This document discusses different types of foundations, focusing on mat/raft foundations. It defines a foundation as connecting a structure to the ground and transmitting the structure's weight. Shallow foundations have a depth less than or equal to their width, while deep foundations have a greater depth. Mat/raft foundations are continuous footings used when individual footings would cover too much area or to reduce settlement over compressible soil. The document discusses design approaches like rigid and flexible, and using finite element analysis to model mat foundations considering soil properties, applied loads, and structural responses.

1. COMPUTER AIDED ADVANCED
FOUNDATION DESIGN

2. WHAT IS A FOUNDATION ?
A foundation is the part of a structure which transmits the
weight of the structure to the ground. All structures
constructed on land are supported on foundations. A
foundation is a connecting link between the structure
proper and the ground which supports it.

3. TYPES OF FOUNDATION
SHALLOW FOUNDATION
• DEPTH OF FOUNDATION≤ BREADTH
OF FOUNDATION
DEEP FOUNDATION
• WHEN DEPTH OF FOUNDATION IS
MORE THAN WIDTH OF FOUNDATION

4. Karl von Terzaghi (Father of soil mechanics)
was the first to present a comprehensive theory for
the evaluation of the ultimate bearing capacity of
rough shallow foundations. This theory states that
a foundation is shallow if its depth is less than or
equal to its width.
 Later investigations, however, have suggested
that foundations with a depth, measured from the
ground surface, equal to 3 to 4 times their width
may be defined as shallow foundations.

5. SHALLOW FOUNDATION
A shallow foundation is a type of foundation which
transfers building loads to the earth very near the surface,
rather than to a subsurface layer or a range of depths as
does a deep foundation.
Shallow foundations include spread footing foundations,
mat-slab foundations, slab-on-grade foundations, pad
foundations, rubble trench foundations and earthbag
foundations.

6. WHY MAT/RAFT FOUNDATION IS
REQUIRED ?
If the loads transmitted by the columns in a structure are
so heavy or the allowable soil pressure so small that
individual footings would cover more than about one-half
of the area, it may be better to provide a continuous
footing under all columns and walls. Such a footing is
called a raft or mat foundation.
Raft foundations are also used to reduce the settlement
of structures located above highly compressible deposits.

7. SHAPES OF RAFT/MAT
FOUNDATION
 A raft foundation may be rectangular or circular and may be
with or without an open as shown

8. DESIGN APPROACH AND
CONSIDERATIONS
Basically there are two approaches which have
been suggested for analyzing the behavior of raft
foundation.
Flexible Foundation Approach
Rigid Foundation Approach

9. RIGID APPROACH
In Rigid Approach It is presumed that raft is rigid
enough to bridge over non uniformities of soil
structures.
Pressure distribution is considered to be either
uniform or varying linearly.
Design of rigid raft follows conventional methods
where again following two approaches have been
suggested.
Inverted Floor System
Combined Footing Approach

10. FLEXIBLE APPROACH
In flexible foundation approach, Raft is considered
to distribute loads in the area immediately
surrounding the column depending upon soil
characteristics.
In this approach differential settlement Is
comparatively larger but bending moment and
shear force to which raft is subjected is low.
Analysis is suggested basically on two approach
(1) Flexible Plate supported on elastic foundation
i.e., Hetenyi’s Theory.
(2) Foundation supported on bed of uniformly
distributed elastic springs with a spring constant
determined using coefficient of sub-grade reaction
i.e., Winkler’s Foundation

11. FINITE ELEMENT ANALYSIS
Finite element analysis (FEA) is a computerized
method for predicting how a product reacts to real-
world forces, vibration, heat, fluid flow, and other
physical effects.
 Finite element analysis shows whether a product
will break, wear out, or work the way it was
designed.

12. FEM IN RAFT FOUNDATION
 Finite element method transforms the problem of plates on
elastic foundation into a computer oriented method of matrix
structural analysis method.
 In this method, plate is idealized as a mesh of finite element
interconnected only at the nodes(corners), and the soil may
be modeled as a set of isolated springs or as an elastic
isotropic half space.
 The matrix structural analysis is extended to include the
influence of the superstructure, the foundation and the soil
can be accounted for.
 It is possible to consider different values of subgrade
modulus in different areas of raft foundation

13. OUR CONSIDERATIONS
We took a raft of (30 x 35)m, which is distributed
into (500x500)mm division as shown.

14. PLATE THICKNESS
 We took a plate thickness of 300mm

15. SELF WEIGHT AND APPLIED
LOADS
 Self weight has a factor of (-1).
 Concentrated loads of columns on selected nodes is 250t.

16. SUPPORTS
 Here soil spring constant is applied to each node at an influence area of
1m x 1m.
 In our case spring is released in Mx, My, and Mz direction and Fx , Fy
and Fz has a value of 937.5 KN/m.

17. PLATE STRESS COUNTOUR
 Shear Force in X- Direction i.e., SQX(LOCAL) which has
maximum and minimum value of 1.34 & 1.33 N/mm2

18. MOMENT STRESS CONTOUR
 The moment in x-direction i.e., Mx has maximum and
minimum values of -87.7 kNm/m to 247KNm/m respetively.

19. GLOBAL MOMENT CONTOUR
 It has a maximum value of 87.7KNm/m and minimum value
of -248KNm/m.

20. MOMENT STRESS CONTOUR
 The moment in y–direction i.e., My has maximum and
minimum values of -75.8 to 248 KNm/m respectively.

21. SPECIAL THANKS
 To Our Head Of Department of Civil Engineering,
Dr. ARUP SAHA CHAUDHURI ,PhD(IITB),MIBC, MASCE-
SEI(USA)
THANK YOU