Useful for Masters course

1. FOUNDATION
GEOLOGY

2. Major Building Parts
Superstructure
Substructure
Foundation
R. R. Gadgil, Dept. of Earth Science, Goa University 2

3. Introduction
■ “The foundation of a building is that part of walls, piers and columns in direct
contact with the ground and transmitting loads to the ground.”
■ Every building needs a foundation of some kind.
■ Because of the variety of soil, rock, and water conditions that are encountered
below the surface of the ground and the unique demands that buildings make
upon their foundations, foundation design is a highly specialized field
combining aspects of geotechnical and civil engineering.
R. R. Gadgil, Dept. of Earth Science, Goa University 3

4. Purpose of foundation
■ To distribute the load of the structure over a large bearing area so as to bring the
intensity of load within the safe bearing capacity of soil.
■ To load the bearing surface at a uniform rate to avoid differential settlement.
■ To prevent the lateral movement of supporting material.
■ To attain a level and firm bed for building operations.
■ To increase the stability of the structure as a whole.
R. R. Gadgil, Dept. of Earth Science, Goa University 4

5. General Considerations
■ The exploratory program for the foundation of a building essentially depends on 2 factors:
1. The weight of the building and other forces acting on it and
2. The service of the building or the purpose for which it is being built.
 As to the service of the building, there are four categories, namely,
1. Residential buildings including housing projects
2. Commercial buildings
3. Industrial buildings
4. Pumping and power plants.
 Monuments and palaces do not fall into any category.
R. R. Gadgil, Dept. of Earth Science, Goa University 5

6. Structural Loads
■ The weight of the building itself including its appurtenances is the Dead load (DL).
■ The weight of the loads applied to the building intermittently is the live load (LL).
■ For rough computations, the live load is sometimes estimated at 50% of the dead
load.
■ Generally the dead and the live loads are transmitted to the foundations vertically.
■ Such is the case when all the loads are taken up by the walls and columns and then
passed to the footings.
■ Roofs over large halls, such as auditoriums or mess halls are sometimes designed in
the form of arches.
R. R. Gadgil, Dept. of Earth Science, Goa University 6

7. Structural Loads
■ Besides dead and live loads, there are lateral forces acting on the building.
■ These are mainly wind and earthquake forces which act laterally and tend to
push the building sideways, where they must be resisted by the shearing
strength of the materials in which the foundations are built and the passive
resistance of the soil mass supporting the building.
■ The resultant of vertical and lateral forces at the foundation generates two
types of stresses- compressive and shear, the former causing reduction in
volume of the material and consequent settlement leading to deformation,
while the latter tends to slide the base of the foundation over the
underlying material.
■ Other forces acting on some buildings are vibrations.
■ These are particularly evident in power and pumping plants or in industrial
buildings containing large machines.
R. R. Gadgil, Dept. of Earth Science, Goa University 7

8. Types of foundation
■ There are two basic types of foundations
1. Shallow foundation
2. Deep foundation
■ Shallow foundation
– The foundation provided immediately below the lowest part of the structure
near the ground level, transferring load directly to the supporting soil, is
known as shallow foundation.
– Shallow foundation is provided when stable soil with adequate bearing
capacity occur near to the ground level.
– Requirements
 Suitable soil bearing capacity
 Undisturbed soil or engineered fill
R. R. Gadgil, Dept. of Earth Science, Goa University 8

9. 1) Types of Shallow foundation
a) Spread footing or open trench foundation
b) Raft/mat foundation
c)Rubble trench foundation
R. R. Gadgil, Dept. of Earth Science, Goa University 9

10. Footing
■ Footings distribute the load to the subsoil over an area sufficient to suit the
pressures to the properties of the soil or rock.
■ Their size is therefore governed by the strength of the foundation
materials.
■ If the footing supports a single column it is known as a spread or pad
footing whereas a footing beneath a wall is referred to as a strip or
continuous footing.
■ Settlement of a footing due to a given load per unit area of its base, is a
complex function of the dimensions of the base and of the compressibility,
permeability and Poisson's number of the foundation materials located
between the base and a depth which is at least equal to three times the
width of the base.R. R. Gadgil, Dept. of Earth Science, Goa University 10

