Design of Foundation of Transmission Towers in Different Soils

1. All foundation shall be of RCC. The design and construction of RCC structures shall be carried out as per IS:456 and minimum grade of concrete shall be M-20.
2. Limit state method of design shall be adopted.
3. Cold twisted deformed bars as per IS:1786 or TMT bars shall be used as reinforcement.
4. Foundations shall be designed for the critical loading combination of the steel structure and or equipment and/or superstructure.
5. If required protection to the foundation, shall be provided to take care of any special requirements for aggressive alkaline soil, black cotton soil or any soil which is detrimental/harmful to the concrete foundations.
6. All structures shall be checked for sliding and overturning stability during both construction and operating conditions for various combination of loads.
7. For checking against overturning, weight of soil vertically above footing shall be taken and inverted frustum of pyramid of earth on foundation should not be considered.
8. Base slab of any underground enclosure shall also be designed for maximum ground water table. Minimum factor of safety of 1.5 against bouncy shall be ensured.
9. The tower and equipment foundations shall be checked for a factor of safety of 2.2 for normal condition and 1.65 for short circuit condition against sliding, overturning and pullout

Guidelines for Classification of Foundations of Transmission Towers in Different Soils

The transmission tower may be situated in different locations. Power System transmission networks are being sprading all over the world. The soil condition of different places are also different. Depending upon the nature of soil, the types foundation of transmission towers should be selected and constructed accordingly. We have tried to give you a clear and brief Guidelines for classification of Foundations of transmission towers in different Soil conditions.

SL	Name of soil encountered	Type of foundation to be adopted
1	In good soil (silty sand mixed with clay)	Normal Dry
2	Where top layer of Black Cotton soil extends upto 50% of the depth with good soil there after.	Partial Black Cotton
3	Where top layer of black cotton soil exceeds 50% and extends upto full depth or is followed by good soil.	Black Cotton
4	Where top layer is good soil upto 50% of the depth but the lower layer is a black cotton soil	Black Cotton
5	Where subsoil water is met at 1.5 ml or more below the ground level in good soil	Wet
6	Good soil locations which are in surface water for long period with water penetration not exceeding 1.0 m below ground level (e.g. paddy fields).	Wet
7	In good soil where subsoil water is encountered between 0.75m and 1.5m depth from ground level.	Partially submerged
8	In good soil where subsoil water is encountered within 0.75 m depth from ground level	Fully Submerged
9	Where top layer of normal dry soil extends upto 85% of the depth followed by fissured rock without presence of water.	Dry Fissured Rock
10	Where top layer is lissured rock followed by good soil/sandy soil with/without presence of water	Special foundation
11	Where normal soil/tissured rock extends upto 85% of the depth followed by hard rock	Dry fissured Rock with under cut in Fisured Rock combined with anchor bar for hard rock design
12	Where fissure rock os encountered with subsoil water within 0.75m or below 0.75m from G.L. (Top layer may be either a good soil or black cotton soil)	Submerged Fissured Rock
13	Where Hard Rock is encountered at 1.5 m or less below ground level.	Hard Rock
14	Where Hard Rock is encountered from 1.5 m to 2.5m below G.L. (Top layer being good soil)	with chimney for Normal Soil Hard Rock Foundation
15	Where hard rock is encountered from 1.5m to 2.5 m below G./L. (Top layer either in Black cotton) soil or fissured Rock	Hard Rock Foundation design with chimneys designed for wet black cotton soil.
16	Where fissured rock is encountered at the bottom of pit (with black cotton soil at top)	Composite Foundation
17	Where hard rock is encountered at bottom with water and black cotton soil at top and hard rock layer depth is less than 1.5 m.	Hard Rock
18	Sandy soil with clay content not exceeding 10%	Dry Sandy soil foudation
19	Sandy soil with water table in the pits	Wet sandy soil design to be developed considering the depth of water
20	Where top layer upto 1.5 m below G.L. is normal dry soil and thereafter hard soil/murrum	Normal dry with undercut
21	Where bottom layer is marshy soil with top layer of good soil/fissured rock/black cotton	Soil investigation is to be carried out and special foundation design to be developed.
22	Where the top layers are a combination of clinker mixed with firm soil, gravel and stone chips upto 60% of foundation deapth from ground level followed by hard murrum	Normal dry with undercut
23	Where top layers are combination of hard murrum, soft rock etc. followed by yellow/black clayee soil	Special foundation design is to be developed after carrying out soil investigation.

Any other combination of soil not covered above shall require development of special foundation design.