high.

18. ADVANTAGES
 Raft or mat foundation is economic due to
combination of foundation and floor slab.
 It requires little excavation.
 It can cope with mixed or poor ground condition.
 It reduces differential settlement.

19. DISADVANTAGES
 In some cases, large amount of reinforcement is
required for raft foundation which increases the cost
of project.
 Special attention on raft foundations is required in
case of concentrated loads.
 If they are not treated properly, there is a chance of
edge erosion.
 Skilled workers are required to construct the raft
foundations.

20. DESIGN OF RAFT FOUNDATION
According to IS – 2950:1965, the design criteria of
raft footings are given below:The maximum
differential settlement in foundation on clayey soils
and sandy soils should not exceed 40mm and 25mm
respectively.The maximum settlement should
generally be limited to the following values:
• Raft foundation on clay – 65 to 100 mm.
• Raft foundation on sand – 40 to 65 mm.
There are two methods for the design of raft foundations.
They are:
1. Conventional Method
2. Soil Line Method.

21. Costs of raft foundations
As with any form of construction, providing accurate costs for
a ‘typical’ project is very difficult, since so much depends on
the specific site conditions and the project specification.
However, since excavations are generally much less than with
traditional foundations (if balancing rafts are excluded), spoil
removal costs are generally much lower. Site excavation also
takes less time, as does preparation and concrete pouring
times, which reduces labour costs. In general, for sites
without complex structural load requirements, raft
foundations cost significantly less than traditional
approaches, and should at least be considered for every
project.The final choice of the engineer will depend on
structural characteristics (e.g. weight of the superstructure,
arrangement of vertical elements in plan) as well as on the

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