Lecture 2:

Deep Foundations

Engr. Muhammad Asif UCE&T, B Z University, Multan.

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Foundations

Types of Foundations

Type of foundation depends on whether the load transfer is at deeper depths or shallower depths - Need for these two types (soil strength, ground water conditions, foundation loads, construction methods and impact on adjacent property)

Deep Foundation

Deep foundations penetrate through upper layers of incompetent soil in order to transfer load to competent bearing soil or rock deeper within the earth.

These foundations penetrate incompetent soil until a satisfactory bearing stratum is reached. This is usually at depths >3 m below finished ground level.

Deep foundations usually L/B > 5 L = pile length, B = dia. or breadth of pile

Deep Foundations

Extend several dozen feet below the building1. Piles2. Piers3. Caissons (kay-son])4. Compensated Foundation

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1. Piles

PilePiles 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.

orPile foundations are the part of a structure used to carry and transfer the load of the structure to the bearing ground located at some depth below ground surface.

Piles

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Piles Foundation is used when:

i. The soil near the surface doesn’t have sufficient bearing capacity (weak) to support the structural loads.

ii. The estimated settlement of the soil exceeds tolerable limits.

iii.Differential settlement due to soil variability or non-uniform structural loads is excessive.

iv.Excavations to construct a shallow foundation on a firm soil are difficult or expensive.

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Load Can Be Transferred By Pile to The Ground By 2 Way i.e.:

a. End Bearing Piles Pile will transmit load

into the firm soil layer of the ground such as rock, gravel, very dense sand.

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Base enlargement tool

b. Friction Piles Pile transmit the load

from the structure to the penetrable soil by means of skin friction or cohesion between the soil & the embedded surface of the pile.

There are 2 types of End Bearing Piles. That is preformed Timber Pile & In-Situ Reinforced Concrete Pile

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Friction Pile May Be Used To Support DownWard Load19

Types Of Piles

a) Concrete Pilesi. Cast-In-Place Concrete Pilesii. Precast Concrete Pilesiii. Drilled Shafts

b) Steel Piles (Driven piles)

i) H-Piles ii) Cylindrical iii) Tapered

c) Timber Piles (Driven piles)

d) Composite Piles

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A. Concrete Piles

a) Concrete Pilesi. Cast-In-Place Concrete Pilesii. Precast Concrete Pilesiii. Drilled Shafts

i. Cast In Place Concrete Piles

a. Formed by driving a cylindrical steel shell into the ground to the desired depth and cavity of shell is filled with fluid concrete.

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ii. Precast Concrete Piles

a. Usually have square/circular/octagonal cross sections.

b. Fabricated in a construction yard from reinforced or pre-stressed concrete.

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Solid Precast Concrete (Displacement Pile)

iii. Drilled Pile

A Drilled Pile removes soil from the ground and the resulting round hole is filled with concrete or grout.

B. Steel Piles

i. It comes in various shapes & sizesii. Steel H-Piles are rolled steel sectionsiii. Steel pipe piles are seamless pipes that can be

welded to yield lengths up to 70m.iv. They are usually driven with open ends into the

soil.v. A conical tip is used where the piles have to

penetrate boulders & rocks.

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Combination Piles(Precast Concrete & “H” Pile Points)

Open End Pipe (Non-Displacement Pile)

Timber Piles

C. Timber Piles

i. Have been used since ancient timesii. Easy to cut and splice.

iii. Requires no special handling.

iv. The pile length is usually limited to less than 100 feet.

v. Timber piles have a limited load-carrying ability.

vi. Subject to insect attack and organic decay.

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Types Of Pile Chosen DependsOn Following Factors:

a) What type of pile is readily available

b) Location & type of structure (magnitude of loading)

c) Ground Condition (soil type)

d) Cost

e) Durability

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2. Piers

2. Piers

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

It’s a vertical bridge support.

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Pier45

Pier 46

Pier47

3. Caissons

Caissons

Caisson Foundation

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What Is Caissons?

It’s a prefabricated hollow box or cylinder.

It is sunk into the ground to some desired depth and then filled with concrete thus forming a foundation.

Most often used in the construction of bridge piers & other structures that require foundation beneath rivers & other bodies of water.

This is because caissons can be floated to the job site and sunk into place.

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Basically it is similar in form to pile foundation but installed using different way used when soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat.

It’s 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.

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Reinforced Concrete Caissons55

Caissons59

Chenab Bridge Multan

4.Compensated foundations

4. Compensated foundations

Compensated foundations are deep foundations in which the relief of stress due to excavation is approximately balanced by the applied stress due to the foundation. The net stress applied is therefore very small. A compensated foundation normally comprises a deep basement.

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