FOUNDATION
The foundation is that part of wall, pier and columns in direct contact with and transmit the road
to the ground.
The importance of foundation to the building structure means the functional requirement of
foundation.
1. To spread the loads from the building over a sufficient area of soil.
2. To sustain and transmit to the ground safely and without causing any deflection, the
combine, dead, or wind load.
3. To be able to resist the movement of subsoil caused by swelling or shrinkages
4. To be able to resist any attack by sulphates or other dangerous chemicals present in the
soil.
5. If code of practice 101 on permissible bearing pressure for different types of soil is
adhere to; it increases the life span of the building structure.
Factors to be considered in the choice of foundation
a. Types of building or the clients choice.
b. Nature of loading:- concrete wall exert greater pressure and subsequently differential
movement to a greater extent than brick wall or block wall.
c. Site condition as regards subsoil:- if the soil is stable.
d. Cost
e. Types of soil- loaming soil, clay, sand, gravel.
SETTLEMENT:- Movement of the subsoil.
Causes:
i.
Changes in moisture content of the subsoil.
ii.
Frost action.
Remedy:
Can be avoided if foundation is taken deep down enough.
LOADS ON FOUNDATION
Loads on the foundation consist of the accumulation of the dead load, imposed load, and wind
load. The dead loads is the force due to the static weight of the walls, partitions, floors, roofs,
and finishes including other permanent construction.
Imposed load is the load assumed to be produced by the intended users of the building. These
include distributed load, concentrated load, impact, inertia, and life load.
Wind load is the load due to the effect of wind pressure.
TYPES OF FOUNDATION FOR DIFFERENT TYPES OF SOIL
a. The choice of foundation depends among other factors on the type of soil in which the
foundation is to be constructed.
b. Rock – this is a very hard foundation soil. This kind of soil has a high load bearing
capacity.
c. It can support strip foundation for a load bearing wall and a pad foundation for isolated
loads as those transmitted by columns and similar components.
d.
Gravel and sand – compact; suitable for pad, and strip foundation
e.
Clay – stiff; suitable for pad and strip foundation. The depth of the foundation should be
taken to a depth where it will not be affected by swelling and shrinkage which sometimes
occur in clay soil.
f. Sandy clay – Firm; suitable for strip and pad foundation. The foundation should be made
wide enough taking into consideration the load bearing capacity of the soil.
g.
Clay and silt – Soft; suitable for raft foundation because of the low load bearing capacity
of the soil.
SOILS ARE ALSO CLASSIFIED INTO TWO CATEGORIES WHICH ARE
●
Cohesive soils in which the particles stick together and examples include silt,
clay, clay and silt
● Non – Cohesive soil in which the particles do not stick together but are loose
especially when they are dry. Examples include rock, gravel and sand
The total load on the building
The total on a building is made up of dead load, line load and super imposed load. It is assumed
that a building imposes a uniform loading all around its perimeter. This is not strictly correct as
line load form floors and superimposed loading from the roofs will only be carried by the walls
that support these elements. To reduce the complexity of calculating individual loadings for
section of the external walls, it is easier to take the worst case scenario (i.e. the wall experiencing
the greatest sum of loads) and assume that all other walls are similarly loaded. If the load is
assumed to be uniform, then it is not necessary to add up all the loads for the entire perimeter of
the building. If a representative 1 metre length of the external wall is taken, then need the total
load on that section of wall can be determined and the foundation needed to support that section
of the wall can be determined and the foundation needed to support that section of wall can be
calculated. This design can then be applied to the rest of the foundation for the house.
The nature and bearing capacity of the sub-soil
The nature and bearing capacity of the subsoil ( the soil beneath the top soil) varies with the
types of soil, it is the degree of compressibility and he amount of moisture in the soil. Also,
cohesive soils, clays specification, can be subject to seasoned movement up to a depth of 1metre.
These soils exhibit shrinkage/contraction and swelling/expansion during dry and wet seasons
respectively.
The amount of settlement produced by the loading
Soil is compressible to varying degrees. As load is applied to the foundation the soil beneath the
foundation will be compressed, the water and air in the voids between the soil particles will be
squeezed out and the foundation will settle. This consolidation process will continue until the
forces between the particles are equal to the applied load. The speed of the consolidation is
determined by the speed of the migrating of water and air from between the soil particle.
Foundation built on sand on sand settles relatively rapidly, while foundation built on clay soil
settles more slowly and can last for a number of years.
Soils that are close to the surface are likely to be more compressible than those at greater depth,
as deeper soil have been compressed by the weight of the overlaying soil. If the applied load on a
clay soil is reduced due to excavation, water tends to move to the unloaded areas and swelling
the soil will occur.
Peat and other soils containing a lot of organic matter shrink and swell easily as their water
content changes. They are very compressible and settle reality even under light loading. Made up
ground behaves in a similar manner unless the materials is well graded, carefully placed and
properly compacted in thin layers. Shallow foundation should not be used on sites consisting of
made up ground. Slight settlement should not cause problems to the structure of the building.
Excessive settlement may cause shear failure of the soil.
