Foundations transmit loads from buildings to the ground below. There are different types including shallow foundations like spread footings and deep foundations like piles. Spread footings support individual columns on isolated slabs while combined footings link two columns. Strap footings connect isolated footings with a beam. Mat foundations are large slabs supporting many columns. Pile foundations transfer loads to deeper soils through friction or bearing. Foundations can fail due to unequal settlement, subsoil moisture movement, lateral pressures, or weathering effects. Proper design and construction seeks to prevent these failures.

1. Foundation

2. Definition
• Sub-structure of the building
• The lower portion of the building, usually located below the ground
level
• Transmits the load of the super-structure to the sub-soil

3. Types of foundations
Shallow
Foundations
Foundations
Deep
Foundations
Strap Footings
Mat
Foundations
Combined
Footings
Spread Footings
Well
Foundations
Pier
Foundations
Pile
Foundations

4. Spread Footing
• A spread footing (or isolated or pad) footing is provided to support an
individual column.
• A spread footing is circular, square or rectangular slab of uniform
thickness.
• Sometimes, it is stepped or sloped to spread the load over a large
area.

5. Combined Footing
• A combined footing supports two columns.
• It is used when the two columns are so close to each other that their
individual footings would overlap.
• A combined footing may be rectangular or trapezoidal in plan.

6. Strap Footings
• A strap (or cantilever) footing consists of two isolated footings
connected with a structural strap or a lever.
• The strap connects the two footings such that they behave as one
unit.
• The strap beam does not remain in contact with soil, and thus does
not transfer any pressure to the soil

7. Mat or Raft Foundations
• A mat or raft foundation is a large slab supporting a number of
columns and walls under the entire structure or a large part of the
structure.
• A mat is required when the allowable soil pressure is low or where
the columns and walls are so close that individual footings would
overlap or nearly touch each other.
• Mat foundations are useful in reducing the differential settlements on
non-homogeneous soils or where there is a large variation in the
loads on individual columns.

8. Pile Foundations
• Pile foundation is that type of deep foundation in which the loads are
taken to a low level by means of vertical members which may be of
timber, concrete or steel.
• Pile foundation may be adopted
(i) instead of a raft foundation where no firm bearing strata exists at any
reasonable depth and the loading is uneven,
(ii) when a firm bearing strata does exist but at a depth such as to make strip
or spread footing uneconomical, and
(iii) when pumping of sub-soil water would be too costly or timbering to
excavations too difficult to permit the construction of normal foundations.

9. • Piles used for building foundation may be of four types:
(i) End bearing pile - used to transfer load through water or soft soil to a
suitable bearing stratum; used to carry heavy loads safely to hard strata.
Multi-storeyed buildings are invariably founded on end bearing piles, so that
the settlements are minimized.
(ii) Friction pile - used to transfer loads to a depth of a friction-load- carrying
material by means of skin friction along the length of the pile. Such piles are
generally used in granular soil where the depth of hard stratum is very great.

10. (iii) Combined end bearing and friction pile - transfers the super-
imposed load both through side friction as well as end bearing. Such
piles are more common, specially when the end bearing piles pass
through granular soil.
(iv) Compaction piles - used to compact loose granular soils, thus
increasing their bearing capacity; themselves do not carry a load; may
be of weaker material (such as timber, bamboo sticks etc.)

11. Pier Foundation
• A Pier foundation consists of a cylindrical column of large diameter to
support and transfer large super-imposed loads to the firm strata below.
• The difference between pile foundation and pier foundation lies in the
method of construction.
• Though pile foundations transfer the load through friction and/or bearing,
pier foundations transfer the load only through bearing.
• Generally, pier foundation is shallower in depth than the pile foundation.
• Pier foundation is preferred in a location where the top strata consists of
decomposed rock overlying a strata of sound rock. In such a condition, it
becomes difficult to drive the bearing piles through decomposed rock. In
the case of stiff clays, which offer large resistance to the driving of a
bearing pile, pier foundation can be conveniently constructed.

12. • Pier foundations may be of the following types:
(i) Masonry or concrete pier- used when a good bearing stratum
exists upto 5 m below ground level
(ii) Drilled caissons- compressed member subjected to an axial load at
the top and reaction at the bottom
• Drilled caissons may be of three types :
(i) concrete caisson with enlarged bottom
(ii) caisson of steel pipe with concrete filled in the pipe
(iii) caisson with concrete and steel core in steel pipe

14. Well Foundations/ Caissons
• Well foundations or caissons are box like structure-circular or
rectangular-which are sunk from the surface of either land or water
to the desired depth.
• Large in diameter than pier foundations or drilled caissons
• Used for major foundation works:
• Bridge piers and abutments in river, lakes etc
• Breakwaters and other structures for shore protection
• Large water front structures such as pump houses, subjected to heavy vertical
and horizontal loads
• Hollow from inside; may be filled with sand and plugged at bottom

15. • Load is transferred through perimeter wall, called steining
• Not used for buildings

16. Causes of failure of foundation
1. Unequal settlement of sub-soil
• may lead to cracks in structural components and rotation thereof
• may be due to
 non-uniform nature of sub-soil throughout the foundation
unequal load-distribution on the soil strata
eccentric loading
• remedial measures include
 resting of foundation on rigid strata
 proper design of the base of footing so that it can resist cracking
 limiting the pressure in the soil
Avoiding eccentric loading

17. 2. Unequal settlement of masonry
• Due to the presence of mortar joints between ground level and concrete
footing which may either shrink or compress
• May also lead to cracks in super-structure
• Remedial measures
Using mortar of proper strength
Using thin mortar joints
 restricting the height of masonry to 1 m per day if lime mortar is used and 1.5 m per day
if cement mortar is used
Properly watering the masonry
3. Sub- soil moisture movement
• Major cause of failure of footing on cohesive soil
• Caused due to fluctuation in sub-soil water level
( When water table drops down, shrinkage of soil takes place. Due to this, there
is lack of sub-soil support to the footings which crack, resulting in the cracks of
the building; During upward movement of moisture, the soil swells resulting in
high swelling pressure. If the foundation and super-structure is unable to resist
swelling pressure, cracks are induced)

18. 4. Lateral pressure on the walls
• Walls transmitting the load to foundation may be subjected to lateral
pressure from pitched roof or an arch or wind. Due to this foundation will be
subjected to a moment or resultant eccentric load. If the foundation has not
been designed to take this moment, it may fail.
• Failure can either be in the form of overturning or by generation of tensile
stresses on one side and high compressive stresses on other side of footing.
5. Lateral movement of sub-soil
• Caused when
Very soft soil are move out or squeeze out laterally under vertical loads, specially at
location where ground is sloping
A big pit is excavated in the near vicinity of the foundation in granular soil
• Results in excessive settlements and collapse in structure
• Remedial measure- sheet piles should be driven to prevent lateral movement
or escape of soil

19. 6. Weathering of sub-soil due to trees and shrubs
• Roots of shrubs near the wall absorb moisture from the foundation soil
resulting in the reduction of their voids, which further results in depressions
in ground.
• If roots penetrates below the level of footing, settlements may increase,
resulting in foundation cracks.
7. Atmospheric action
• Causes:
Moisture movement due to rain or droughts may cause trouble to shallow foundations
Foundation may scoured if the building lies in low lying area
stagnant water near the foundation may decrease the strength of footing
• Remedial measures
Filling foundation trenches with good soil and compacting it
Providing gentle slope away from the wall
Providing narrow, sloping strip of impervious material along exterior walls