Building Materials and Technology
UNIT I
Topic: Foundations
Classification Based on Structure
Load Bearing Structures
• A structure comprising of slabs, beams and load bearing walls is known as a
Load Bearing Structure.
• Loads from slab/roof are transmitted through walls to the
• sub-soil below the ground through their foundations.
• In load bearing structures, thickness of the walls decreases from ground to first
and then to second floor to reduce the load on the vertical walls.
• As a result, compared to upper floors, lower floors will have less carpet area.
• Such types of structures are suited where hard strata of soil is available at low
depth.
• Individual residential bungalows, tenements, low rise buildings (up to 3 storey),
are constructed as load bearing structures.
Framed Structures
• It is a structure comprising of slabs resting on beams and beams are
supported by a network of columns and whole load of the structure is
transferred to the sub-soil below the ground through columns and their
footings.
• Walls don’t bear any load and rest on plinth beams without foundations.
• This type of structure has more flexibility.
• R.C.C. is the most suitable material to withstand external loads like
compressive, tensile, torsion and shear along with moment.
• All columns, beams and slabs are connected rigidly and are constructed
monolithically.
• Carpet
area
is
almost same for all floors and
is
more than load bearing structure.
• Generally, all multistoried buildings or high rise buildings have framed
structure.
Composite Structures
• The structures constructed with combination of both load bearing as well as
framed structure is called composite structure.
• The load of slabs is transmitted to the sub-soil below ground by load bearing
walls and columns through their foundations.
• In this type of structure, external walls are treated as load bearing walls and
all intermediate supports are in the form of R.C.C. columns.
• This type of structure is preferred in buildings having large spans such as
workshops, halls, warehouses, godowns, etc.3
• This type of structure have advantages of both load
• bearing and framed structure.
Building Components
Building Components
➢ Sub-Structure
• Shallow Foundation
o
o
o
o
Spread Footings
Combined Footings
Strap Footings
Mat/Raft Foundations
• Deep Foundation
➢ Super Structure
•
•
•
•
•
•
•
•
•
Plinth
Walls, Columns & Beams
Floors
Sills, Lintels and Weather sheds
Doors, Windows & Ventilators
Roofs & Slabs
Parapet
Stairs, Lifts, Ramps
Building Finishes
Sub – Structure / Foundations
W H AT IS A F O U N D AT I O N ?
The low artificially built part of a structure which transmits the load of the structure
to the ground is calledfoundation.
OR
A foundation is a structure that transfers the load to the ground.
• A structure essentially consists of two parts, namely the super structure which is above
the plinth level and the substructure which is below the plinthlevel.
• Substructure is otherwise known as the foundation, and this forms the base for any
structure. Generally, about 30% of the total construction cost is spent on the
foundation.
• The soil on which the foundation rests is called the “foundation soil”.
OBJECTIVES OF A F O U N D AT I O N
A foundation is provided for the following purposes:
• To distribute the total load coming on the structure on a larger area.
• To support the structures.
• To give enough stability to the structures against various disturbing
forces, such as wind and rain.
• To prepare a level surface for concreting and masonry work.
• “To ensure that the structural loads are transmitted to the subsoil safely, economically
and without any unacceptable movement during the construction period and
throughout the anticipated life of the building or structure”
IMAGES
TYPES OF F O U N D AT I O N
Foundation
Shallow Foundation
Shallow Foundation:
a) Isolated Footing
b) Combined Footing
c) Raft/Mat Foundation
d) Wall or strip footing
e) Stepped footing and soon.
Deep Foundation
Deep Foundation (if D>W)
a) Pile Foundation
S H A L L O W F O U N D AT I O N
Shallow foundations are constructed where soil layer at shallow depth (up to 1.5m) is
able to support the structural loads (Depth of foundation islessthan or equal to its
width)
i. Isolated Footing
•
•
•
•
•
In framed structures where several columns are to be
constructed, isolated footings can be adopted.
The columns involved can be provided with masonry or
concrete footing.
If masonry footing is provided, steps are given and the
foundation area is thus increased sothat the stresses
developed at the base is within thelimit.
Concrete can be molded to any shape and hence a concrete
footing may be a sloping one to provide sufficient spread.
Economical when columns are placed at longer distances.
• Spread Footing:-Spread footings are those which spread the super-imposed
load of wall or column over larger area. Spread footing support either column
or wall.
• It may be following kinds
• Single footing for column: In which the loaded area of column has been
spread to the large size through single spread. The base is generally made of
concrete.
• Stepped footing for column: This type of footing provided for heavily loaded
column which required greater spread with steps. The base is generally made
of concrete.
• Sloped footing for column: In this type of footing concrete base does not have
uniform thickness but is made sloped.
• Wall footing without step: It consist of concrete base without any steps
including masonry wall.
• Stepped footing for wall: It consist of masonry wall have stepped footing with
concrete base .
