FOUNDATION :The building foundation or sub structure is that part of a structure which is placed below the surface of the ground and which transmits the superstructure load to the underlying soil ultimately. It is the part of a structural system that supports and anchors the superstructure of a building. Foundation is the most important part of a building. Building activity starts with the formation of foundation. Main activities of building foundation are:-

To distribute building load to soil beneath To distribute the load uniformly To grapnel the structure to the ground to resist movement due to lateral force To prevent sinking of the structure.

Any part of a structure that serves to transmit the load to the earth or rock can be called foundation. The higher and heavier the building is to be, the wider and deeper the supports of footings for the foundation have to be.

PURPOSE OF FOUNDATION

To pass out building weight to the soil beneath evenly;To prevent differential settlement of building;To provide a plane surface for the convenience of construction;To make building substantial and durable by continuing the structure in the soil.

TYPES OF FOUNDATION :Depending on the depth of the load-transfer member below the super-structurethe depth of the load-transfer member below the super-structure and the the type of transfer load mechanismtype of transfer load mechanism foundation can be classified into two types:

a) Shallow Foundationb) Deep Foundation.

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The classification on foundation has been provided in flow chart as follows:

Classification of Foundation

a) SSHALLOWHALLOW F FOUNDATIONOUNDATION : A shallow foundation is a type of foundation which transfers building loads to the earth very near the surface. The objective of shallow foundation is to distribute the structural concentrated load over a wide horizontal area at a little depth rather than a range of depths. Shallow foundation is often selected when the soil has a good bearing capacity and the structural load will not cause excessive settlement of the underlying soil layers. In general, shallow foundations are more simple and cost effective to construct than deep foundations because little soil is removed or disturbed. Shallow foundation construction is typically utilized for most residential and light commercial raised floor buildings.

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Foundation

Shallow

Spread Footing

s

Wall

Footings

Isolated

Column

Footings

Grillage Founda

tion

Combined

Footings

Mat Founda

tion

Eccentrcally

Loaded Footing

s

Deep

Pile Foundation

Pre-cast Piles

Cast-in -situ Piles

Cofferdams Caisson

SPREAD FOOTINGS : Spread footing is a common term used to refer shallow foundations which transfer’s concentrated loads to a wide range area. It includes reinforced concrete footings, wall footings, isolated column footings. Spread footing provides a stable base or platform at a low depth and it prevents the structure from settling into the ground.

Spread Footing

ISOLATED COLUMN FOOTINGS : Isolated column footings are used to support single columns. Each individual isolated footing provides support for each individual column, pier, post or other single concentrated load. So, they act as a base for a column. They transfer the superimposed structural load to a wide range of soil. They are the most economical types of footings and are used when columns are spaced at relatively long distances. They can be square, rectangular or even circular in plan view.

Isolated footings can be of brick masonry, stone masonry or of Reinforced Cement Concrete (R.C.C.) depending on amount of load, project expanses and materials available. The base of the footing depends on the load bearing capacity of soil and the superimposed load. They are also known as pad footing or individual column footing. Generally isolated footings are used in case of reinforced concrete structure buildings.

WALL FOOTINGS : This type of shallow footing supports a wall by providing footing beneath the entire wall structure. This type of footing is used when columns are made of bricks.

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GRILLAGE FOOTINGS : In case of low bearing capacity of soil and avoiding deep excavation grillage foundation can be used. Grillage foundation can reduce the superimposed load of column within the safe bearing capacity of the soil. The grillage can be made of steel or timber.

COMBINED FOOTINGS : A combined footing is nothing but a combination of pad footings having a common base. A combined footing is constructed when (i) a column lies very close to the property line and (ii) to prevent overlapping of footings when columns are very adjacent. They can be rectangular or trapezoidal in plan. Combined footing proves to be more cost effective than a single column footing.

MAT FOUNDATION : A mat is a slab that supports multiple columns. It is typically used when the bearing capacity of soil is very low. When required footings will cover more than half the area beneath a structure, it is often desirable to enlarge and combine the footings to cover the entire area. A mat foundation may be cheaper than individual footings because of reduced forming costs and simpler excavation procedures. Although mat foundations are more difficult and more costly to design but they prove to be more effective.

