1. BUILDING CONSTRUCTION
DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY
CIVIL ENGINERING DEPARTMENT
Prepared By: Jani Gaurang N.
BUILDING CONSTRUCTION
SHALLOW FOUNDATION
DARSHAN INSTITUTE OF ENGINEERING AND TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
DARSHAN INSTITUTE OF ENGINEERING AND TECHNOLOGY CIVIL ENGINEERING DEPARTMENT

2. CLASSIFICATION OF FOUNDATION
SHALLOW FOUNDATION
DEEP FOUNDATION
DARSHAN INSTITUTE OF ENGINEERING AND TECHNOLOGY CIVIL ENGINEERING DEPARTMENT

3. DEFINTION OF SHALLOW FOUNDATION
According to terzaghi, a foundation is shallow if the depth is equal to or less than its width.
A shallow foundation is also known as an open foundation.
Those foundation, which have depth even greater than its width, but constructed by way of open excavation also come under shallow foundation.
A shallow foundation is placed immediately below the lowest part of the super-structure supported by it.
DARSHAN INSTITUTE OF ENGINEERING AND TECHNOLOGY CIVIL ENGINEERING DEPARTMENT

4. The footing is commonly used in conjunction with shallow foundation
The footing is commonly used in conjunction with shallow foundation. A footing is a foundation unit constructed in brick work, masonry or concrete under the base of a wall or column for the purpose of distributing the load over a large area.
Spread footing
Combined footing
Strap footing
Mat or raft
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5. SPREAD FOOTING
The spread footing are those which spread the superimposed load of wall or column over a larger area.
Single footing for a column
Stepped footing for a column
Slopped footing
Wall footing without step
Stepped footing
Grillage foundation
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6. Single footing for a column
A spread footing for a single column is either known as the isolated footing
In this case, the footing may consist of simple concrete block projection out from the column face on all sides.
The base dimensions of the concrete base should not be less than twice the appropriate lateral dimension of the column in that direction.
The thickness of concrete block should atleast be equal to side offset from the column face.
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7. STEPPED FOOTING FOR A COLUMN
If the column load is more or if the safe bearing pressure of the soil is less, the base area will be large.
In such a case, it is necessary to provide masonry offsets, to achieve larger spread, before the load is transferred to the concrete base.
The height and width of each offset should be so proportioned that the rate of spread does not exceed the permissible value for the masonry.
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8. SLOPPED FOOTING FOR A COLUMN
These are also known a isolated or individual column footings.
They have the projection in the concrete base.
Due to the low bending strength the footings constructed with bricks, stone or plain concrete require considerable depth to be safe to carry heavy loads.
The depth of plain concrete footing can be reduced much, by providing reinforcements at its base to taken up tensile stresses.
RCC column footings may be circular, rectangular or square in plan.
The footing is reinforced both-ways by means of mild-steel ribbed bars placed at right angles to one another at equal distances apart.
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9. WALL FOOTING WITHOUT STEP
It is also called strip footings, which provides a continuous longitudinal bearing.
When the wall carries light loads and the safe bearing pressure is very high, width of the footing may be very small. In such a case, the wall directly rests on the concrete base and no masonry offsets are provided.
As a rule, the width of concrete base should not be less than twice the width of the wall and the thickness should be atleast equal to offset.
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10. STEPPED FOOTING FOR WALL
When the wall carries heavy loads and the safe bearing pressure of the soil is not very high, the base width required may be much greater.
In such a case, the masonry offsets are provided to achieve larger spread, before the load is transferred to concrete base.
In case of typical wall footings, the lowest course of bricks will have twice the width of the wall above at plinth level.
The depth of concrete bed is normally not less than 15 cm and its projection on both the side of wall base may be 10 to 15 cm.
The depth of concrete bed should not be less than its projection beyond the wall base.
If the walls having thickness ‘t’ cm. the foundation width is equal to ‘(2t+30)’ cm. and the depth of foundation is equal to or more than the width of foundation or approximately equal to ‘3t’ cm.
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11. COMBINED FOOTINGS
A combined footing is a single footings, which supports two columns.
When the columns are very near to each other.
When the bearing capacity of soil is less, requiring more area under individual footing.
When the end column is near a property line so that its footing can not spread in that direction.
Purpose of combined footing is to get uniform pressure distribution under the footing.
For this, the CG of the footing area should coincide with the C.G. of the combined loads of the two columns.
If the outer column, near the property line, carries heavier load, provision of trapezoidal column becomes essential to bring the C.G. of footing in line with the C.G. of two column loads.
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13. STRAP FOOTING
A strap footing consists of two or more footing of individual columns, connected by a beam, called a strap.
When a column is near or right next to adjacent property limit, a square or rectangular footing concentrically located under the column would extend into the adjoining property.
The strap beam, connecting the spread footing of the two columns, does not remain in contact with soil and thus does not transfer any pressure to the soil.
The function of the strap beam is to transfer the load of heavily loaded outer column to the inner one.