• Grillage Foundation
• It is special type of isolated footing generally provided for heavily
loaded steel column and used in those location where bearing capacity
of soil is poor.
• The depth of such foundation is limited to 1 to 1.5 m.
• The load of steel column is distributed over very large area by means
of two or more tiers of steel joints.
• Each layer being laid at right angle to the layer below it.
S H A L L O W F O U N D AT I O N
ii. Combined Footing
•
•
•
This type of footing is adopted when the space between two
columns is sosmall that the foundation for individual columns
will overlap.
Combined footings are proportioned in such a way that the
center of gravity of the loads coincides with the centre of
gravity of the foundation. Hence these footings have either a
trapezoidal or a rectangular shape.
Generally, preferred when the soil is wet (marshy areas)to
reduce the pressure on the ground
“Footings are structural members used to support columns and
walls and to transmit and distribute their loads to the soil in such
a way that the load bearing capacity of the soil is not exceeded.
Also excessive settlement, differential settlement or rotation are
prevented and adequate safety against overturning or sliding is
maintained.”
Combined Footing:
• A spread footing which supports two or more columns is termed as combined footing.
• The combined footing may be of following kinds.
• Rectangular combined footing: The combined footings will be provide in rectangular in
shape if columns carry equal loads. The design of rectangular combined footing should
be done in such way that centre of gravity of column coincide with centroid of footing
area.
• Trapezoidal combined footing: If columns carry unequal loads the
footing is of
trapezoidal shape are provided.
• Combined column-wall footing: It may be required to provide a combined footing for
column and wall. Such combined footing are shown in fig.
S H A L L O W F O U N D AT I O N
iii. Raft or Mat Foundation
• Used to spread the load of the structure over
a large base to reduce the load per unit area
being imposed on the ground
• Particularly useful where low bearing capacity
soils are encountered & where individual
column loads are heavy.
• Used when the subsoil is weak.
Raft foundation:
• A raft Foundation is a combined footing that covers the entire area beneath a structure
and support all the wall and column.
• They are used in areas where the soil masses contains compressible lenses or the soil is
sufficiently erratic so that differential settlement would be difficult to control.
• Raft foundation may be divided in to three types based on their
design and construction.
• Solid slab system
• Beam slab system
• Cellular system
• All the three types are basically the same, consisting of a large, generally unbroken area
of slab covering the whole or large part of structure.
S H A L L O W F O U N D AT I O N
iv. Strip Footing
•
•
•
Used when the soil has good bearing capacity
The width of footing deeps on the structural
load
Transmits the weight of load bearing wall
across the area of the soil
S H A L L O W F O U N D AT I O N
v) Strap Footing:
• If a Independent footing of two columns are connected by a beam, it is called a strap
footing.
• A strap footing may be used where the distance between the column is so great that
trapezoidal footing becomes quite narrow.
• The strap does not remain in contact with soil and does not transfer any pressure to
the soil.
DEEP F O U N D AT I O N
The shallow foundations may not be economical or even possible when the soil bearing
capacity near the surface is too low. In those cases deep foundations are used to
transfer loads to a stronger layer, which may be located at a significant depth below the
ground surface.
Pile Foundation
•
Can be defined as a series of columns constructed or inserted
into the ground to transmit the loads of a structure to a lower
level of subsoil
•
Can be used when suitable foundation conditions are not
presented at or near ground level.
DEEP F O U N D AT I O N
Deep foundation type is of a foundation. Different from shallow foundation by depth they are
embedded into earth. There are some reason for set up deep foundation, they are low
bearing soil conditions, heavy super structure load and high (height) rise structure. There are
variety of foundation available, pile foundations (end bearing pile, friction pile, tensile pile,
sheet piles, soldier type pile and etc), slurry walls
Pile Foundation
Pile foundations are deep foundations.
PILE F O U N D AT I O N
Pile Foundation classifications
Piles may be classified by their basic design function (end-bearing, friction or a
combination) or by their method of Installing (replacement (driven) or displacement
(bored)).
PILE F O U N D AT I O N C L A S S I F I C AT I O N BY INSTAL L I N G
Displacement Piles/ Driven Piles
Displacement Piles which are driven are termed ‘Displacement Piles’ because their
installation methods displace laterally the soils through which they are introduced Installation
techniques Dropping weight The dropping weight or drop hammer is the most commonly
used method of insertion of displacement piles
Example:
• Steel pile, concrete spun piles
• Precast Concrete Piles , Timber piles
PILE F O U N D AT I O N C L A S S I F I C AT I O N BY INSTAL L I N G
PLACEMENT OF PILE
INSTALLATION OF PILE
REPETITION OF PROCESS
PILE F O U N D AT I O N C L A S S I F I C AT I O N BY INSTAL L I N G
Replacement Piles / Bored Piles
Replacement Piles that are formed by creating a borehole into which the pile is then cast or
placed, are referred to as ‘Replacement Piles’ because existing material, usually soil is
removed as part of the process.
