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Function of Foundation

Foundations are designed to transfer all

types of loads coming from structure to

the soil safely

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Foundation classification

(A)Shallow Foundation: D/B 1

D- Depth of Foundation

B- Width of foundation

- Load transfer at shallow depthExamples: Strip footing, Spread footings- Both isolated

and combined raft or mat foundation

(B) Deep Foundation: D/B>1

- Load transfer at deeper depth

Examples: Pile foundation and Well foundations

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Transmission line tower foundations

(A)Normal open cast foundation:Normal depth 3m

(B) Special open cast RCC foundation

- Shallow depth foundation

- Shallow depth with raised chimney

foundation

- Unequal chimney foundation- Unequal leg with raised chimney

foundation

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Design requirements

(A)Classification of foundations

(B) Foundation loads

(C) Soil parameters

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Classification of foundations

Foundations are classified based on soil type andsubsoil water level and listed below:

- Normal dry

- Sandy dry

- Wet

- Partially submerged (PS)

- Fully submerged (FS)

- Black cotton soil (WBC)

- Dry fissured rock (DFR)

- Wet fissured rock (WFR)- Submerged fissured rock (SFR)

- Hard rock (HR)

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Typical foundation shape

(Dry/Wet/PS/FS/WBC)

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Typical foundation shape (Fissured

rock)

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Normal fissured rock foundation

and foundation with unequal

chimney extn.

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Foundation loads

Following Ultimate foundation loads acting at thetower base (along the tower slope) are

considered:

- Down thrust (Compression)

- Uplift (Tension)

- Transverse side thrust

- Longitudinal side thrust

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Soil parameters

Following soil parameters are needed for foundation

design:

- Ultimate bearing capacity of soil

- Soil density (Dry/Submerged)

- Angle of repose

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Stability Checks

- Check for bearing capacity

- Check for uplift capacity

- Check for overturning

- Check for sliding

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Steel Design for Chimney and RCC pad

Design is based on IS-456:2000Two step approach:

- Design of chimney:

Based on compression with bending

Based on Uplift with bending

Minimum percentage of steel required is

0.8%

- Design of RCC pad:

General three step pad is used Reinforcement requirement is based on

bending at the max. bending moment at

the face of Chimney & max. bending

moment at the face of first pad

Minimum percentage of steel required is 0.12%

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