FOUNDATIONS

Building is consisting of two major parts:

The superstructure which is the above-ground portion ofthe building

The substructure: which can be further understood as

- the basement which is the habitable below-ground

portion

- The foundations which are the components of the

building that transfer its loads into the soil.

The foundation of building is the lowest part of thesubstructure which is in direct contact with the ground and

transmits loads to the soil, providing a proper base for the

super-structure.

Super structure

Sub structure

Foundation

Basement

Ground Level

Components of a building

Purpose of foundations:

It is often misunderstood that the foundation is providedto support the load of the structure. In fact it is thedevice to transmit the load of the structure to the soilbelow. Foundation is provided for the following mainpurposes:

To distribute the weight of the structure over large areaso as to avoid over-loading of the soil beneath.

To load the substratum evenly and thus to preventunequal settlement.

To provide a level surface for building operations.

To take the structure deep into the ground and thus toincrease its stability, preventing overturning.

Foundation loads:

A foundation supports a number of different kinds ofloads:

The dead load of the building, which is the sum of theweights of the frame, the floors, the roofs, and walls, the

electrical and mechanical equipment and the foundation

itself.

The live load, which is the sum of the weights of thepeople in the building, the furnishing and the equipments

they use, and snow, ice and water on the roof.

Wind loads, which can apply lateral, downward, anduplift loads to a foundation

Foundation loads:

Horizontal pressures of earth and water againstbasement walls.

In some buildings, horizontal or inclined thrusts fromarches, rigid frames, domes, vaults or tensile structures.

On some building sites, buoyant uplift forces fromunderground water, identical to the forces that cause a

boat to float.

During earthquakes, horizontal forces caused by themotion of the ground relative to the building.

Requirements of foundations:

A satisfactory foundation for a building must meetthree general requirements:

The foundation, including the underlying soil and rock,must be safe against a structural failure that could

result in collapse.

During the life of the building, the foundation must notsettle in such a way as to damage the structure or

impair its function.

The foundation must be feasible both technically andeconomically, and practical to build without adverse

effects to surrounding property.

Types Of Foundations:

There are two basic types of foundations:

Shallow foundations are those that transfer the loadthe load to the earth at the base of the column or wall ofthe substructure. Shallow foundations are generallyless expensive than deep ones and can be used wheresuitable soil is found at the level at the bottom of thesubstructure, whether this be a few feet or severalstories below the surface.

Deep foundations transfer the load at a point somedistance below the substructure. Deep foundations,either piles or caissons, penetrate through upper layersof incompetent soil in order to reach competent bearingsoil or rock deeper within the earth.

Factors affecting the selection:

The primary factors that affect the choice of afoundation type for a building are:

- Subsurface soil and ground water conditions

- Structural requirements, including foundation loads,

configurations and depth.

Factors affecting the selection:

The secondary factors that affect the choice of afoundation type for a building are:

- Construction methods, including access and working

room

- Environmental factors, including noise, traffic, and

disposal of earth and water

- Codes and regulations

- Impacts on adjacent property

- Time available for construction

- Construction risks

DEEP FOUNDATIONS

In case, the stratum of good bearing capacity is notavailable near the ground, the foundation of the

structure has to be taken deep with the purpose of

attaining a bearing stratum which is suitable in all

respects.

In addition, there may be many other conditions whichmay require deep foundations for ensuring stability

and durability of a structure. For example, the

foundation for a bridge pier must be placed below the

scour depth, although suitable bearing stratum may exist

at a higher level.

DEEP FOUNDATIONS

The most common forms of construction pertaining to deep foundations are:

Piles

Cofferdams

Caissons

Raft foundations

Out of these, pile foundation is more commonly used in building construction.

Pile foundation

The word pile is used to describe columns, usually ofreinforced concrete, driven or cast in situ in the ground in

order to carry foundation loads to some deep

underlying firm stratum or to transmit loads to the

subsoil by the friction of their surfaces in contact

with the subsoil.

The main function of the pile is to transmit loads to lowerlevel of ground by a combination of friction along their

sides and end bearing at the pile point or base.

Pile Foundations Applications:

Following are the situations in which a pile foundation ispreferred:

The load of the super structure is heavy and itsdistribution is uneven.

The top soil has poor bearing capacity.

The subsoil water level is high so that pumping of waterfrom the open trenches for the shallow foundations is

difficult and uneconomical.

There is a large fluctuations in subsoil water level.

Pile Foundations Applications:

If deep strip foundation is attempted, timbering of sidesis difficult to maintain or retain the soil of sides of the

trench.

The structure is situated on the sea shore or river bed,where there is danger of scouring action of water.

Canal or deep drainage line exist near the foundations.

The top soil is of expansive nature.

