Foundation

Basic Civil Engineering-Foundation

By

Mr.B.Ramesh, M.E.,(Ph.D), Research Scholar, CEG, Anna University, Chennai. Associate Professor, Department of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119

Foundation

Foundation  Introduction :  A structure essentially consists of two parts, namely the super structure which is above the plinth level and the substructure which is below the plinth level.  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 foundation : 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. 2 May 2012

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Foundation…  Types of foundation : The two main types of foundation are :  Shallow foundation (spread foundation) and  Deep foundation

Types of Pile foundation :  Friction pile  Load bearing pile

Note : Under-reamed piles is a type of load bearing pile

Types of Shallow foundation :  Isolated footing (single footing, Column footing)  Combined footing Types of Deep foundation : o Pile foundation o Pier foundation 2 May 2012

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Foundation…  Shallow foundation (spread foundation):  Depth of foundation is less than or equal to its width.  Isolated footing (single footing, column 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 so that the stresses developed at the base is within the limit. 2 May 2012

 In case of masonry footing, the projection of each step must be ½ brick thick and each step is made of 1 or 2 bricks put together.  Concrete can be moulded to any shape and hence a concrete footing may be a sloping one to provide sufficient spread. 5

Foundation…  Isolated footing (single footing, column footing)…

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Foundation…  Isolated footing (single footing, column footing)…

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Foundation…  Isolated footing (single footing, column footing)… wall footing

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Foundation…  Combined footing:  This type of footing is adopted when the space between two columns is so small that the foundation for individual columns will overlap.  Combined footings are proportioned in such a way that the centre 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.

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Footing :

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 9 is maintained.

Foundation…  Size of footing :  The area of footing can be determined from the actual external loads, such that the allowable soil pressure is not exceeded.

Area of footing 

Total load including self - weight allowable soil pressure

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Foundation…  Pile :  A slender, structural member consisting steel or concrete or timber.  It is installed in the ground to transfer the structural loads to soils at some significant depth below the base of the structure.

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Foundation… Pile caps are thick slabs used to tie a group of piles together to support and transmit column loads to the piles.

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Foundation…  Deep foundation :  Deep foundation consists of pile and pier foundations.  This consists in carrying down through the soil a huge masonry cylinder which may be supported by the sides of soil or may be supported on solid rock (hard stratum).  Pile foundation : o Pile is an element of construction used as foundation. It may be driven in the ground vertically or with some inclination to transfer the load safely. 2 May 2012

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Foundation…  Pile foundation… o Loads are supported in two ways. o If the load is supported by the effect of friction between the soil and the pile skin, it is called friction pile. o Friction piles may be made of cast iron, cement concrete, timber, steel, wrought iron and composite materials. o If the load is supported by resting the pile on a very hard stratum, it is called load bearing pile. o Load bearing piles are steel sheet piles, concrete piles and timber piles. 2 May 2012

oPiles may be cast-in-situ or precast. oThey may be cased or uncased. 14

Load Bearing Pile

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Friction Piles

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Load bearing pile

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Friction Pile

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Batter Piles

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Foundation… Under-reamed piles:  Structures built on expansive soils often crack due to the differential movement caused by the alternate swelling and shrinking of soil. Under-reamed piles provide a satisfactory solution to the above problem.  The principle of this type of foundation is to transfer the load to a hard strata which has sufficient bearing capacity to take the load.  Single and double under-reamed piles may also be provided for foundations of structures in poor soils overlying firm soil data. In such soils if double under-reamed piles are provided, both the under-reams shall rest within the firm soil data. 2 May 2012

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Cast in situ

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Foundation… PILE FOUNDATION IS USED WHEN:

 The soil near the surface doesn’t have sufficient bearing capacity (weak) to support the structural loads.  The estimated settlement of the soil exceeds tolerable limits  Differential settlement due to soil variability or non-uniform structural loads is excessive.  Excavations to construct a shallow foundation on a firm soil are difficult or expensive.