Examples: Bored Piles
PILE F O U N D AT I O N C L A S S I F I C AT I O N BY INSTAL L I N G
PILE F O U N D AT I O N C L A S S I F I C AT I O N BY F U N C T I O N I N G
These piles
transfer
their load
on to a firm
stratum like
rock.
Installed
when soil
bearing
capacity is
low. Pre
fabricated
files and
auger type
pile can be
used.
Friction
piles carry
the major
part of
loads only
by means
of friction
developed
between
pile shaft
and soil.
DEEP F O U N D AT I O N
DEEP F O U N D AT I O N
DEEP F O U N D AT I O N
More About the Deep foundation
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Deep foundation are those in which is very large in the depth of
foundation comparison to its width.
Deep foundation may be of following types
Pile foundation
Pier foundation
Caissons or Well foundation
• Pile Foundation
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•
•
•
•
•
Pile Foundation is that type of foundation in which the loads are taken to a low level
by means of vertical members which may be timber, concrete or steel.
Pile foundation may be adopted when no firm bearing strata is available and the
loading is uneven.
Piles may be of following types
End bearing piles
Friction Pile
Compaction pile
• End bearing piles: This types of piles are used to transfer load
through water or soft soil to a suitable bearing stratum.
• Friction Pile: Friction piles are used to transfer loads to a
depth of friction load carrying material by means of skin
friction along the length of piles.
• Compaction pile: Compaction piles are used to compact
loose granular soils, thus increasing their bearing capacity.
Pier foundation:
• A Pier Foundation consist of cylindrical column of large
diameter to support and transfer large superimposed load to the
firm strata below.
• Generally, pier foundation is shallow in depth than the pile
foundation.
• Well Foundation:
• Well Foundation or Caisson are box like structures which are
sunk from the surface of either land or water to the desired
depth.
• They are much larger than the pier foundation or drilled
caissons.
• Caisson foundations are used for major foundation works like
• Bridge piers
• Docks
• Large water front structure such as pump house.
• Foundations on Black Cotton Soil
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Black cotton soils and other expansive soils have typical characteristics of shrinkage and swelling due to
moisture movement through them.
When moisture enter between the soil particles under some hydrostatic pressure, the particles separate
out, resulting in increase in the volume.
This increase in volume is commonly known as swelling. If this swelling is checked or restricted high
swelling pressure, acting in the upward direction, will be induced.
This would result in several cracks in the walls and may some times damage the structural such as lintels,
beams, slabs etc.
During summer season, moisture moves out of the soil and consequently, the soil shrinks.
Shrinkage cracks are formed on the ground surface. These shrinkage cracks some times also known as
tension cracks, may be 10 to 15 cm wide on the ground surface.
Black cotton soils and other expansive soils are dangerous due to their shrinkage and swelling
characteristics.
In addition, these soils have very poor bearing capacity, ranging from 5 t/m2 to 10 t/m2.
For designing footings on these soils, the following points should be kept
in mind:
• The safe bearing capacity should be properly determined, taking into account
the effect of sustained loading. The bearing capacity of these soils may be
limited to 5 to 10 t/m2.
• The foundation should be taken at least 50 cm lower than the depth of
moisture movement.
• Where this soil occurs only in top layer, and where the thickness of this layer
does not exceed 1 to 1.5 m, the entire layer of black cotton soil should be
removed, and the foundation should be laid on non-shrinkable nonexpansive soil.
• Where the soil is highly expansive, it is very essential to have minimum
contact between the soil and the footing. This can be best achieved by
transmitting the loads through deep piles.
• Where the bearing capacity of soil is poor, or soil is very soft, the bed of the
foundation trench should be made firm or hard by ramming mooram.
Types of foundation in black cotton soils.
Foundation in black cotton soils may be of the following types:
1.Strip foundation. For medium loads, strip foundation may be provided, along
with special design features.
2.Pier foundation Piers are dug at regular interval and filled with cement concrete.
The piers may rest on good bearing strata.
3.Under-reamed pile foundation. An under-reamed pile is a pile of shallow depth (1
to 6 m) having one bulb at its lower end.
• Under-reamed Pile Foundation
• Under-reamed piles are bored cast-in-situ concrete piles having bulk shaped
enlargement near base.
• These piles are commonly recommended for providing safe and economical
foundations in expansive soils such as black cotton soil having poor bearing capacity.
• In these type of foundation the structure is anchored to the ground at a depth where
ground movement due to changes in moisture content negligible.
• A pile having one bulk is known as single under-reamed pile. It is seen that the load
bearing capacity of the pile can be increased by increasing the number of bulk at the
base.
• In such a case the pile is named as multi-under-reamed pile. The increase in the
bearing capacity of the pile can also be achieved by increasing the diameter and the
length of the pile.
•
The method of construction of under-reamed pile is very simple. The holes for casting
piles in the ground may be bored by using hand augers.