Beam is design to carry shear and bending action.
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15. GRILLAGE FOUNDATION
A grillage foundation is an isolated footing generally provided, when heavy structural load from column, piers or steel stanchions are required to be transferred to a soil having poor or low bearing capacity.
Steel grillage foundation.
Timber grillage foundation.
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16. Steel grillage foundation is constructed of steel beams, structurally known as a rolled steel joints(R.S.J.).
In case double tier grillage, the top tier of grillage beam is laid at right angle to the bottom tier.
The joints or beam of each tier are held in position by 20 mm dia. Spacer bars with 25 mm dia. Pipe separators.
The grillage beams are embedded in concrete and minimum clearance of 8 cm is kept between the grillage beams so that concrete can be easily poured and compacted.
It should be noted that the concrete filling does not carry any load, it simply keeps the beam in position and prevents their corrosion.
Minimum concrete cover of 10 cm is kept on the outer sides of the external beams.
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17. The concrete is then placed between and around the beams.
The excavation for foundation is done to the desired depth and the bed is well levelled.
The foundation bed is then covered with a 15 cm. thick rich mix of concrete, which is well compacted to make it impervious.
The grillage beams are then placed on this bed at specified distance using separators. The top surface of grillage beams is kept in a horizontal plane and rich cement grout is filled all around the lower flange of the beams to secure them to the concrete bed.
The concrete is then placed between and around the beams.
The second tier of beams is then placed at right angles to the first tier and the entire space is filled with concrete.
The steel stanchion is then erected on the base plate fixed on the second tier beams, with the help of side angles and gusset plates. These connecting elements are also embedded in the concrete.
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19. This type of foundation is provided for heavily loaded masonry walls or timber column.
This foundation is specially useful in waterlogged areas, where the bearing capacity of soil is very low and where the steel beams may get corroded due to subsoil water but loading on the soil is limited to 50 to 60 kn/m².
The foundation uses timber planks and timber beams in the place of steel joists. And no concrete is embedded between the timber joints.
The bottom concrete provided in steel grillage foundation is replaced by timber platform constructed of timber planks.
The excavation for foundation is done to the desired depth. The bottom layer of timber planks of size 20 to 30 cm. wide and 5 to 7.5 cm thick is laid.
Over this platform, a tier of wooden beams, 15X10 cm. in size, spaced at 30 to 50 cm. is laid at right angles to the direction of the planks.
The top of this layer, a timber beam of same section as that of the wooden post is placed at right angles.
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21. RAFT FOUNDATION
A raft or mat is combined footing that covers the entire area beneath a structure and supports all the walls and columns.
When the allowable soil pressure is low and building loads are heavy, the use of spread footing would cover more than one-half of the area and it may prove more economical to use mat or raft foundation.
It is also suitable where the differential settlement is not under control.
It also reduce the settlement on highly compressible soils, by making the weight of structure and raft approximately equal to the weight of the soil excavated.
A raft consist of flat concrete slab with uniform thickness throught the area.
If the column loads are heavy, the slab under the column is thick.
If the column spacing is large, and/or the column loads are heavy, thickened bands may be provided along the column lines in both the direction.
If the loads are extremely heavy, two way grid structure made of cellular construction may be used.
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22. Rafts are designed as reinforced concrete flat slabs.
NOTE: IMPORTANT FOR DESIGN CRITERIA
Rafts are designed as reinforced concrete flat slabs.
If the C.G. of loads coincide with the centroid of the raft, the upward load is regarded as a uniform pressure equal to the downward load divided by the area of the raft.
The weight of the raft is not considered in the structural design because it directly rest on the subsoil.
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24. FOUNDATION IN BLACK COTTON SOILS
Construction of building on black cotton soils is very much dangerous due to its volumetric changes.
due to the pressure of fine clay particles, the black cotton soil will swell, when it comes contact with water.
The different settlement of the buildings, caused by the movement of the ground due to the alternate swelling and shrinkage, results in formation of cracks.
The cracks thus formed are sometimes 15 to 20 cm. wide and 2.5 to 4.0 m. deep.
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25. Precaution for foundation in black cotton soils
The safe bearing capacity should be properly determined, taking into account the effect of sustained loading. Because of the SBC of soils is 50 to 100 kn/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 the black cotton soil should be removed, and the foundation should be laid on non-shrinkable non-expansive soil.
Where the depth of clay layer is large, the foundation or footing should be prevented from coming in contact with the soil. This can be done by excavating wider and deeper foundation trench and interposing layer of sand/mooram around and beneath the footing.
Where the soil is highly expansive, it is very essential to have minimum contact between the soil and the footing.this can be achieved by transmitting the loads throught deep piles.
Where the bearing capacity of soil is poor, the bed of the foundation trench should be made firm or hard by ramming mooram and ballast.
The foundation should be constructed during dry season.
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26. STRIP FOUNDATION It suitable in medium loads.
A 60 cm thick layer of cohesion less sand is placed below the foundation concrete, and is compacted.