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Distribution of Soil Pressure

Soil pressure distribution in noncohesive soil. 2 May 2012

Soil pressure distribution in cohesive soil. 21

Foundation…  Bearing capacity of soil :  It is defined as the maximum load per unit area which the soil will resist safely without displacement  The bearing capacity of the soil can be found by loading the soil, noting the settlement and by dividing the maximum load by the area on which the load is applied. The maximum load is obtained from the graph between the settlement and load.  Safe bearing capacity of soil = [Ultimate bearing capacity of soil] / [Factor of safety]  On completion of a structure there may be some displacement in the position of the foundation.  For ordinary framed structures of concrete, the permissible angular distortion is 1/500 and the desirable value is 1/1000.  The maximum differential settlement should not exceed 25 mm in case of foundations on sandy soil and 40 mm in case of foundations on clayey soil. 2 May 2012

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Foundation… Bearing capacity of soil…  In case of non-cohesive soils, such as sand and gravel, the allowable bearing capacity should be reduced by 50%, provided that the water table is above or near the bearing surface of the soil.  The bearing capacity of reclaimed soils or shrinkable soils can be taken as 50kN/m2 in the absence of the site data.

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Artificial methods to improve bearing capacity of soil      

By increasing the depth of foundation. By draining the sub-soil water. By compacting the soil. By confining the soil mass. By cement grouting. By injecting chemicals like silicates etc.

Essential requirements of a good foundation:  The foundation should be so located that it is able to resist any unexpected future influence which may adversely affect its performance.  The foundation should be stable or safe against any possible failure.  The foundation should not settle or deflect to such an extent that will impair its usefulness. 2 May 2012

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PIERS  It’s a vertical bridge support.  It’s a foundation for carrying a heavy structural load which is constructed in site in a deep excavation.  Among the things to be taken in consideration during construction of pier are as follows: a) Drilling through wet or caving soils may need use of temporary steel casing. May also require the use of a tremie & a pump to dewater the hole & place concrete. This is more expensive and require a large diameter hole.

b) For the purpose of reinforcing, it’s difficult to get bars to the full depth of the pier with the proper concrete cover in deep holes. Use centralizes. Use large diameter bars versus more bars. 2 May 2012

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c) Don’t leave holes open for any length of time even in dry condition. Have concrete on site and fill right after drilling and cleaning.

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Foundation… Videos and Animations 1.

Basement forming

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Foundation

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Types of foundation

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Pile foundation

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Sheet piling

6.

Raft foundation

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TYPES OF PILES 

Concrete Piles i) Cast-In-Situ Concrete Piles a) Cased cast-in-situ b) Uncased cast-in-situ ii) Precast Concrete Piles

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Steel Piles i) H-Piles

ii) Cylindrical piles

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Timber Piles

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Composite Piles

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iii) Tapered piles

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TYPES OF PILE CONSTRUCTION a)

Displacement Piles It cause the soil to be displaced radially as well as vertically as pile shaft is driven or jacked into the ground. b) Non Displacement Piles (Replacement piles) It cause the soil to be removed and the resulting hole filled with concrete or a pre cast concrete pile is dropped into the hole and grouted in. Displacement Pile Non Displacement pile

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METHOD OF INSTALLATION 

Dropping Weight or Drop Hammers - commonly used method of insertion of displacement piles

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Diesel Hammers -Most suitable to drive pile in non cohesive granular soil

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Vibratory Hammers or vibratory method of pile driving -very effective in driving piles through non cohesive granular soil

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Jacking Method Of Insertion

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Pile installation using Drop Hammer

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Pile Driving Rig - temporarily support the pile that being driven and to support the pile hammer. 2 May 2012

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Jacking Method Of Insertion  Jacked Piles are most commonly used in underpinning structures  By excavating underneath a structure short lengths of pile can be inserted and jacked into the ground using the underside of the existing structure as a reaction.

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NON DISPLACEMENT PILES

There are four types: a) b) c) d)

Small Diameter Cast-In- Situ Large Diameter Cast-In-Situ Partially Preformed Piles Grout or Concrete Intruded Piles

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Thank You

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