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.