11. Spread footing
■ A spread footing foundation, which is typical in residential
building, has a wider bottom portion than the load-bearing
foundation walls it supports. This wider part "spreads" the
weight of the structure over more area for greater stability.
■ The design and layout of spread footings is controlled by several
factors, foremost of which is the weight (load) of the structure it will
support.
■ These foundations are common in residential construction that
includes a basement, and in many commercial structures. But for high
rise buildings they are not sufficient.
R. R. Gadgil, Dept. of Earth Science, Goa University 11

12. Spread footing in stone masonry
R. R. Gadgil, Dept. of Earth Science, Goa University 12

13. Isolated Spread footing
R. R. Gadgil, Dept. of Earth Science, Goa University 13

14. Spread footing
■ There are 3 basic types of spread footings
1. Individual footings: Generally supporting a column (either an outside wall
column or an interior column)
2. Continuous or wall footings
3. The mat or raft foundations
R. R. Gadgil, Dept. of Earth Science, Goa University 14

15. Spread footing
■ Individual footings may be an one of 3 varieties
1. Shallow slab type
2. Battered type
3. Stepped type
R. R. Gadgil, Dept. of Earth Science, Goa University 15

16. Continuous footing
■ It is merely a wall with a
widened base resting on the
subsurface material.
■ This type is used preferably
for light buildings such as
residences, one-story school
buildings or small, one-story
warehouses.
R. R. Gadgil, Dept. of Earth Science, Goa University 16

17. Continuous footing
R. R. Gadgil, Dept. of Earth Science, Goa University 17

18. Isolated stepped type footing
R. R. Gadgil, Dept. of Earth Science, Goa University 18

19. Raft/mat/slab foundation
■ Raft foundation is a thick concrete slab reinforced with steel which
covers the entire contact area of the structure like a thick floor.
■ The reinforcing bars runs normal to each other in both top and bottom
layers of steel reinforcement.
■ Sometimes inverted main beams and secondary beams are used to
carry column loads that require thicker foundation slab considering
economy of the structure.
■ Used generally for higher loads and prevention of excessive
settlements.
R. R. Gadgil, Dept. of Earth Science, Goa University 19

20. Raft foundation
R. R. Gadgil, Dept. of Earth Science, Goa University 20

21. Raft/mat/slab foundation
■ The chief function of a raft is to spread the building load over as great an area of
ground as possible and thus reduce the bearing pressure to a minimum.
■ In addition a raft provides a degree of rigidity which reduces differential
movements in the superstructure.
■ The settlement of a raft foundation does not depend on the weight of the building
which is supported by the raft.
■ It depends only on the difference between this weight and the weight of the soil
which is removed prior to the construction of the raft.
■ A raft can be built at a sufficient depth so that the weight of soil removed equals
the weight of the building.
■ Hence such rafts are sometimes called floating or semi-floating foundations.
■ The success of this type of foundation structure in overcoming difficult soil
conditions has led to the use of deep raft and rigid frame basements for a number
of high buildings on clay.
R. R. Gadgil, Dept. of Earth Science, Goa University 21

22. ---Raft Foundation Construction
Method – YouTube
---Construction of Mat Footing
R. R. Gadgil, Dept. of Earth Science, Goa University 22

23. Deep Foundations
■ When the superficial layers of the ground are not sound
enough or are highly compressible, shallow foundations may
not guarantee either an adequate factor of safety or settlement
within the allowable values for the structure.
■ In special circumstances, where differential settlements have to
be strictly delimited, shallow foundations may not be
appropriate solution.
■ Here the loads have to be supported by transferring them to a
deeper soil or rock with a higher bearing capacity.
■ Hence, pile foundations have to be used.
R. R. Gadgil, Dept. of Earth Science, Goa University 23

24. Deep Foundations -
Purpose
transfer building loads deep into the earth
Basic types
– Drilled (& poured)
– Driven
R. R. Gadgil, Dept. of Earth Science, Goa University 24

25. R. R. Gadgil, Dept. of Earth Science, Goa University 25

26. Piles
■ A pile is basically a structural foundation component
where length is its maximum dimension.
■ A deep foundation is normally considered to be a pile
when its total length is equal to or greater than eight
times its width or minimum dimension.
■ They can be classified into 2 basic groups
1. Precast driven piles
2. Cast-in-situ bored piles
R. R. Gadgil, Dept. of Earth Science, Goa University 26

