Function of Foundation

Foundations are designed to transfer all types of loads coming from structure to the soil safely

Foundation classification

(A)Shallow Foundation: D/B≤ 1

D- Depth of FoundationB- Width of foundation- Load transfer at shallow depth Examples: Strip footing, Spread footings- Both isolated

and combined raft or mat foundation

(B) Deep Foundation: D/B>1

- Load transfer at deeper depthExamples: Pile foundation and Well foundations

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

Design requirements

(A)Classification of foundations

(B) Foundation loads

(C) Soil parameters

Classification of foundations

Foundations are classified based on soil type and subsoil 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)

Typical foundation shape (Dry/Wet/PS/FS/WBC)

Typical foundation shape (Fissured rock)

Normal fissured rock foundation and foundation with unequal chimney extn.

Foundation loads

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

- Down thrust (Compression)

- Uplift (Tension)

- Transverse side thrust

- Longitudinal side thrust

Soil parameters

Following soil parameters are needed for foundation design:

- Ultimate bearing capacity of soil

- Soil density (Dry/Submerged)

- Angle of repose

Stability Checks

- Check for bearing capacity

- Check for uplift capacity

- Check for overturning

- Check for sliding

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%