• After boring is carried out at the required depth, the base of the bore hole is enlarged in
the form of a bulb near its base by use of a tool, known under-reamer.
• After the pile holes are ready for concreting, reinforcement cage are lowered in the holes
and concrete is poured.
• The piles should be cast at least 200 to 400 mm above the cut-off level. Later on, when
the concrete is hardened, the extra length of each pile is broken and the pile top is
brought to the desired level.
• Thus, besides relative saving in direct cost (when compared with conventional isolated
footings) it is possible to have overall saving in time of completion of a work by
adopting under-reamed piles.
F U N C T I O N S OF F O U N D AT I O N
• Distribution of loads
• Stability against sliding & overturning
• Minimize differential settlement
• Safe against undermining
• Provide level surface
• Minimize distress against soil movement
Functions of foundations :
1. Reduction of load intensity.
Foundations distribute the load of the superstructure to a larger area so the total intensity
of load doesn't exceed the SAFE BEARING CAPACITY of soil.
2. Even distribution of load.
Foundations distribute the non uniform load of the super structure evenly to thesubsoil.
3. Provision of level surface.
Foundations provide a levelled and hard surface over which a super-structure can be built.
4. Lateral stability.
It anchors the super-structure to the ground thus imparting stability to thebuilding.
5. Safety against undermining.
It provides safety against undermining or scouring due to burrowing animals & flood water.
6. Protection against soil movements.
Special measures prevent or minimise the distress (cracks) in superstructure, due to
expansion or contraction of sub-soil.
BASIC DESIGN PROCEDURE
Sizing the chosen
foundation in the
context of loading,
ground bearing
capacity & any likely
future movement of
the building /
structure
Choosing the foundation type, should consider:
1.Soil condition.
Calculation of anticipated structural
loading
Assessment of site conditions in
the context of the site & soil
investigation report
2. Type of structure.
3. Structural loading.
4. Economic factors.
5. Time factor relative to the proposed contract
period.
6. Construction problem.
F O U N D AT I O N FAILURE
Essential requirements for a good foundation
1. The foundation shall be constructed to sustain load and transmit these to subsoil in such a way
that pressure on it will not cause settlement which would impair the stability of the building.
2. Foundation should be rigid so that the differential settlements are minimised. Specially for the case
when superimposed loads are not evenly distributed.
3. Foundations should be taken sufficiently deep to guard the building against
damage or distress caused by swelling or shrinkage of sub-soil.
4. Foundations should be so located that its performance may not be affected due to any unexpected
future influence.
Sub-soil exploration
•
Since the foundations have to transfer the load of the sub-soil, surface conditions at any given site must
be adequately explored to obtain the information required for the design and construction of the
foundations.
•
Sub- soil exploration is done for following purposes :
a) For New Structures :
1. The selection of type and depth of foundation.
2. The determination of the bearing capacity of the selected foundation.
3. The prediction of settlement of the selected foundation.
4. The determination of the ground water level.
5. The evaluation of earth pressure against walls, basements etc.
6. The provision against constructional difficulties.
7. The suitability of soil and degree of compaction of soil.
b) For Existing Structures :
1. The investigation of safety of the structure.
2. The prediction of settlement.
3. The determination of remedial measures if the structure is unsafe or will suffer detrimental settlement.
Site Exploration
The objective of the site exploration is to provide reliable, specific and detailed information about the soil
and ground water conditions of the site for a safe and economic design of foundations.
The exploration should yield precise information about the following :
1. The order of occurrence and extent of soil and rock strata.
2. The nature and engineering properties of the soil and rock formation.
3. The location of ground water and its variation.
Methods of site exploration
The various methods of site exploration may be classified as follows :
a) Open Excavation.
b) Boring Methods.
1.Auger Boring.
2.Auger and Shell Boring.
3. Wash Boring.
4.Percussion Boring.
5.Rotary Boring.
c) Sub-surface soundings.
d) Geo-physical methods.
1.Seismic refraction method.
2.Electrical resistivity method.
a) Open Excavation (Open Trial Pits)
•
Trial pits are the cheapest method of excavation in shallow deposits.
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In this method, pits are excavated at site, exposing the sub-soil surface thoroughly. Soil samples are
collected at various levels.
•
The biggest advantage of this method is that soil strata can be inspected in their natural condition and
samples can be taken conveniently.
•
The method is generally suitable for shallow depths, say upto 3 m.
The cost of open excavation increases rapidly with the depth.
Trial Pits
b) Boring Methods
1. Auger Boring
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Augers are used in cohesive and other soft soils above water table level.
Augers are either mechanical or manually operated.
Hand augers are used for upto an depth of 6m.
Mechanically operated augers are used for greater depth and they can also be
used in gravelly soils.
Augers are of two types :
a) Spiral augers.
b) Post-Hole auger.
Samples recovered from soil brought up by augers are badly disturbed and are useful for identification
purposes only.