27. Piles
R. R. Gadgil, Dept. of Earth Science, Goa University 27

28. Precast Piles & Bored Piles
■ These are straight pieces of timber, reinforced concrete or steel which are
driven into the ground by blows or vibrations until they reach the required
depth.
■ They are also known as displacement piles, because of this method of
installation, since when they are driven into the ground they displace the
soil occupying that place.
■ Bored piles are sunk by boring holes of the required depth and diameter.
■ A steel cage of reinforcement is installed as required and then the hole is
filled with concrete.
■ As a volume of soil has to be excavated and removed to construct the
piles they are also known as substitution piles.
R. R. Gadgil, Dept. of Earth Science, Goa University 28

29. Precast Concrete Plies
R. R. Gadgil, Dept. of Earth Science, Goa University 29

30. Single Piles
 A column is not normally supported by a single
pile, and in fact this system should not be used
except in the case of a large-diameter column with
a high bearing capacity.
R. R. Gadgil, Dept. of Earth Science, Goa University 30

31. Column or End-bearing
pile
 In this type of pile all or most of the
load received at the top is transmitted
integrally to the tip, with only a small
part of the load transmitted by shaft
(along its sides).
 An example of this would be a pile
driven into very soft soil with the tip
resting on very hard, competent ground.
R. R. Gadgil, Dept. of Earth Science, Goa University 31

32. Friction pile
 On the other hand, in this type of pile,
most of the load is transmitted to the
ground through the skin friction of the
shaft.
 In this case, the load reaching the tip
may be smaller than that transmitted
along the length of the pile shaft.
 Normal situations fall somewhere
between the two…..
R. R. Gadgil, Dept. of Earth Science, Goa University 32

33. Caissons or Piers
 A drilled-in pier or caisson is a variation
of individual footing.
 Such piers vary in diameter from 6” to
6’ and more, the most common sizes
being 24 and 36”.
 The hole for the pier is usually drilled
with special machines and after the
excavation is made, the hole is filled
with concrete and sometimes reinforced
with vertical steel bars.
R. R. Gadgil, Dept. of Earth Science, Goa University 33

34. Caissons or Piers
 If it is necessary to spread the load
carried by the pier over wider area then
the actual drilled diameter of the pier,
the bottom of the hole is “underrimmed”
or “belled” i.e. the lower end is enlarged
into a cone or bell-like design.
R. R. Gadgil, Dept. of Earth Science, Goa University 34

35. Caissons or Piers
 (i) concrete caisson with
enlarged bottom (b), (ii)
caisson of steel pipe with
concrete filled in the pipe (c)
and (iii) caisson with concrete
and steel core in steel pipe (d).
R. R. Gadgil, Dept. of Earth Science, Goa University 35

36. Concrete piles
 Concrete piles may be precast or cast-in-place.
 Precast piles are generally octagonal or square in cross
section and are reinforced with steel bars.
 For cast-in-place piles, a thin steel shell with a steel
core inside is driven into the ground, after which the
steel core is removed and the shell is filled with
concrete.
R. R. Gadgil, Dept. of Earth Science, Goa University 36

37. Steel piles
 They are of 2 types: Concrete filled steel pipes (tubes) and steel H
piles.
 Open end heavy steel tubes upto 30 in. in diameter are driven
through soft deposits to rock, the material inside the tube is
washed out with water or compressed air jet, then the tube is
redriven to final position and filled with concrete.
 The H piles, have all 3 components of the letter H equal.
 These piles, like the steel tubes, are used to carry the weight
of the building to deeper, more reliable strata.
R. R. Gadgil, Dept. of Earth Science, Goa University 37

38. Pile Foundations
R. R. Gadgil, Dept. of Earth Science, Goa University 38

39. R. R. Gadgil, Dept. of Earth Science, Goa University 39

40. Site Cast Concrete Piles
Cased Piles Uncased Piles
R. R. Gadgil, Dept. of Earth Science, Goa University 40

41. Negative Friction (drag)
 When a fill or load is placed on
compressible soil deposit, consolidation
of the compressible material takes
place, earthquake liquefaction or
groundwater extraction.
 The downward movement of soil
develops skin friction between the pile
and the surrounding soil and it is termed
as negative skin friction.
 Negative skin friction can be developed
from lowering on water level in
compressible soils such as clay, peat, mud
and soft soil and also due to increase in
stress by some means (e.g. filling).
R. R. Gadgil, Dept. of Earth Science, Goa University 41