Settlement must also be uniform throughout the building; otherwise damage may result from
different settlement. The amount of different movement between parts of a building must be kept
within acceptable limits.
Types of Foundation and Their Application
Foundations are classified into two broad categories:
· Shallow foundations
· Deep foundations
Shallow Foundations
Shallow foundations transfer load of the building to a sub-soil at a level close to the surface.
They are nearly always the cheapest to construct and generally used where sufficient depth of a
strong soil exists near the surface of the ground. The foundation needs to be designed so that the
soil is not overstressed so that the pressure on the subsoil beneath the foundation is equal at all
points in order to avoid unequal settlement. The common types of shallow foundations are
· Strip foundation
· Pad foundation
· Raft foundation.
Strip Foundation
Reinforced concrete strip foundations are used to support and transmit the loads from heavy
walls. The minimum thickness of a strip foundation is 150 mm. This may be of course, greater
where the projection of the edge of the foundation from the base of the wall is greater than
150mm A typical strip foundation is as shown in figure 8.1. The width of the strip should be at
least equal to three times the thickness of the wall it supports.
Fig. 8.1 Strip foundation
The thickness of the foundation should be at least equal to the projection of the foundation from
the base of the wall so that the load transmitted from the wall will not be dispersed over an area
outside the width of the foundation as illustrated in figure 8.2. This is to prevent the shear failure
of
the
foundation.
(a) Lines of shear failure on wide and thin foundation
(b) design of foundation to prevent shear failure
Fig. 8.2 Design of strip foundation to avoid shear failure
The effect of the wall on the relatively thin foundation is to act as a point load and the resultant
ground pressure will induce tension on the underside across the width of the strip. Tensile
reinforcement is therefore required in the lower face of strip with distribution bars in the second
layer running longitudinally especially in soft soils and in wide strip foundation. The
reinforcement arrangement for a wide strip foundation is as shown in figure 8.3
Fig. 8.3 Wide strip foundation
In firm clays the sub-soil is capable of carrying substantial loads and may only require a
foundation to be slightly wider than the wall it is supporting. The foundation still needs to be
deep enough to overcome the problems of seasonal changes in moisture content of the soil. A
deep, narrow foundation, of about 350mm in width and up to 1.50m in depth as shown in figure
8.4 could be constructed.
Fig. 8.4 Deep strip foundation
On a sloppy site it is necessary to step the foundation parallel to the slope of the ground. This is
called a stepped foundation. To prevent differential settlement in a stepped foundation, the height
of the step should not exceed the thickness of the foundation. At each step the higher foundation
should overlap the lower foundation for a distance equal to the thickness of the foundation. An
illustration of a stepped foundation is given in figure 8.5
Fig. 8.5 Stepped foundation
Pad foundation
This type of foundation is used to support and transmit the loads from piers and columns. The
most economic plan shape is a square but if the columns are close to the site boundary, it may be
necessary to use a rectangular plan shape of equivalent area. The reaction of the foundation to
the load and ground pressures is to cup, similar to a saucer, and therefore main steel is required
in both directions. A typical example of a reinforced pad foundation is shown in figure 9.5.
Fig. 9.5 Pad foundation
Raft foundation
The principle of any raft foundation is to spread the load over the entire area of the site. This
method is particularly useful where the column loads are heavy and thus requiring large bases or
where the bearing capacity is low, again resulting in the need for large bases.
Raft foundation can be considered under three headings:
· solid slab rafts,
· beam and slab rafts, and
· cellular rafts.
(a) Reinforce concrete solid slab raft foundation
(b) Cellular raft foundation
Pile Foundation
Pile foundation is used to transfer heavy load to a soil layer of adequate strength located far
below the ground surface that it will be uneconomical to carry out excavation to reach such a
depth. This is used where the soil condition is poor close to the earth surface. Pile foundations
could be made using concrete, steel or timber. The concrete could be precast or cast insitu.
Piles are classified by their method of construction into
· Bored or Replacement Piles which are concrete cores poured into holes in the ground at
measured intervals.
· Driven or Displacement Piles which are driven into the ground at spaced intervals.
By the way they carry their loads pile foundations are classified into two types. These are
· End bearing pile, and
· Friction pile
(a) End bearing piles are those piles that are driven down to soil layer of adequate strength in
such a way that the end of the piles actually bear on this particular soil layer.
(b) Friction piles are used in situations where the soil layer of adequate strength is located far
below the ground surface such that it becomes uneconomical to produce piles that will go
to such a depth. This type of piles carries their load by the friction that exists between the
shaft of the pile and the surrounding soil particles. The surface of the friction piles are
made rough so that the desired frictional force could be produced as the pile is driven into
the ground.
Test Questions
1. Use drawing to describe the following types of foundation
Pad foundation
Stepped foundation
Deep strip foundation
2. State the principle behind Raft foundation
3. Mention five types of foundation for different types of soil
4. Write short note on the following
i.
Total load on the building
ii.
The nature and bearing capacity of the sub soil
iii.
The amount of settlement produce by the loading