3. Functions of foundations:
• Foundations are provided for the main following
reasons;
• To distribute the weight of the structure over a
large area so s to avoid overloading of the soil
beneath
• To load the substratum evenly and thus prevent
unequal settlement
• To provide a level surface for building operations
• To take the structure deep into the ground thus
increasing its stability and prevent overturning
4. Functional requirements
• To receive the loads from the superstructure
and transmit it safely to the natural
foundation so as not to cause any settlement
or other movement
• Be at such a depth or may be constructed to
avoid damage due to swelling, shrinkage or
freezing of the subsoil.
• Be capable to resist attack by sulphates or
other deleterious matter present in the soil
5. Basic types of foundation
• Natural Foundation; this is the ground
underneath the base of the building after
excavation is complete
• Artificial Foundation; this is the structure that
lies between the building and natural
foundation
6. Loads acting on foundations and their
effects
• Dead loads are the loads or weight of all the fixed
part of the structure like walls, foundation, roof
e.t.c and services like sanitary appliances
• Live or superimposed loads; These are movable
weights of people, furniture and machinery that
occupy the building.
• Wind loads: These are pressure on walls and
roofs due to wind pressure. Pressure from wind
load on foundations is more important to tall
buildings.
• Seismic loads
7. SITE INVESTIGATION
• To determine suitability of foundation to be
used in accordance with the type of soil
• Involves boring of holes and taking of samples
to the laboratory for analysis
8. The purposes of site or soil investigation
or exploration, can be summarized as to:
1. know the qualities and thickness of underground soils so as
to predict the likely behavior of the foundation.
2. assign suitable safe bearing capacity to the underlying soils.
3 determine the depth up to which the foundation must be
carried.
4. find out the depth of the underground water table and if
need be, to decide the ways and means to be adopted, to
solve the problem of ground water.
5. select safe and most economical type of foundation.
6. make allowance for the likely settlement of foundation.
7. foresee other likely difficulties, which may come across
during construction, and to remain prepared to face them.
9. CHOICE OF EXPLORATION METHODS
• This is influenced by:
• Nature of ground: in clayey soils, boring is
suitable for deep boring and pits for shallow
borings.
• In sand soil, boring with special instruments to
be able to take samples is essential
• In hard rocks, boring is recommended.
10. Cont-
• Topography :In hilly country, the choice between
vertical openings (for example, boring sand trial
pits) and horizontal openings (for example,
headings) may depend on the geological
structure, since steeply inclined strata are most
effectively explored by headings and horizontal
strata by trial pits or borings.
• Swamps and areas overlain by water are best;
explored by borings which may have to be put
down from a floating craft.
11. Cont-
• Cost: For deep exploration, borings are usual, as
deep shafts are costly. However, if the area is
vast, geophysical methods or sounding methods
may be used in conjunction with borings.
• For shallow exploration in soil, the choice
between pit and borings will depend on the
nature of the ground and the information
required for shallow exploration in rock;
• the cost of boring a core drill to the site will only
be justified if several holes are required;
otherwise trial pits will be more economical.
12. Methods Of Soil Exploration
1. INSPECTION:Carried out prior to design of
structure
• Exposed surfaces of ground, old cuts etc are
examined, behavior of existing structures in
the vicinity are studied.Experience of other
engineers working in the area are collected
• These help to form a fairly correct opinion of
the nature of the soil and its likely behavior in
future.
• The method is quick and cheap but does not
give correct/reliable information of the site
13. 2. TEST PITS
• Used to identify type of soil at short depths
• Pits are dug at frequent intervals wide enough to
allow a person to enter.
• Size of pits depend on type of examination and
depth, normally 1.5X1.5m and about 1.5-2.5m
deep.
• Sufficient no of pits should be dug to obtain enough
information.
• Different soil layers are identified and soil samples
are taken at different levels for laboratory analysis
• The method is cheap, laborious but cannot be used
at greater depths
•
14.
15. 4. Auger Boring
• Commonly used for testing foundations
of ordinary buildings
• Various types of augers are available for
boring.
• This is the most commonly used
method and are widely used to extract
soils from different depths.
• Augers may be in the form of post hole
type, screw type or shell type.
16.
17. 5 Deep boring:used in case of very large/tall
heavy structure.
Two kinds:
• Percussion boring; a cutting tool is dropped
repeatedly on the ground and the pulverized
material is brought to the ground by adding
water into the core.
• The paste is dried,and classified
• Method suitable for hard materials like rocks
20. Rotary or core drilling.
In this method a hollow tube having
• cutting edge is driven by rotary motion.
Water is used to case cutting of the core of
the material.
• Rotary or core drilling when done in soft soils,
no core is possible to be obtained and slurry
formed by soft cut material and water has to be
pumped out of the hole being bored.
21. • Solid core is obtained when rotary or core
drilling is done in hard formations.
• In case soft loose falling material is
encountered, casing tube may have to be
used to prevent falling of loose material in
the bored hole when tube is being
withdrawn .see fig.
• After carrying out boring to the required
depth, core chart of the ground formations
is developed
23. 6 Test piles:
· Wooden or steel piles are driven into ground under
blows at specific weight and the driver can judge the
hardness of soil by considering the no. of blows required
to drive the piles or rod for specific length into the
ground.
· The method merely gives load resistance of the soil
therefore it is not complete.
24. 7. Wash boring-
· Casing tube is
driven into the
ground aided with
water jet.
