THIAGARAJAR COLLEGE OF ENGG

BASIC OF CIVIL ENGG

Types of foundations for different soil characteristics.

Classification of soil. Based on size:

1) Gravel – 4.75mm – 80mm. Coarse grained soils. 2) Sand - 0.075mm – 4.75mm. 3) Silt - .002mm – 0.075mm. Fine grained soils. 4) Clay - <0.002mm. Silt and clay are plastic soils i.e., they can be easily shaped or moulded.

According to the Indian Standard system, the soils are classified broadly classified into: 1) Coarse grained soils : In these soils, more than half the total materialsby mass is larger than 75 micron IS sieve size. 2) Fine grained soils : In these soils, more than half the total material’sby mass is smaller than 75 micron IS sieve size. 3) Highly organic soils and other miscellaneous soil : These soilscontain large percentage of fibrous organic matter such as peat and the particles of decomposed vegetation.

Engineering properties of soil.These properties help an engineer to determine the type of foundation to be used and alternative methods to be followed if quality of soil in a particular area is poor.

* Compressibility : => Settlement of structures is governed by compressibility of soil. * Permeability : => Coarse grain soils are more pervious than fine grained soils. => Important in the design of earth dams. * Shear strength : => Ability of soil to withstand shear stress. => Governs bearing capacity and stability of earth slopes. => Dense gravel and sand possess high shear strength and hence are used to construct earth slopes.

Index properties.Properties that give a rough idea about the engineering properties are known as index properties.

For coarse grained soils, particle size and density are index properties. * coarse grained soils will have high permeability. * coarse grained soils with high density will have less compressibility and more shear strength.

For fine grained soils, consistency and moisture content are index properties. * Clay with soft consistency has high compressibility and less shear strength whencompared to clay with stiff consistency. * As moisture content increases, compressibility increases and hence shear strength decreases.

Bearing Capacity of the soil.Bearing capacity is the maximum pressure which a soil can withstand without undergoing shear failure and excessive settlement. Gravels and sand in a dense state will have high bearing capacityand on the other hand, soft clay and clay silt mixture will have poor bearing capacity.

* Ultimate bearing capacity : The ultimate bearing capacity of the soil is defined as the loadcarrying capacity of soil without occurrence of shear failure.

* Safe bearing capacity : The safe bearing capacity of the soil is determined by dividing the Ultimate bearing capacity of soil by suitable factor of safety. safe bearing capacity = ultimate bearing capacity . factor of safety

Factor of safety is taken as 2 for normal applications and between 2 and 5 for special structures.

Factors affecting bearing capacity.

Bearing capacity depends on a number of factors given below:

1. Type of soil and it’s physical properties such as density, shear strength, etc.2. Total and differential settlement.3. Position of water table.4. Physical features of foundation.

Foundations.* Foundation is that part of a structure which transfers the load from the super structure to the soil on which it rests.* It can be built by various types of materials – generally brick, stone, concreteSteel, timber, etc.* The shape and size of the foundation depends on type of structure andtype of the soil on which the structure rests ultimately.

Function of a foundation are (i) to transmit load of the structure over larger area to prevent over loading of soil beneath, (ii) to prevent unequal settlement ,(iii) To provide a level surface for building operations, (iv) to increase stability o f the superstructure.

Shallow foundations:

They are located just below the lowest part of the wall or column which they support. They areClassified into (a) footings and (b) rafts.

(a)Footing: A footing is a foundations unit constructed under the base wall or column. They are Classified into (i) Spread footings: are those which spread the load over a large area by supportingeither a column or a wall. It is further classified into (a) continuous footings: => provide a continuous longitudinal bearing. => simple footings are used when BC of the soil is high. => stepped footings are used when BC of the soil is low. (b) isolated footings: => supports a single column. => square or circular in plan depending upon space constraints. => footings may be simple, stepped or sloped.

(c) Combined footings:

=> supports two or more columns. => maybe rectangular or trapezoidal. Combined footings are used under the following circumstances: * When the bearing capacity of soil is so low that individual column footing work out to be of uneconomic size. * When the exterior column is close to adjoining property line or other similar restrictions. * When the columns are placed so closely that separate footing would overlap. * If the column loads are unequal and the external column near the property line is heavily loaded , trapezoidal footings are used. * If the column loads are equal, rectangular footings are used.

(ii)Strap footings: If the independent footings of two columns are connected by a beam, it is called strap footing. It may be used where the distance between the columns is much more such that combined footing becomes more narrow with high bending moments.

(iii) Mat and Raft foundations: => It is a combined footing that covers the entire area beneath the structure and supports all the walls and columns. => When the BC of the soil is low, mat and raft foundation is used. => Used where soil contains compressible mass or the soil is sufficiently erratic, to minimise the differential settlement.

Deep Foundations.Deep foundations are those which derive ground support at a great depth below the super structure.These foundations are employed where the soil immediately below the structure is not capable ofsupporting superimposed structural load. The general forms of deep foundations are piles, piers and Caissons or well.

Deep foundations may be employed in the following situations: => Firm soil layer at a greater depth underlying softer material. => Possibility of scouring underlying layer. => The top soil is of expansive nature. => Structure is highly sensitive to unequal settlement. => Offshore construction or foundations in marshy areas. => Resisting overturning or lateral forces.

Pile foundation:

Pile is a slender column which receives the structural load for transferring it to the underlying Layer/ layers.

Classification of Piles: Depending on the material, functions and method of installation, piles are classified as under. (a)Materials of construction : Timber piles, steel piles, concrete piles and composite piles. (b)Based on function : End-Bearing piles, friction piles and combined end bearing and friction piles. (c)Based on method of installation : Driven piles and bored piles – cast-in-situ or pre – cast.

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