When the soil swells, the sand is also grains would yield by moving up, thus relieving the swelling pressure.
The alternate layer of sand and mooram and act as a spring which can compress or expand along with the sub-soil movements.
If the soil is soft and has poor bearing capacity, a 30 cm thick layer of ballast and mooram should first be rammed into the soil.
Over the top of it, a min. of 30 cm thick layer of coarse grained sand may be placed.
After compacting the base of the trench, 25 to 30 cm wide strips of concrete, 25 to 30 cm thick, may first be laid and compacted.
The space between the two strips of concrete may be kept equal to width of the bottom layer masonry.
On the top of this, the foundation concrete layer, preferably of reinforced concrete is laid.
In addition 80 mm dia. Pipes spaced at 1.5 to 2 m etc. are placed through masonry and concrete bed, so as to reach the bottom sand fill.
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28. PIER FOUNDATION WITH ARCHES
Piers are dug at regular interval and filled with cement concrete.
The piers may rest on good bearing strata.
Piers are connected by concrete or masonry arch, over which the wall may constructed.
A concrete beam may be provided over the arch if the arch is constructed of masonry.
The arches are constructed with a gap above the ground level and this gap would permit free vertical movement of soil during swelling and shrinkage operations.
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30. 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.
If the bulb is taken at a level lower than the critical depth of moisture movement in expansive soils, the foundation will be anchored to the ground and it would not move with the movement of the soil.
Dia. Of piles between 15 to 50 cm.
The piles spacing may vary between 2 to 4 m. the piles are connected by a rigid capping beam, with reinforced and wall may be also constructed above it.
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32. CAUSES OF FAILURE OF FOUNDATIONS
Unequal settlement of the sub-soil
the nature of the soil may not be uniform.
loading conditions may be non-uniform.
pressure on the soil may exceed the safe-bearing pressure.
Unequal settment of masonry
the mortar used in the masonry may shrink or compress, when excessive load comes over it before setting, which results into unequal settlement of masonry.
moratar should provide easy workability and should not be too lean or too stiff.
the masonry should be constructed to the same uniform level throught the building.
the height of the masonry should not be more than 1.5m per day.
the curing of masonry should be done properly for at least 10 days.
Horizontal movement of soil adjoining the structure
the clayey and black cotton soils undergo volumetric changes with the changes in atmospheric conditions.
the load on the soil should be limited to 5.5 tonnes/m².
foundation concrete block or masonry should be provided with a layer of sand or morrum on either side or at the bottom to prevent the intimate contact of soil.
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33. Shrinkage due to withdrawn of moisture from soil below the foundation
the roots of trees grown near the wall foundation, absorbs the sub-soil moisture, resulting in differential shrinkage below the foundation and forms the cracks in foundation.
the safe distance should be maintained.
Lateral pressure on the walls
the walls of the building may be subjected to lateral pressure due to slopped roof or an arch pressure or due to a violent storm or a wide cantilever projection.
Action of atmosphere
the salts in the rainwater entering into the ground, may react chemically with the material of foundation and disintegrate it.
if the foundation is not taken to a sufficient depth, the rainwater may scour the soil and expose the foundation.
due to the change in the sub-soil water table, the expansion or shrinkage of the soil may takes palce, which causes the cracks in the foundation.
foundation should be taken to a sufficient depth to avoid the adverse effect of atmosphere.
the adequate drainage provision of sub-soil water should be made by providing proper ground slope.
foundation should be provided with dense cement concrete or stone masonry, where the ground water and soil contains excessive salts.
The sides of foundation trenches should be well filled and consolidated and providing plinth protection all along the external walls to keep the rainwater away from the building.
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34. SETTING OUT OF FOUNDATION TRENCHES
The setting out foundation trenches means the marking of the excavation lines and centerlines on the ground before excavation is started.
The setting out plan or foundation layout plan is a dimensioned ground floor plan, usually drawn to scale of 1:5.
For setting out the foundations of any building, the centerline of the longest outer wall of the building is first marked on the ground by stretching a string between wooden pegs driven at the ends. This serves as the reference lines for marking the center lines of all the walls of the building.
The centerline of the wall, which is perpendicular to the long wall, is marked by setting up a right angle. The right angle is set up forming triangle with sides 3,4, and 5 units long. If we fix the two sides of right angled triangle to be 3m. And 4m., then the third side, i.e. the hypotenuse should be taken as 5m. The dimension should be setout with a steel tape.
The alternate method of setting out right angle is by the use of theodolite.
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35. Similarly, the outer line of the foundation trench of each cross-wall can be set out, which are marked by stretching the string joining the corresponding pegs at the two extremities of the line, with the help of dry lime-powder.
In an accurate method, brick pillars or platforms constructed at both ends of each wall. The platform are about 20 cm. thick and 15 cm. wider than the centerline of the building walls are carefully laid by means of small nails fixed on the top of the wooden pegs driven in the trench width and are plastered at the top. The tops of platforms should be equal to plinth height of the building.
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