FoundationFoundation

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Types of Foundations Foundation can be divided into two broad classification –

shallow and deep foundation. They further divided into sub-divisions as follows:

NO FOUNDATION SUDDIVISION1 Strip and Pad foundation

(Shallow)Wide strip, isolated pad, continuous pad, combined pad, cantilever and balanced base foundations

2 Raft foundation(Shallow)

Solid slab raft, beam and slab raft, cellular raft

3 Pile foundation (Deep) Friction piles and End bearing piles

4 Pier foundation Masonry, Mass concrete, Cylinders and monoliths

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Strip and Slab Foundations

• WIDE STRIP FOUNDATION- is used to carry load-bearing wall including brick or concrete wall of multi-story buildings as Figure 2.16A.

• ISOLATED PAD FOUNDATIONS-is used in framed buildings (Figure 2.16B). Reinforced concrete pad- is a square or rectangular slab of concrete carrying a single column –figure 2.17A,B.

• CONTINUOUS PAD FOUNDATION-supports a line of columns and is one form of combined foundation (figure 2.16C & 2.19).

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Strip and Slab Foundations

• COMBINED COLUMN FOUNDATION-it is used when there exists some restriction on spread of independent foundation at right angle to a line of column (figure 2.16D & E and figure 2.20).

• BALANCED FOUNDATION –these consist of the cantilever foundation and the balanced base foundation which may be used in the following circumstances instead of a combined column foundation”

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Type of Foundations

Figure 2.16D-combined

Figure 2.16C-continuous

Figure 2.16A-wide strip Figure 2.16B-

Pad

Figure 2.16E-combined

Figure 2.16F-cantilever

Figure 2.16G-balanced base

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Pad or Slab Foundation

Figure 2.17 A & B –Pad or slab foundation

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Continuous Column Foundation

Figure 2.19-Continuous column foundation

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Pad Foundation

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Pad Foundation

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Wide strip foundation

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Wide strip foundation

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Combination of Pad and Strip Footing

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Raft / Mat Foundations1. Raft foundation is

fundamentally a large combined slab foundation designed to cover the whole or a large part of the available site.

2. A raft is used to distribute a large load to a large area

3. A raft may be used when the soil is weak and columns are so closely spaced in both directions.

4. A raft may be also used to provide a stiff foundation to avoid a differential settlement.

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Pile Foundations• Pile foundations is defined as a form of

foundation in which the building loads are taken to a hard level by means of piles.

• Pile is used when •no firm bearing strata exist at reasonable

depth and the applied loading is uneven, making raft inadvisable

•when a firm bearing stratum does exist but at a depth such as to make strip, slab or pier foundation uneconomical, that is at depths over 3 to 4.5 m.

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Pile Foundations• Piles to depth of 18m are common and in

exceptional circumstances they are used to depth of 30m.

• All piles are divided into (i) friction piles and (ii) end bearing piles and (iii) combination of friction and end bearing piles.

• Friction piles transfer their load to the surrounding soil by means of friction between their surface and the soil. Friction pile can be used in conjunction with a raft as per Figure 2.23. Friction piles are also used as a tension in holding a building down against the wind uplift.

• End bearing piles transfer load from weak soil to a firm soil such as rock.

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Pile Foundation

Figure 2.23

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Pile Groups• The bearing capacity of a pile depends upon

its length, its cross-sectional area and the shear strength of the soil into which it bears and it is frequently necessary to use a group of piles rather than a single pile in order to obtain adequate bearing capacity. Such a group is terms a “pile cluster” Figure 2.24A.

• For reasons of stability a group of at least three piles is often used under any heavy load rather than a single pile.

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Pile Groups• This also provide a higher safety of factor if

one pile fails.

• The tops of piles in a cluster are connected by a block of concrete called pilecap, reinforced to transmit safely the column load to the heads of the piles in the cluster (figure 2.24B).

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Pile Clusters

Figure 2.24 A & B

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Type of Pile1. Piles are divided into:

(i) Driven piles using hammers(ii) Formed on site by placing concrete and reinforcement in a borehole.

2. Driven piles in divided into timber, steel or precast concrete.

3. Precast concrete piles is either reinforced or prestressed.

4. Steel piles is divided into pipe pile or H-pile.5. Timber pile is used for light weight building.

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Pile Driving• The pile is supported between guides or

“leader” in a vertical steel piling rig/frame (Figure 2.25) sufficiently high to take the pile and to allow room above the cap for a hammer or for the drop of a weight, by means of which the pile is driven into the ground.

• Pile driving frames are normally mobile and arranged to rake over to drive piles at an angle when required.

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Piling Rigs

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Bored Piles• On sites where piling is to be carried out

close to existing premises, driven piles is unsuitable because of amount of vibration.

• Therefore, bored pile is recommended.• The use of bored piles is usually

cheaper than any form of driven pile when a few only are required.

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Bored Piles• The diameter of bored pile normally

between 450mm to 1000mm.• The bearing capacity of bored pile is

higher than precast piles.• The pile hole may be bored either by

rotary drilling or by percussive boring.

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Rotary Drills

Figure 2.28 Rotary Drill

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Bored Piles

Figure 2.29 Bored piles

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Bored Piles

Figure 2.29 Bored piles