In general, Pile foundation provides a commonsolution to all difficult foundation site problems. And

can be most effectively used for buildings, trestles,

bridges and water front installations (piers, docks,

etc.)

Classification of Piles

Piles can be classified on following criteria:

On the basis of mode of load transfer:

- End bearing

- Friction

- Composite

On the basis of function or use:

- Sheet pile

- Anchor Pile

- Batter Pile

- Fender Pile

- Compaction pile

Classification of Piles

On the basis of cross sectional area:

- circular

- square

- tubular

- octagonal

- H-piles

On the basis of size of the piles:

- Micro(mini) Piles (< 150 mm)

- Small diameter Piles (>150 and < 600 mm)

- Large diameter Piles (>600 mm )

Classification of Piles

On the basis of material:

- Timber

- steel

- PCC

- RCC

- composite

On the basis of method of construction/ execution

- Driven/displacement precast piles

- Driven/displacement cast-in-situ pile

- Bored/replacement precast pile

- Bored/replacement cast-in-situ pile

Bearing Piles:

Piles that transfer loads by end

bearing to compact gravel, hard clay

or rock are termed end bearing piles.

Bearing piles are driven into the

ground until a hard stratum is

reached. Such piles act as pillar

supporting to super structure and

transmitting the load down to the

level at which it can be safely borne

by the ground. Thus bearing piles,

by themselves do not support the

load, rather they act as a medium to

transmit the load from the foundation

to the resisting substructure.

Types of piles: on the basis of load transfer:

Types of piles

Friction piles:

Piles that transfer loads

mainly by friction (skin

friction) to clays and silts

are termed as friction piles.

The friction piles are driven at

a site where the soil is weak

or soft to a considerable

depth i.e. in the type of soil

whose strength does not

increase with depth or, where

rate of increase in strength

with depth is very slow.

Types of piles

Sheet piles:

Sheet piles differ from

bearing or friction piles in

that they are rarely used to

furnish vertical support but

are used to function as

retaining wall. Sheet piles

are used for retaining soil

that is liable to escape

laterally when subjected to

pressure or to enclose the

area required for some

foundation and protect it

from the action of running

water or leakage

Types of piles: on the basis of function or use:

Sheet pile walls are constructedby driving prefabricated sections

into the ground.

Soil conditions may allow for thesections to be vibrated into ground

instead of it being hammer driven.

The full wall is formed byconnecting the joints of adjacent

sheet pile sections in sequential

installation.

Sheet pile walls provide structuralresistance by utilizing the full

section.

Steel sheet piles are mostcommonly used in deep

excavations, although reinforced

concrete sheet piles have also being

used successfully

Sheet piles

Steel sheet piling is the most common because of several advantages over other

materials:

1. Provides high resistance to driving stresses.

2. Light weight

3. Can be reused on several projects.

4. Long service life above or below water with modest protection.

5. Easy to adapt the pile length by either welding or bolting

6. Joints are less apt to deform during driving.

Sheet pile disadvantages are:

1. Sections can rarely be used as part of the

permanent structure.

2. Installation of sheet piles is difficult in

soils with boulders or cobbles. In such

cases, the desired wall depths may not be

reached.

3. Excavation shapes are dictated by the

sheet pile section and interlocking

elements.

4. Sheet pile driving may cause

neighborhood distrurbance

5. Settlements in adjacent properties may

take place due to installation vibrations

Sheet pile walls are constructed

by:

1. Laying out a sequence of sheet pile

sections, and ensuring that sheet

piles will interlock.

2. Driving (or vibrating) the individual

sheet piles to the desired depth.

3. Driving the second sheet pile with

the interlocks between the first sheet

pile and second "locked"

4. Repeating steps 2 & 3 until the wall

perimeter is completed

5. Use connector elements when

more complex shapes are used.

Anchor piles:

When piles are used to

provide anchorage

against horizontal pull

from sheet piling walls or

other pulling forces, they

are termed as anchor

piles.

Batter piles:

When piles are driven at

an inclination to resist

large horizontal or

inclined forces, the piles

are termed as batter

piles.

Screw anchors can be used in

a variety of situations,

including slope

restoration/stabilization.

Once the fault line has been

found, anchors can be

screwed in almost horizontally

into more stable soil.

Once installed, an appropriate

retaining wall is attached,

which helps maintain the

slope integrity

Fender piles:

When the piles areused to protect

concrete deck or other

water front structures

from the abrasion or

impact that may be

caused from the ships

or barges (when they

are tied up at the deck)

they are called fender

piles.

The fender piles areordinarily made up of

timber.

Compaction piles:

When piles are driven ingranular soil with the aim of

increasing the bearing

capacity of the soil, the piles

are termed as compaction

piles.