· The material
inside the casing is
brought to surface
for examination (by
jet of water at
pressure of
0.4kg/cm).
25. BEARING CAPACITY OF SOILS
• bearing value or bearing power is defined
as the ability of the soil to support the
load coming over its unit area without
causing excessive settlement and
displacement.
26. Ultimate Bearing Capacity
is the minimum load, which will cause failure
of a foundation (or it is the maximum load
that the soil can carry, if exceeded the
foundation soil will fail)
27. Improving Bearing Capacities of Soil
Confining the soil
Increasing depth of foundation
Increasing width of foundation
Blending/soil stabilization
Draining the soil
Sand piles
Grouting
28. Foundation Types
Classification: they are classified as
Shallow Foundations; Are Those Foundations
constructed at a short depth in the ground.
• this type of foundation is suitably used where the
nature of the ground soil is strong enough to support
the weight of the structure. foundations under this
group are spreaded footing foundations, raft, pier, pad
and short bored pile foundation.
Deep foundations; are the types of foundations used
in areas with soils of poor bearing capacities or in
water logged areas. these are constructed deep into
the ground so as to transmit the structural load to a
greater depth having soil with better bearing capacity.
the type of foundation in this group is pile foundation.
29. TYPES OF SHALLOW FOUNDATIONS
1 STRIP FOUNDATION.
- Consists of a foundation wall resting on a mass
concrete bed. The concrete bed is provided to
spread the load of the wall over a wider area on
the soil, also to prevent the wall from sinking into
the ground.
- The foundation depth below the ground
depends on ground condition.
30. Top soil is more liable to shrinkage and
volumetric change.
-The recommended minimum depth ranges
between 600mm and 1000mm.
- The minimum depth (D) of the concrete base is
150mm, and should project beyond the wall at a
distance (P) equal to the thickness (D) of the
concrete.
31. example
This is because if
the
concrete is
crushed as
it is between the
wall pressing down
and the subsoil
it can, if the
concrete is weak,
crack or fail at an
angle of roughly
450mm from the
horizontal
100 Over site
concrete with screed
150mm hardcore
wall
D [min 150mm]
GL
Concrete base
B
. . . . . . . .. . . ..
. … . . .. … …
32. Deep Strip FoundationUsed where the bearing
capacity of subsoil and
loads on foundation require
the strip to be no
wider/little wider than wall
thickness and subsoil
necessitates deep
foundation e.g clay soil. It
is constructed by
excavating a deep trench
and filling it with concrete
up to a level just below
finished ground level.
Normally cheaper to fill the
trench with concrete rather
than widen the trench to
obtain a working space.
Wall
GL
Concrete
Mass
concr
ete
1:3:6
As
for
trad
ition
al
strip
33. RAFT FOUNDATIONS; (known as mat
foundation). Used in areas where the subsoil is
very weak –usually soft natural ground or fill, or
on ground that is liable to subsidence as in
mining areas.
The whole area is covered by reinforced
concrete; which is thickened under the load
bearing walls.
35. PAD FOUNDATION
This is a square or rectangular slab
of concrete carrying a single
column.
Reinforcement is placed at the
bottom in both directions to
resist the bending stresses set
up by the cantilever (double
cantilever) action of the slab
about the column base.
Shear reinforcement is not
provided.
The thickness of slab may be
reduced towards the edges to
economize concrete.
•
Column
Column
reinforc
ement
R
e
i
n
f
o
r
c
Base
reinforcement
{both directions}
Blinding layer
. . . . . . . .
36. 8. PIER FOUNDATIONS.
Used when good bearing stratum exists up to
4.5m below ground level.
Brick, masonry or mass concrete foundation piers
in excavated pits are used.
The type of foundation prevents excessive
excavation. Holes are excavated on the ground
(the smallest hole in which hand excavation can
be carried out is about 1m2)
37. Short Bored Pile Foundations
The Foundation is economically used for small
buildings, and on shrinkable clays where adjacent
trees or the felling of trees make appreciable volume
change in the subsoil for some short length
Piles are cast in hand or machine augered pits. The
piles support reinforced concrete ground beams off
which walls are constructed.
38. Size and spacing of the piles depend on
loads and the building plan. Piles are cast
below angles and inter-sections of external
and internal load-bearing walls and spaced
between these points where the loads and
the span of ground beam requires additional
piles.
39. SHORT BORED PILE FOUNDATION
Wall
Concrete floor
Hardcore
Reinf. Conc ground beam
Bored & cast in situ concrete piles
at 1800-2500mm c|c
Diameter250-300mm
40. SITEWORKS
Once the main contractor/builder as been awarded
the tender to carry out the construction, he starts
to prepare the site by:
Carrying out site clearance,
Setting out the site as well as set out the
temporary services that he will need on site
41. • Finding for the site boundaries as shown on the
drawings, and where required put up security
fence around the site boundaries to control
movement into /from the site.
Organizing for the location of temporary huts and
temporary access road to site basing on the
given drawing and specifications.
42. SITE CLEARANCE.
Clearing the site involves removal of shrubs,
debris from demolished structures cut down
unwanted trees as well s removal of stones and
boulders especially where the building is to be
erected.
Remove unwanted top vegetable soil at
minimum depth of 150mm as it is unsuitable for
construction works
43. A well-managed site should have the following
facilities;
i. A site office with secure storage and workshops
ii. A toilet and canteen/eating place
iii Supply of fresh water
iv. Electric and telephone services
v. Space for storage of materials and tools
vi.Contractors name board
