FOUNDATION

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FOUNDATION: 

FOUNDATION

Foundation: 

Lowermost part of a building which transmits the load of building to the underlying earth. Foundation

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Major functions of the foundation It distribute non-uniform load of the superstructure evenly on the subsoil hence it minimize chances of differential settlement. It provided stability against scouring flood water. It provided against sliding. It provides a level surface for the construction of the superstructure

Types of Foundation: 

Wall footing Isolated footing Combined footing Raft or mat foundation Pile foundation Types of Foundation

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Wall footing/spread footing Footing provided under a wall. light load- simple footing is provided. Heavy load- stepped footing is provided. Simple footing Stepped footing

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Isolated footing/column footing It is used to support isolated columns Size of footing is very large- stepped or sloped foundation Reinforced cement concrete footing is provided for heavy loads. Load transmitted through the foundation to soil should be less than bearing capacity of soil.

Combined footing: 

It is constructed for two or more column Isolated footings of individual column overlaps or when external column is situated near the boundary. Footing: Rectangular or trapezoidal. Rectangular footing- provided when load acting on two columns are almost equal. Trapezoidal footing- Provided when load acting on the column is very high When one of the column is very close to the boundary. Combined footing

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Combined footing

Mat foundation /raft foundation/flat foundation: 

It is a concrete slab which cover the entire area below the building. Column loads are heavy- reinforced concrete slabs are provided Mat foundation are used When Soil is soft clay or made up land Highly compressible soil to reduce settlement. It can face large settlement without causing any harm to the super structure. Mat foundation /raft foundation/flat foundation

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Mat foundation /raft foundation/flat foundation

Pile foundation: 

Long vertical member made up of timber, steel, concrete. It may be vertically or slightly inclined. Pile is inclined-battened pile. Pile foundation

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Based on material Wooden pile Concrete pile Steel pile RCC pile Pre-stressed concrete pile

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Depending on load carrying capacity Load bearing pile . End bearing pile Friction pile Non-load bearing pile. Sheet pile

Concrete piles: 

Precast concrete piles Cast-in-situ piles Concrete piles

Pre-cast concrete piles: 

Pre-cast concrete piles Cast in a yard, cured and then driven into the ground Square section with chamfered corners Diameter of piles – 25cm to 60cm Length of piles – 3 to 30m( < 5m are not used) At lower end, shoes are provided. Shoes of 20cm depth and 15cm width

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Advantages It can be examined and make necessary repairs work before end use Positioning of reinforcement can maintained Piles can driven under water also.

Cast in situ piles: 

Cast in situ piles Cast at the place where they have to function by driving a casing into an excavated hole Filling up the casing with concrete. Casing is kept in position after placing the concrete – cased cast-in-situ piles Casing is withdrawn after placing concrete – uncased cast-in-situ piles

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BEARING CAPACITY

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Bearing capacity:- The ability of the soil to support the imposed load without excessive settlement or failure. Ultimate bearing capacity :- The gross pressure intensity at which the soil fails. Safe bearing capacity :- maximum pressure which the soil can carry without the risk of shear failure. Factor of safety is 2 to 3

Determination of ultimate bearing capacity in the field: 

Simplest and widely used field test- plate load test Determination of ultimate bearing capacity in the field

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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 )

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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 1/5 th of ultimate bearing capacity whichever is less

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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

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. Methods for improving bearing capacity of soil

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By confining the soil Sheet piles are driven around the structure to form an enclosure 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.

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Thank you