Post – hole auger
Helical auger
(Mechanical)
2. Auger and Shell Boring
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•
•
Cylindrical augers and shells with cutting edge or teeth at lower end can be used for making deep
borings.
Hand operated rigs are used for depth upto 25 m and mechanized augers are
used for 50 m depth.
Augers are suitable for soft to stiff clays, shells for very stiff and hard clays, and shells or sand pumps for
sandy soils.
3. Wash Boring
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For test boring over 3 meter in depth, this method can be conveniently used. In this method a hollow steel pipe
known as casing pipe or drive pipe is driven into the ground for a certain depth.
Then a pipe usually known as water jet pipe or wash pipe, which is shorter in diameter, is lowered into the casing
pipe. At its upper end, the wash pipe is connected to water supply system while the lower end of the pipe is
contracted so as to produce jet action. Water under considerable pressure is forced down the wash pipe.
The hydraulic pressure displaces the material immediately below the pipe and the slurry thus formed is forced up
through the annular space between the two pipes. The slurry is collected and samples of material encountered
are obtained by settlement. In this process the particles of finer material like clay, loam etc. do not settle easily
and the larger and heavy particles of the soil may not be brought up at all.
Moreover, the exact position of a material in the formation cannot be easily be located. However the change of
stratification can be guessed from the rate of progress of driving the casing pipe as well as the color of slurry
flowing out.
Yet the results obtained by wash boring process give fairly good information about the nature of the sub-soil
strata.
This method can be adopted in soft to stiff cohesive soils and fine sand.
4. Percussion boring:
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This method consists of breaking up of the sub-strata by repeated blows from a bit or chisel. The material thus pulverized is
converted into slurry by pouring water in the bore.
At intervals the slurry is bailed out of the hole and dried for examination.
This method can be adopted in rocks and soils having boulders.
However this method is not recommended for loose sand or clayey soils.
5. Rotary drilling:
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When rocks or hard pans are to be penetrated for examination, core drilling is done toget
undisturbed samples of the formation.
In this process a hole is made by rotating a hollow steel tube having a cutting bit at its base. The cutting bit makes an
annular cut in the strata and leaves a cylindrical core of the material in the hollow tube.
Two types of cutting bits are generally used, namely, diamond bit and shot bit.
Diamond bit consists of industrial diamonds set in the face of the bit and in shot bit, chilled
shot is used as an abrasive to cut the hardpan.
When core samples of small diameter are needed, diamond bit is preferred.
Percussion boring
Rotary Boring machine
c) Sub-surface sounding
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The sounding method consists of measuring the resistance of the soil with the depth
by the means of penetrometer under static and dynamic loading.
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The penetrometer may consist of sampling spoon or cone or any other shaped tool.
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The resistance to penetration is correlated with some engineering properties of soil such as density index,
consistency, bearing capacity etc.
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Thus in this method by using sounding , the resistance of soil is measured which is useful for general exploration
of erratic soil profiles , for finding depth to bed rock or stratum.
•
We can have an approximate induction of strength and other properties of soil.
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The two commonly used tests are standard penetration test and the cone penetration test.
d) Geo Physical Methods
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Geo physical methods are used when the depth of exploration is very large, and also when the
speed of investigation is of primary importance.
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Geo physical investigations involve the detection of significant differences in the physical properties of geological formations.
•
The most commonly used methods of geophysical investigation are :
1. Seismic Refraction Method :
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The seismic refraction method is based on the property of seismic waves to refract (or be bent) when they travel from one
medium to another of different density or elasticity.
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In this method, shock waves are created into the soil at their ground level or a certain depth below it.
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The radiating shockwaves are picked up by the vibration detector (Geophone or seismometer)
where the time of travel of shock waves get recorded.
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Direct waves or primary waves travel directly from shock point along the ground surface to be picked up by
geophone.
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Refracted waves travel through the soil and also get refracted at the interface of two soil strata. The refracted
waves are also picked up by the geophone.
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If the underlying level is denser the refracted waves travel much faster and at longer
distances, the shock waves reach faster than the direct waves.
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Hence by distance-time graphs and analytical methods, the depth of various strata can be evaluated by using the
time of travel of primary and refracted waves.
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Seismic refraction method is fast & reliable in establishing the profile of different
strata.
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Different material such as gravel, clay hardpan or rock have characteristic properties and hence can be identified
by distance-time graphs.
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But for exact recognition and exploration, boring or sounding methods should be supplemented along.
2. Electrical resistivity method
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The electrical resistivity method is based on the measurement and recording of changes in
the mean resistivity of various soils.
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Each soil soil has its own resistivity depending upon its composition , compaction,water content etc.
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In this method , four metal spikes serve as electrodes which are drive into the ground along a straight line at equal
distance.
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A direct voltage is imposed between the outer two electrodes, and potential dropis measured between the
inner electrodes.