ECCENTRICALLY LOADED FOOTINGS : It is a spread or wall footing that must resist a moment in addition to the axial column load.

b) DDEEPEEP F FOUNDATIONOUNDATION : A deep foundation is used when the bearing capacity of soil is low near ground. When a building structure transmits excessive loads to a soil with low bearing capacity near ground, settlement of the foundation takes place which endangers the stability of the structure. In that case deep foundation is used to ensure good bearing capacity of soil at a considerable amount of depth. A deep foundation requires considerable amount of materials and earthwork resulting increased cost and effort. Piles, cofferdams and caissons are some the familiar forms of deep foundations.

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IISOLATEDSOLATED C COLUMNOLUMN F FOOTINGOOTING : : Depending on structural load and soil isolated footing can be of following types:

BBRICKRICK P PILLARILLAR F FOOTINGOOTING: When the structural columns are made of bricks this type of isolated footing is often used. This is the most economic type of isolated column footing. The width and the depth of the footing depend on load to be carried and bearing capacity of the soil. The width is found by offsets running symmetrical round the column.Stone Pillar FootingStone Pillar Footing: Stone pillar footings are stronger than brick pillars. Construction of this type of footing is similar to brick pillar footing. The width and the depth of this footing is a bit bigger because of the stone sizes. This type of footing is weak in resisting bending. So they are not used against long heavy structural loads.R.C.C. Column FootingR.C.C. Column Footing: In case of column subjected to heavy loading and bending this type of column footing provides a superior solution in shallow foundation. Made with reinforcement and concrete this type of footing is high on strength and bending. The footing is reinforced cross ways by re-bar placed at right angles to one another. This type footing is capable of transmitting massive loads with a reduced footing depth. This type is used as a most effective form of isolated footing. Due to its strong structure this footing is used against high building structures provided that the soil has sufficient bearing capacity.

INDEPENDENT COLUMN FOOTING REQUIREMENTS : The main requirements of an independent footing are as follows:

i. Must distribute loads evenly to the soil;ii. Must be well drained to prevent wash-out;

iii. Must bear on engineered soil.

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STEPS OF ISOLATED FOOTING

SOIL EXCAVATION

LEVELLING & DRESSING OF SOIL SURFACE

PLACEMENT OF BRICK FLAT SOLING LAYER

GIVING A CEMENT CONCRETE LAYER ON IT

PLACEMENT OF CEMENT CONCRETE BLOCK

PLACING OF REINFORCEMENT

SHUTTERING

CONCRETE CASTING

REMOVAL OF FORMWORK

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CURING

BACKFILLING OF EXCAVATED AREA

SOIL EXCAVATION

To set a footing, the first step is to excavate the soil of respective area. At first the area should be located. The depth of excavation depends on the desired strength as strength increases with depth. We see the tools that are used in excavation below,

PICS

In most cases, baskets & spades are used for excavation. Deep excavation damages adjacent constructions. In this phase, shore piles or other preventive means are used. Soil sloping can also reduce this damage. The picture below shows how workers dig soil during excavation,

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Fig: Excavation

The area of footings are chosen according to design. First the C.G of footings is located. The excavated soil is kept near the footing as later it can be used for backfilling.

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PIC: excavated soil nearby the footing

LEVELLING & DRESSING OF SOIL SURFACE

After excavation, the most important step is to level the soil surface. The load of the construction should be vertical & for this, leveling & dressing is must. Otherwise the construction might be subjected to tilting to one side. Sand layer & brick chips are often used for leveling & dressing.

In our site LEVELLING & DRESSING were done by sand layer.

PLACEMENT OF BRICK FLAT SOLING LAYER9

Now a hard & plane surface is needed. For this purpose we place a brick layer on previous layer. It is called BRICK FLAT SOLLING. In this layer bricks are arranged in a regular combination. The upper surface of the layer should be uniformly planed. Below we see a brick flat soling.

PICS

CEMENT CONCRETE LAYER

Generally a C.C (cement concrete) layer having 3” height is kept on the brick layer. It is named as CEMENT CONCRETE LAYER. It provides a smooth, uniform & strong surface for reinforcement frame. The mix ratio for C.C layer is cement:sand:aggregate=1:3:6.

But sometimes it is avoided to reduce expenses.

PLACING OF CEMENT CONCRETE BLOCK

Usually some CC blocks are kept on the CC layer to maintain the clear cover between CC layer & reinforcement. It is also very important to prevent the reinforcement from corrosion. There is no standard size of blocks. It varies with size of footing & clear cover.

