FOUNDATIONS

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Introduction

• DEFINITION: Foundation is the lowest part of the structure on which the building rests

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

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

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

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

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

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.

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.

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.

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.

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

2. TEST PITS

• Used to identify type of soil at short depths

• Pits are dug at frequent intervals wide enough toallow a person to enter.

• Size of pits depend on type of examination anddepth, normally 1.5X1.5m and about 1.5-2.5mdeep.

• Sufficient no of pits should be dug to obtain enoughinformation.

• Different soil layers are identified and soil samplesare taken at different levels for laboratory analysis

• The method is cheap, laborious but cannot be usedat greater depths

•

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.

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

Percussion boring tool

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.

• 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

Typical drilling units

6 Test piles:· Wooden or steel piles are driven into ground underblows at specific weight and the driver can judge thehardness of soil by considering the no. of blows requiredto drive the piles or rod for specific length into theground.· The method merely gives load resistance of the soiltherefore it is not complete.

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).

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.

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)

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

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.

TYPES OF SHALLOW FOUNDATIONS1 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.

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.

exampleThis 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

. . . . . . . .. . . ..

. … . . .. … …

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

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.

Concrete raft

Edge beamReinforcement bars placed in both directions

GL

. .

. .

. . . . . . .

. . . . . . .

Wall

PAD FOUNDATION

This is a square or rectangular slabof concrete carrying a singlecolumn.

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 reinforcement

Reinforc

Base reinforcement {both directions}

Blinding layer

. . . . . . . .

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)

Short Bored Pile FoundationsThe 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 lengthPiles are cast in hand or machine augered pits. The piles support reinforced concrete ground beams off which walls are constructed.

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.

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

SITEWORKSOnce 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

• 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.

SITE CLEARANCE.Clearing the site involves removal of shrubs,debris from demolished structures cut downunwanted trees as well s removal of stones andboulders especially where the building is to beerected.

Remove unwanted top vegetable soil atminimum depth of 150mm as it is unsuitable forconstruction works

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

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