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The mean resistivity Ω (ohm-cm) is calculated by : Ω =
2ΠD E/I
D = Distance between electrodes. (cm)
E = Potential drop between inner electrodes. ( volts) I = Current between
outer electrodes. (ampere)
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The depth of exploration is roughly proportional to the electrode spacing.
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So to study greater depths, the electrode spacing is increased gradually and maderoughly equal to depth of
exploration required. This method is know as resistivitysounding.
Settlement of foundations
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The vertical downward movement of the base of a structure is called settlement.
•
Its effect upon the structure depends on its magnitude, its uniformity, the time over which it takes place, and the
nature of the structure.
•
Settlement of foundation may occur due to :
1. Elastic compression of the foundation and underlying soil.
2. Inelastic compression of underlying soil, which is much larger than the elastic compression.
3. Ground water lowering. Due to changing water level soil tends to compact and causes settlement of ground surface.
Lowering of water level in fine grained soil causes settlement.
4. Vibrations due to pile driving, blasting and oscillating machineries may cause settlement of granular soils.
5. Seasonal swelling and shrinkage of expansive clays.
6. Ground movement on earth slopes, such as surface erosion or landslide.
7. Other causes such as adjacent excavation, mining subsidence, underground erosion
etc.
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A certain amount of elastic and inelastic settlement of foundations is unavoidable, and should be taken into
account in design.
•
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If the settlement is uniform over the whole are of building and is not excessive, it does little damage.
If however, the amount of settlement varies at different points under the building, differential settlement occurs
which results into stresses being setup in the building.
•
It is suggested that the allowable pressure should be selected such that the
maximum settlement of an individual foundation should be 2.5 cm.
CAUSES OF FAILURES OF FOUNDATIONS AND REMEDIAL MEASURES
1. Unequal settlement of subsoil :
Unequal settlement of the sub-soil may lead to cracks in the structural components. unequal settlement occurs due to :
i) Non uniform nature of soil .
ii) Unequal load distribution on soil strata.
iii) Eccentric loading.
Remedy :
i) Resting foundation on rigid strata.
ii) limiting pressure in soil.
iii) Avoiding eccentric loading.
2. Unequal settlement of masonry :
The portion of masonry, situated between the ground level and concrete footing has mortar joints which may either shrink or
compress, leading to unequal settlement of masonry.
Remedy :
i) Use mortar of proper strength.
ii) Using thin joints.
iii) Properly watering the masonry.
iv) Limiting height of masonry to 1 m per day for lime mortar and 1.5 m per day for cement mortar.
3. Sub-soil moisture movement :
•
•
When the water table drops down, shrinkage of sub-soil takes place and hence lack of sub- soil support is encountered and
cracks develop in thebuilding.
When water table rises swelling takes place inducing swellingpressure.
4. Lateral pressure on the walls :
•
•
The walls transmitting load to foundation may be subjected to lateral pressure or thrust from a pitched roof or arch or
wind action.
The foundation may fail by overturning or generation of high tensile stresses on one side and
high compressive stress on the other side.
5. Lateral movement of sub-soil :
•
•
•
This is applicable to very soft soil which moves out laterally due to verticalpressure.
Such a situation may arise if ground is sloping or in granular soil where a big pit is being
excavated nearby.
Due to excessive settlement , the building may even collapse.
Remedy:
•
In such a situation, sheet piles shouldbe driven to prevent lateral movement or escape of soil.
7. Atmospheric action:
•
•
•
Atmospheric agents such as sun, wind, rain may adversely affect the behavior of the foundation.
If depth of foundation is shallow , moisture movements may cause scouring.
If water is stagnant, it will result into dampness which ultimately decreases the strengthof
the footing or foundation wall.
Remedy:
It is recommended to provide suitable plinth protection along external walls by:
i) Filling back the foundation trenches with good soil and compactingit.
ii) Provide gentle ground slope away from the wall.
iii) Providing a narrow , sloping strip of impervious material along the exteriorwalls.
• The soil supporting a building must be strong enough to carry the super
imposed load. After the prelimanary and detailed investigation of the
type of soil, depth of bed rock, Elevation of ground water etc., the next
step is to select a suitable foundation to be used for the building.
• The depth to which the foundation is to be taken and its bottom
dimension so that it can be safely transmit the load from building to under
lying soil with out any failure or significant settlement.
• For the determination of this, a knowledge of the safe allowable
pressure on the soil is necessary.
• The ability of the soil to support the super imposed load without excessive
settlement or failure is called Bearing capacity.
Ultimate bearing capacity:-The gross pressure intensity at the base of
the foundation which would cause shear failure of the soil.
Safe bearing capacity:- maximum pressure which the soil can carry
without the risk of shear failure.
ultimate bearing capacity
Safe bearing capacity =
Factor of safety
Determination of ultimate bearing capacity in the field
Simplest and widely
field test- plate load test
used
A square pit of sides equal to five times the width
of test plate is dug up to the required depth.