Sometimes brick or half brick can be used instead of CC block. But bricks are not uniformly smooth plane. Its strength is also less than CC block. So, it is better to use CC blocs as clear cover. Generally its size is 3”x3”x3”.

PLACING OF REINFORCEMENT

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Now it is a very important step to place reinforcements. First we should maintain available clear cover by using CC blocks. We must tie rebar’s with G.I wires or welding. It is done so that rebars remains intact in all the time even during the casting. First a single frame of horizontal reinforcement is kept upon the CC block. The number & diameter of reinforcements depends upon the strength of construction. And it is determined by design engineer.

The rebars are tied by hand at the outer place if the case is respectively small. But for large case, it is done on the brick flat soling. Because it is difficult to move & place a large case.

Fig: Formation of a case Fig: Tied by G.I wire

Fig: Preparing column

The case must remain horizontal. It is done by plumb bob. The figure below we see how a case is being horizontal by plumb bob.

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Fig: Making horizontal

After placing the case, the centre of case is marked as the column can be placed.

Fig: Fixing the column position

Now the column is placed upon the marking space. It must be vertical. Then it is also tied with case. After making vertical the column is supported to remain fixed.

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Fig: Placing of column & making it vertical

SHUTTERING

Shuttering means the solid boundary around the concrete. It resists water flow. It also bears the load of concrete. So a shuttering should be enough water tight & strong. The amount of

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water should remain constant in order to get a proper hydration. There are two kinds of shuttering,

1. Wooden shuttering &

2. Steel shuttering

Wooden shuttering is more chip than steel shuttering. But it is less water proof & less strong. It also lasts a short time. On the other hand steel shuttering is strong enough. It also can be used more times. But it is costly. Side shuttering can be removed after 3 days as after this time concrete get required strength.

Fig: Wood shuttering

The minimum requirements of a shuttering:

*It should be enough strong to bear the load

*It should be water tight

*It should be economical

*Its inner surface should be plane

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*Its joints should be made carefully

*It should retain the concrete shape from all types of distortions.

Concrete Casting

Concrete:

Concrete is any product or mass by the use of a cementing medium. Generally, this medium is the product of reaction between hydraulic cement and water including aggregates and admixtures.

Composition:

Concrete= Cement +Fine aggregates + Coarse aggregates + water

Roll of Components

Cement : Used as binding material. Fill voids between C.A and F.A. Fine aggregates: Fill voids among C.A. Coarse aggregates: Carry the load. Water: Used in hydration process to develop strength. Impart workability.

Range of Proportion: Cement: 7-15% Aggregates: 60-70%, F.A = 35-45% , C.A = 25% Water = 14-21%

Mixing Ratio:

Cement : F.A : C.A = 1 : 2 : 4 W/C = 0.45

Concrete casting:

Working procedure:

It is very important to make plan before casting .The factors which are to be considered before casting is stated as follows:

After the completion of footing casting, column is casted. After casting compaction is done with the help of a vibrator which is a type of mechanical compactor. The vibrator is kept immersed in one place until the surrounding concrete is fully compacted. This is usually judged by observing the formation of a circular shaped laitance around the compacted area.

Mixing Machine is used to mix cement, coarse aggregates and fine aggregates in correct

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proportion to prepare concrete. While using the mixing machine, coarse aggregate is added first followed by fine aggregates and then cementitious material. Water is added at regular time intervals while the materials are mixed. The hydration process of concrete starts as soon as water is added, therefore concrete should be mixed, transported, placed and compacted within the setting time of cement. Usually the concrete is casted within 30 minutes after mixing.

Safety purposes:

Sufficient safety arrangement should be ensured for roadside people.

To keep the surrounding building safe it should be well fenced during casting work.

CURING

Curing of concrete is one of the essential requirements of the process of concrete. The object of curing at normal temperature is to keep concrete saturated, or as nearly saturated as possible, until the originally water-filled space in the fresh cement paste has been occupied to the desired extent by the products of hydration of cement. In the case of site concrete, active curing nearly always ceases long before the maximum possible hydration has taken place. The strength of concrete increases more rapidly in the first few days after setting and afterword’s the rate of increase in strength goes on retarding. The period for which curing should be continued depends upon atmospheric conditions such as temperature, humidity and wind velocity. It should be carried out at least 21 days after the casting of concrete is done efficient curing increases both impermeability and durability of concrete and increases resistance of concrete to abrasion and reduces shrinkage. Correct curing also increases resistance of concrete to abrasion and reduces shrinkage.

