DEEP FOUNDATIONS

PREPARED BY:

SAQIB NAWAZB.ARCH. 3RD YR.

A foundation is the lowest and supporting layer of a structure.

Requirements (Functional)

A properly designed foundation is one that transfers the structural load throughout the soil without overstressing of soil which can result in either excessive settlement or shear failure,both of which can damage the structure.

Based on their point of transfer of load, Foundations are generally divided into two categories:

SHALLOW FOUNDATIONS&

DEEP FOUNDATIONS.

a) Shallow Foundation System A shallow foundation is a type of foundation which transfers building

loads to the earth very near the surface. Shallow foundations include spread footing foundations, slab-on-grade foundations, rubble trench foundations, and earth bag foundations.

b) Deep Foundation System A deep foundation is used to transfer a load from a structure through an upper weak layer of soil to a stronger deeper layer of soil. There are different types of deep footings including impact driven piles, caissons,helical piles, and earth stabilized columns.

SHALLOW FOUNDATION DEEP FOUNDATION

LOADS ON FOUNDATION

• Dead Load : Refers to the overall weight of the structure. Includes weight of the materials permanently attached to the structure (such as flooring) and fixed service equipment (such as air conditioning)

• Live load : Refers to the weight of the applied bodied that are not permanent parts of the structure. Applied to the structure during part of its useful life (e.g. people, warehouse goods). Specified by code.

• Wind loads : Acts on all exposed parts of the structure. Calculated using building codes.

• Earthquake Forces : Building code is consulted.

DEEP FOUNDATIONS

“It is a foundation system that transfers loads to a deeper and capable soil layer.”

Piers are foundations for carrying a heavy structural load which is constructed insitu in a deep excavation.

Well foundations are commonly used for transferring heavy loads to deep strata in river or sea bed for bridges, transmission towers and harbour structures.

Piles are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity.

Caissons are a form of deep foundation which are constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson.

WELL FOUNDATIONS

• Well foundations are commonly used for transferring heavy loads to deep strata in river or sea bed for bridges, transmission towers and harbour structures.

PILE Foundations rely on driven piles, often have groups of piles connected by a pile cap (a large concrete block into which the heads of the piles are embedded) to distribute loads which are larger than one pile can bear. 

PILES are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity.

PILES can be of timber, steel or of composite material(steel and concrete).

METHODS OF INSTALLATION

DROPPING WEIGHTDIESEL HAMMERVIBRATORY METHODS OF PILE DRIVINGJACKING METHODS OF INSERTION

DROPPING WEIGHT OR DROP HAMMER is the most commonly used method of insertion of displacement piles.

DIESEL HAMMER produces controlled explosions which raises a ram used to drive the pile into the ground.

VIBRATORY METHODS can prove to be very effective in driving piles through non cohesive granular soils. The vibration of the pile excites the soil grains adjacent to the pile making the soil almost free flowing thus significantly reducing friction along the pile shaft.

JACKING PILES are the most commonly used methods in underpinning existing 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. 

Classification of PilesBased on material •Concrete•Steel•Timber

Based on size• Micro piles dia. < 150 mm.• Small piles dia. 151 to 600 mm.• Large piles dia. > 600 mm.

Based on Load transfer mechanism•End bearing piles •Friction/Floating piles •Bearing cum Friction piles

LOAD TRANSFER MECHANISM OF PILES

PRE CAST /PRE STRESS PILES(CONCRETE)

Size : 150mm to

400mm Lengths : 3m, 6m, 9m and 12m Structural Capacity : 25Ton to 185Ton Material : Grade M-35 Concrete Joints: Welded Installation Method :

Drop Hammer

Jack-In

Advantage of Pre Cast /Pre Stress Piles…..

•Better Bending Resistance

•Higher Axial Capacity

•Better Manufacturing Quality

•Able to maintain Higher Driving Stresses

•Higher Tensile Capacity

•Easier to Check Integrity of Pile

•Similar cost as RC Square Piles

Disadvantage:

•Difficult to transport

•Difficult to achieve desired cut-off.

Steel H PilesSize : 200mm to 400mm

Lengths : 6m and 12m

Structural Capacity : 40Ton to

1,000Ton

Material : 250N/mm2 to

410N/mm2 Steel

Joints: Welded

Installation Method : Hydraulic

Hammer

Jack-In

Advantage:

1. Relatively less hassle during installation and easy to achieve cut-off level.

2. High driving force may be used for fast installation .

3. Good to penetrate hard strata Load carrying capacity is high

Disadvantage:

4. Relatively expensive5. Noise pollution during installation 6. Corrosion7. Bend in piles while driving

Large Diameter Cast-In-Situ Piles (Bored Piles)

Size : 450mm to 2m

Lengths : Varies

Structural Capacity : 80Ton to 2,300Tons

Concrete Grade : M-25,30,35 Grade (See IS-

456-2000)

Joints : None

Installation Method : Drill then Cast-In-Situ

Advantages: Fast method of pile installation Economical, since reinforce-ment is installed after tube

installation, compared to precast piles, where reinforcement must be designed with respect to transport and handling.

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