- 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.
- Limit state method of design shall be adopted.
- Cold twisted deformed bars as per IS:1786 or TMT bars shall be used as reinforcement.
- Foundations shall be designed for the critical loading combination of the steel structure and or equipment and/or superstructure.
- 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.
- All structures shall be checked for sliding and overturning stability during both construction and operating conditions for various combination of loads.
- 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.
- 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.
- 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.