Curing may be done by adopting the following methods:

By covering the surface with moist sand, sacks or sprinkling water at regular intervals to keep the covering wet.

By ponding the surface where the depth of water in the enclosed square is maintained from 5 to 8cm

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Columns, walls or other such vertical surfaces are cured by wrapping jute bags or canvas over the surface and keeping the wrapped material continuously wet by sprinkling water or by some other means.

BACKFILLING

After completion of curing, the excavated soil is filled up. This process is called backfilling. Ideal backfill materials are purely granular soils(clean sand, gravel, or sand and gravel) containing less than about 5% of very fine sand, silt, or clay particles. Normally the soil that has been excavated out is used for this purpose but if the condition of the soil is not good then soil from a different area is used for this purpose. While backfilling, anti termite chemical mixed water is sprayed on the sides and bottom of foundation trench. The filling is done in layers and then it is compacted since loosely placed backfill material will eventually settle under its own weight and will not be capable of supporting structures.

The excavated soil in our project site was full of organic materials and rubbish hence it was dumped and not used for backfilling.

REMOVAL OF FORMWORK

The formwork should be planned and constructed in such a manner that it is possible to remove the different components in a particular order. Duration of times up to which the formwork should be kept in place depends upon many factors such as type of cement used, shape and position of the member loads to be carried by the members and the temperature of the air. As the column shutter does not bear any load it can be removed after 3 days. Its purpose is only to provide shape. If the shuttering bears load it is removed after 14 days (bottom part of beam).All formwork should be eased gradually and carefully in order to prevent the load being suddenly transferred to concrete. The formwork should, under no circumstances be allowed to be removed until the concrete reaches strength of at least twice the stress to which the concrete may be subjected to at the time of removal of formwork. Where the shape of element is such that formwork has re-entrant angles, the form work is removed as soon as possible after the concrete has set, to avoid shrinkage cracking due to the restraint imposed. The forms can be easily stripped by loosening the double wedges. Sometimes the swelling of timber may result in shuttering getting jammed at places. In such situation a small crowbar can be used to strike off the shuttering.

Problems in isolated footing

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1. Segregation: Separation of constituents of a heterogeneous mixture so that their distribution is no longer uniform is termed as segregation. In case of concrete casting when it is thrown from a large height

Figure: segregation.

Segregation occurs due to the difference in particles size (sometimes in the specific gravity of the mix ingredients) .There are two types of segregation. In the first, the coarser particles tend to separate out since travel further long distance. In the second, segregation is occurred by separation of grout (water &cement). Here in our project aggregates were thrown from a remarkable height. As a result the ingredients were separated as they hit the lower rebar layer. This segregation will ultimately decrease the strength of the concrete. So proper arrange should be taken to prevent segregation.

2. Compaction: In our observed site, for compacting concrete they used both rodding & vibration procedure.

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Concrete is being thrown fro about 10’ height

In case of using vibrator, it should kept in mind that vibrator must be vertical. But in our project vibrator was used aligned.

Figure: Aligned vibrating.

3.Improper clear cover maintainance: Clear cover is a very important fact in structure. For the structure under the ground clear should be maintained at least 3”. If clear cover is not

maintained properly then the moisture content around the structure will penetrate into the

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Roding with bamboo

Aligned vibrating

structure & cause corrosion to the reinforcement . Finally collapse the structure.

Figure: Improper clear cover maintainance

4. Water logging during casting: Mix proportion is very much important for the design strength. In our site during casting, water logged continuously. The source of water may be rain water or leakage from nearby pipes or drains.

Figure: Water logging during casting

5. Brick as C.C block & no C.C layer: In isolated footing we normally use BFS & C.C layer to provide a flat surface. But in our project BFS was used with no mortar. Here they also did not use C.C layer as well as C.C block. For maintenance of the clear cover between the BFS bricks were directly used as C.C block that is shown in the following figure.

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Logged water

Improper clear cover

Figure: Brick as C.C block & no C.C layer

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Bricks as C.C block

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