By

Atul R. Dhamne

Orientation

on Construction Technology:

High Towers and Transmission Lines

Voltage (kV)

1977 1990 2000 2002 2012 2017-18 Year

220kV 400kV

500kV HVDC

765kV 800kV HVDC

1200kV

765kV D/C - AC

World’s Highest Voltage level – Test station Charged in Oct.’12

World’s longest multi-terminal

HVDC to harness renewable Hydro Power from North-east

Pursuing Higher Voltage Levels (Ref. PGCIL)

Types of Rocks

Tower foundations are usually built on the upper area of the

rock formations which are often found to be weathered and

disintegrated. The rocks are broadly classified as:

Soft Rock/Fissured Rock : These can be excavated

using normal tools without blasting. These also include hard

gravel, kankar, lime stone, laterite etc.

Hard Rock: these can not be excavated using normal

tools; require chiseling, drilling and blasting. These include

quartzite, granite, basalt, hard marble etc.

Excavation in Hard Rocks In case of excavation in hard rock requiring blasting, extreme care shall be taken with regard to storing, handling and use of explosives. Materials used for blasting such as explosives, detonators, fuses, tamping materials etc. shall be of approved make and of good quality. Proper storage of these materials shall be ensured. The contractor shall familiarize himself with all applicable laws and regulations. Blasting operations shall be carried out under the supervision of competent and licensed person and trained workmen. Proper precautions for safety of persons and property shall be taken. Where blasting is prohibited or not practicable, excavation shall be carried out by chiseling at no extra cost to the owner.

Types of Soil and its Characteristics

Rocks : Broken into regular and irregular sizes by joints

Soil :particulate earth material

Boulder :too large to be lifted by hands

Cobble: particle that can be lifted by a single hand

Gravel : course grained particle larger than 6.4mm

Sand: frictional, size varies from 6.4 to 0.06mm),

Silts: frictional, low surface-area to volume ratio, size

varies from 0.06 mm to 0.002mm

Clays : cohesive - fine grained - high surface-area to

volume ratio, size smaller than 0.002 mm

Peat: soils not suitable for foundations

Soil Parameters For designing the foundations; following soil parameters are required. Limit bearing capacity of soil This parameter is vital from the point of view of establishing

the stability of foundation against shear failure of soil and excessive settlement of foundation when foundation is subjected to total downward loads and moments due to horizontal shears.

Density of Soil This parameter is required to calculate the uplift resistance

of foundation as well as to calculate differential weight of soil and concrete below ground level for checking the bearing pressure under foundation.

Angle of Earth Frustum This parameter is required for finding out the uplift

resistance of the foundation.

Soil Properties Considered for Foundation Designs Sr. No. Types of soil Angle of earth

Frustum (Degrees)

Unit Wt. of

soil (Kg/cu m)

Limit bearing

Capacity (kg/sq.m)

1 Normal dry soil

(a) Without undercut

(b) With undercut

30

30

1440

1600

25000

25000

2 Wet soil due to presence of sub soil

Water/Surface Water

15 940 12500

3 Black cotton soil

(a) In Dry portion

(b) In Wet portion

0

0

1440

940

12500

4 Sandy soil

(a) With clay content 0-5%

(b) With clay content 5-10%

10

20

1440

1440

25000

25000

5 Fissured Rock/Soft rock (with

undercut)

(a) In Dry portion

(b) In Wet portion

20

10

1700

940

62500

62500

6 Hard rock - - 125000

7 Normal hard dry soil (Murrum) with

undercut

30 1600 40000

Testing of Soil

It is desirable to undertake testing of soil for all the tower

locations and report should be obtained about the sub-soil

water table, bearing capacity of soil, possibility of

submergence and other soil properties required for the

correct casting of casing of foundations.

Soil resistivity readings are taken for every 1 kilo metre

along the line in detailed survey. In case soil characteristics

change within 1km, the value shall also be measured at

intermediate locations. The megger reading and soil

characteristics shall also be indicated in the soil resistivity

results.

What is a Foundation?

Function of a foundation is to transfer the structural loads

from a transmission tower safely into the ground.

Detailed geo-technical investigation at specified number of

tower locations need to be done to provide sufficiently

accurate information, both general and specific about the

substrata profile and relevant soil and rock parameters at

site on the basis of which the foundation of transmission line

towers can be classified and designed rationally.

LOAD AND SETTLEMENTS OF FOUNDATIONS

Types of loads on foundations: Dead, live, wind, inclined thrusts and uplift, water table and earthquake forces Types of settlements: Uniform and differential. Differential settlement must be minimized, depends on site soil conditions and distribution of loads on columns supporting the structure

Types of Foundations

a) Shallow Spread Foundations

b) Deep Foundations

Depends on whether the load transfer is at deeper depths or shallower depths

Need for these two types: soil strength, ground water conditions, foundation loads, construction methods and impact on adjacent property

Shallow Foundations

Depending upon the ground water table and type of soil and rock, the shallow foundations are classified as:

Normal Dry Soil Foundations

Wet Soil Foundations

Partially Submerged Foundations

Fully Submerged Foundations

Black Cotton Soil Foundations

Partial Black Cotton Soil Foundations

Soft Rock/Fissured Rock Foundations

Hard Rock Foundations

Sandy Soil Foundations

Soft Rock/Fissured Rock Foundations

When discomposed or fissured rock, hard gravel or any other

similar nature is met this can be executed without blasting.

Under cut foundation is to be used at these locations.

Hard Rock Foundations

Chiseling, drilling and blasting are required for execution.

Soil with negligible cohesion because of its low clay content

(0-10%)

The above categorization of foundations has been done for

economizing the design of foundations; uplift resistance of

foundations is a critical design factor which is greatly affected

by the location of water table and the soil surrounding the

foundation.

Sandy Soil Foundations

Deep Foundations

Extend several dozen feet below the structure:

Pile Foundations

Micro Pile Foundations

Well Foundations

Design Procedure for Foundations

The design of any foundation consists of following stages:

1. Stability Analysis

Check for bearing capacity

Check for uplift resistance

Check for side thrust

Check for over-turning

Check for sliding

2. Structural Design

Transmission Towers in Hilly Terrain

In hilly/mountainous type of terrain or in thickly

populated areas, it is generally not advisable to attempt a

direct route or try to locate towers in long spans.

Small angles of a few degrees cost a little more and add

little to the length of the line.

Suspension towers (A – type) can be provided for line

angles of upto 2 degrees and small angle towers (B –

type) can be provided for angles upto 15 degrees.

Span Length

Span length is the distance between two towers. So greater

the span length lesser will be the cost of transmission line.

That why it is required to keep maximum span length while

keeping the minimum ground clearance .

A span is the part of the line between any two adjacent

towers. A section is the portion of the line route with a single

span or with a number of consecutive spans between two

tension points with "B", "C", or "D" type towers, as

applicable.

Span Length in Hilly Terrain

In hilly areas span length is usually kept lesser than in

planes and it varies from pole to pole means if one pole is on

the edge of a hill then the next pole will be at greater span

length as compared to the pole which is at the start of cliff.

The number of consecutive spans between the section

points should not exceed

15 spans or 5 Km for plain terrain and

10 spans or 3 Km in hilly terrain.

Foundations in Hilly Terrain

Type of the foundation depends upon the area through

which the transmission line is passing through.

For hilly areas there is no need of deep foundations and

hence normal depth of 2-3 meters is sufficient.

Similarly for planes having muddy soil the depth is

about 5 meters and it is done by boring the soil.

Thank you