Test plates are iron plates of size 60cm square for
clayey soil 30cm square for sandy soil.
At the centre of the pit, a square hole of size equal
to the test plate is dug.The bottom of the test plate
should be along the proposed foundation
level.(b1/d1=b2/d2)
•
Seat the plate accurately over the centre of pit
and it should be in contact with the soil over the
whole area
•
A loading post and hydraulic jack is provided
above the test plate.Hydraulic jack support a
gravity loading platform. The loading is done
with sand bags,concrete blocks.
•
Load is increased in regular increments of
250kg or 1/5th of ultimate bearing capacity
whichever is less
•
Each loading increment is kept in postion until no further measurable settlement occurs. Settlement of
the plate is measured by two sensitive dial guage of sensitivity 0.02mm.
•
Plot a graph between settlement and load.
•
From the graph measure maximum load upto which settlement is proportional
• Ultimate Bearing capacity of soil = Maximum load / Area of test plate
• Safe bearing capacity
Safe bearing capacity =
ultimate bearing capacity
factor of safety
Factor of safety may be 2 or 3
Methods for improving bearing capacity of soil
• Increase the depth of foundation
• By draining the soil
– Water content in soil will decreases its bearing capacity
– By draining sandy soil and gravel by gravity pipe drainage
system-improve bearing capacity
• By compacting the soil
– Reduces the open spaces between the individual particles
• By grouting
– Cement mortar can be injected under pressure into the
subsoil to seal off voids in between subsoil and foundation.
• By confining the soil
– Sheet piles are driven around the structure to form an enclosure
– Sheet piles are sections of sheet materials with interlocking
edges that are driven into the ground to provide earth retention and
excavation support. Sheet piles are most commonly made of steel, but
can also be formed of timber or reinforced concrete.
– Which will prevent the movement of soil.
• Chemical treatment
– Chemical solution are injected under pressure into the soil
– Forms a gel and keep soil particles together to form a compact mass.
Types of Loads Acting On Buildings
Types of Loads Acting on a Building
Dead
Load
Live
Load
Wind
Load
Snow
Load
Load
due to
Rain
Earthqua
ke Load
Dead Load
• The dead load includes loads that are relatively constant over time, including the
weight of the structure itself, and immovable fixtures such as walls, roof, immovable
furniture, etc.
• Dead load is permanent, immovable and untransferable load of a structure.
• The dead load of floors, roofs, beams, ceilings, etc. is proportionately transmitted on
the surrounding walls.
Weights of common construction materials of a building
Sr.
No.
Material/Structure
Weight (in kg/m3)
1.
Brick Masonry Walls : 10 cm thick
20 cm thick
192
384
2.
Plain Cement Concrete (P.C.C.)
2300
3.
Reinforced Cement Concrete (R.C.C.)
2400
4.
Bricks
1600 – 1920
5.
Steel
7850
6.
Cement Plaster, 25 mm thick
52
7.
Sand
1760 – 2000
Live Load
• This is the movable, temporary and transferable load on the floor and hence it is
variable.
• The weight of furniture, stored materials, humans, etc.
• are examples of live loads.
• It is also known as superimposed load.
• The live loads are assumed to be acting uniformly over the whole floor area and is
distributed proportionately on the wall foundations.
Wind Load
• Tall buildings are subjected to wind pressure on their exposed faces and inclined
or sloppy roof surfaces.
• The effect of wind pressure is to reduce the pressure on the foundation on the
windward side and to increase the pressure on the leeward side.
• Wind pressure can be measured by the formula :
P = kV2
p = wind pressure in kg/m2
V = velocity of wind in km/hr
k = coefficient whose value depends on various factors such as
wind speed Temperature of air, etc.
k= 0.0006 (as per building code)
Snow Load
• Snow load acts on roofs.
• Actual load due to snow will depend on the shape of the roof and its capacity to
retain the snow.
• Mountain regions in northern parts of India are subjected to snow fall. Houses in
this region experience snow load.
• The
load
of
snow maybe taken as 2.5 kg/m2 per
cm depth
of snow.
Load due to Rain
On surfaces of roofs whose positioning, shape and drainage systems are such as to
make accumulation of rain water possible, the load due to the rain water is known as
the Load due to rain.
Earthquake Load
• Earthquake causes shaking of ground resulting in shaking/motion of building at its
base.
• Forces acts on the building due to the earthquake especially in horizontal
direction.
• This fore can damage or even collapse the building.
• Nowadays earthquake resistant buildings are constructed which can resist the
severe earthquakes also.
Building Components, Their Functions and Dimensions
Super Structure
Plinth
On surfaces of roofs whose positioning, shape and
drainage systems are such as to make accumulation of
rain water possible, the load due to the rain water is
known as the Load due to rain.
Walls, Columns & Beams
On surfaces of roofs whose positioning, shape and
drainage systems are such as to make accumulation of
rain water possible, the load due to the rain water is
known as the Load due to rain.
