Foundation and its functions
Essential requirements
Sub soil exploration and Site exploration
Methods of site exploration
Settlement of foundations
Causes of failure of foundation and remedial measures
2. Contents :
Foundation and its functions
Essential requirements
Sub soil exploration and Site exploration
Methods of site exploration
Settlement of foundations
Causes of failure of foundation and remedial measures
3. What is a foundation ?
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Every building consists of two basic components :
1. The Super-structure
2. The Sub-structure
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The substructure or foundation is that part of the structure which is usually below
the ground level and in direct contact with the soil , through which the load of
superstructure is transmitted to the soil.
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The basic function of a foundation is to transmit the dead loads, superimposed
loads and wind loads from a building (superstructure) to the soil in such a way
that:
a. Settlements are uniform and within permissible limit. &
b. The soil does not fail.
4. 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 the subsoil.
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 the building.
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.
5. 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.
6. Sub-soil exploration
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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.
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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.
7. 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.
8. 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.
9. a) Open Excavation (Open Trial Pits)
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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.
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The biggest advantage of this method is that soil strata can be inspected in their
natural condition and samples can be taken conveniently.
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The method is generally suitable for shallow depths, say upto 3 m.
The cost of open excavation increases rapidly with the depth.
11. 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.
13. 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.
14. 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.
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16. 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 to get
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 hard pan.
When core samples of small diameter are needed, diamond bit is preferred.
18. 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.
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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.
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20. 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.
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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.
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24. 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 drop is
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 made roughly
equal to depth of exploration required. This method is know as resistivity sounding.
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26. Settlement of foundations
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The vertical downward movement of the base of a structure is called settlement.
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Its effect upon the structure depends on its magnitude, its uniformity, the time over
which it takes place, and the nature of the structure.
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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.
27. 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.
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It is suggested that the allowable pressure should be selected such that the
maximum settlement of an individual foundation should be 2.5 cm.
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29. 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.
30. 3. Sub-soil moisture movement :
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When the water table drops down, shrinkage of sub-soil takes place and hence lack of subsoil support is encountered and cracks develop in the building.
When water table rises swelling takes place inducing swelling pressure.
4. Lateral pressure on the walls :
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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 :
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This is applicable to very soft soil which moves out laterally due to vertical pressure.
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:
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In such a situation, sheet piles should be driven to prevent lateral movement or escape of
soil.
31. 7. Atmospheric action:
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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 strength of
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 compacting it.
ii) Provide gentle ground slope away from the wall.
iii) Providing a narrow , sloping strip of impervious material along the exterior walls.
32. A case of sub-soil movement resulting in foundation failure and finally collapse of
the building.