Floors
On surfaces of roofs whose positioning, shape and
drainage systems are such as to make accumulation of
rain water possible, the load due to the rain water is
known as the Load due to rain.
Sills, Lintels & Weather Sheds
On surfaces of roofs whose positioning, shape and
drainage systems are such as to make accumulation of
rain water possible, the load due to the rain water is
known as the Load due to rain.
Doors, Windows & Ventilators
On surfaces of roofs whose positioning, shape and
drainage systems are such as to make accumulation of
rain water possible, the load due to the rain water is
known as the Load due to rain.
Roofs & Slabs
On surfaces of roofs whose positioning, shape and
drainage systems are such as to make accumulation of
rain water possible, the load due to the rain water is
known as the Load due to rain.
Parapet
On surfaces of roofs whose positioning, shape and
drainage systems are such as to make accumulation of
rain water possible, the load due to the rain water is
known as the Load due to rain.
Stairs, Lifts & Ramps
On surfaces of roofs whose positioning, shape and
drainage systems are such as to make accumulation of
rain water possible, the load due to the rain water is
known as the Load due to rain.
Building Finishes
On surfaces of roofs whose positioning, shape and
drainage systems are such as to make accumulation of
rain water possible, the load due to the rain water is
known as the Load due to rain.
Building Components and their
Functions
Sr. No.
Building Components
Functions
Foundation
It transmits the load coming from the
superstructure on to the sub-soil below
it.
Plinth
It protects the building from rain water,
damp or moisture, insects and transmits
the load of superstructure to the
foundation.
3.
Walls
Provided to enclose or to divide the
floor space into rooms as per
requirement and also provide privacy,
security and protection against sun,
rain, etc.
4.
Column
Transmits the load coming from the
beams on the sub – soil below it.
5.
Sill
Supports window frame at bottom.
1.
2.
Sr. No.
Building Components
Functions
6.
Door
Provides access into the room, offers
privacy of sight and sound.
7.
Window
Opening made in wall for providing
light and ventilation.
Ventilator
Small opening made in wall, provided
at lintel level for removal of exhaust air
or foul smell.
Roof/Slab
It is the uppermost part of a building to
cover the space below and protect it
from sun, wind, rain and snow.
Beam
Media by which all loads of slab are
transferred to vertical supports of a
building.
8.
9.
10.
Sr. No.
Building Components
Functions
Lintel
Supports the weight of the wall above
the openings of doors, windows and
ventilator.
Stair
Means of vertical transportation
between the floors. Provides access
between various floors.
Floor
Provides plane surface and supports the
occupants, furniture, fixtures and
equipments of a building.
14.
Watershed/Chajjas
Generally combined with lintels to
protect doors, windows or ventilators
from sun, rain, wind, etc
15.
Parapet
Acts as a protective solid balustrade for
the users.
11.
12.
13.
Building Components and their
Nominal Dimensions
Sr. No.
Building Components
Functions
1.
Foundation
Shallow foundations: Depth≥2T+30
Width=2T+30; T=wall thickness
Deep foundations: Depth-10 to30m
Width as per design
2.
Plinth
Height above ground : 30, 45, 60, 75 or
90
3.
Walls
Load bearing walls : 20, 30, 40 cm
Partition wall : 10 cm
4.
Column
Square : 20 x 20cm, 30 x 30 cm
Rectangular : 20 x 30cm
Circular : 20 Ø, 30 Ø
Footing: 1x1x1 m pit as per design
5.
Sill
Sill height : 70, 80 or 90 cm above floor
Sr. No.
Building Components
Functions
6.
Door
Width : 0.80(min.), 0.90, 1.0, 1.20 m
Height : 1.80 (min), 2.0, 2.10 m
7.
Window
Width : 0.60, 0.70, 0.90, 1.00, 1.20 m
Height : 1.20 m
8.
Ventilator
Width : 0.60, 0.70, 0.90, 1.00, 1.20 m
Height : 0.20 or 0.30 m
9.
Roof/Slab
R.C.C. slab thickness : 10, 12, 15 or 18
cm
10.
Beam
Depth:30, 45 or 60 cm
Width : wall thickness or 30, 45, 60 cm
Sr. No.
Building Components
Functions
Lintel
Length=width of door/window opening
+ min. 10 cm bearing on both ends
Width = thickness of wall
12.
Stair
Tread : 25 cm, 30 cm
Riser : 115 to 20 cm
Width of stair := minimum 1.0 m
13.
Floor
Ground floor = plinth height
Upper floor = slab thickness
Watershed/Chajjas
Tapered in shape: Front : 7 to 10 cm
At lintel side = height of lintel = 10 to
15 cm
Parapet
Height : 1.0m(min.), 1.10, 1.20 or 1.30
m (approx.)
Width : 10, 20 or 30 cm
11.
14.
15.
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