CHAPTER FOUR
Check Survey
4.1 Introduction
A check survey for a transmission line is a final survey that determines the location of the
towers on the ground. It is part of the transmission line survey process after carrying out
preliminary survey and detailed survey and just before the start of foundation works.
o The Check survey is carried out to make a check on detailed survey to reconfirm the
tower positions on ground conforming to the approved profile and tower schedule.
o The co-ordinates of all the tower locations marked during detailed survey are to be
reverified.
o In the process it is necessary to have the pit centre marked according to the excavation
marking plan. The levels, up or down, of each pit center with respect to the center of the
tower location shall be noted and recorded to determine the amount of benching or
earthwork required to meet design requirements of the foundation.
o If the levels of the pit centers are in sharp contrast with the level of the tower centre (say
beyond a slope of 1:4), suitable leg/ Chimney extensions may be deployed as required
for optimization of Benching.
o For undulated/ hilly locations, the levels up or down of each pit centre with respect to
centre of tower location are recorded at intervals of 2m using total stations/ DGPS and
digitized contour plans are made. Based on the digitized elevation plans, the quantities
of benching & protection work, and possible unequal leg extensions / Raised Chimney
are optimized using suitable computer-aided techniques/ software or manual method.
4.2 Pegging of Tower spots on Ground
4.1.1 Checking and Line Alignment
In this operation, traversing is done from the known fixed angle point(the starting point or
any other obligatory point) in the direction of given line deviation i.e. up to a distance equal
to the Section length between the starting point and the next angle point.
4.1.1(a) Procedure of check survey using Total station:
o Traversing is done from AP-1 to AP-2 with the Total station. During traversing, marking
of intermediate station point along road/bund/highway etc may also be done at an
interval of 1 - 2 KM.
o The station points should be marked along the line in intermediate points preferably
near to Tower location.
o After reaching AP-2, traverse loop will be closed by back sighting towards AP-1.
o Alignment from AP-1 to AP-2 will be automatically made in Total station. This
alignment is sometimes also called zero line.
o After the above alignment from AP1 to AP2, enter the distance of first suspension tower
(if coordinate is not known) in the direction of line alignment or enter the coordinate (if
known during detailed survey) of first suspension tower in machine.
o Based on the coordinate/Span length, Suspension tower can be spotted. After that
centre point and line pegs of intermittent towers will be marked.
o Shift the machine to next station point marks during traversing if visibility of point is
not clear and repeated the procedures for other suspension towers.
o This procedure will be repeated for other section of line.
4.1.1(b) Check survey using DGPS:
o Setting of DGPS base station may be done by entering the coordinate of known points
in the machine for AP1.
o Then coordinate of 2nd AP will be entered in rover machine and accordingly machine
will make the alignment from AP1 to AP2. This alignment is sometimes also called zero
line.
o After making the alignment from AP1 to AP2, enter the distance of first suspension
tower (if coordinate is not known) in the direction of line alignment or enter the
coordinate (if known during detailed survey) of first suspension tower in machine.
o Then machine will automatically show the direction and distance of this suspension
tower. Distance of preceding and succeeding line pegs will also be entered in machine
for locating line pegs. When reaching at the said location using rover the Centre peg and
line peg will be marked one by one.
o This procedure will be repeated for other suspension towers of that section.
4.1.2
Figure4.1: Check Survey Report
Pegging of Tower Spots:
Once each angle is fixed in field by the help of permanent peg mark and exact Section length
is known, the surveyor proceeds to mark all intermediate tower positions on the straight line
joining the angle points spaced at distance equal to
individual span length as given on Survey Chart and after the same is duly adjusted for the
closing error.
In order to help in correct aligning of all intermediate towers between 2 angle points, a
number of alignment pegs are given at the time of exact distance measurement of the
Section.
The more the number of alignments pegs the better it will be for the readings as instrument
errors are less if smaller distances are measured in one reading.
These pegs are also very useful when the main tower marking pegs are found missing at a
later date (due to mischief of local people or negligence of excavation marking gang).
4.1.3 Line Peg Marking and Directional Peg Marking:
On the suspension tower, pegs are set along the Centre line of route alignment.
Figure 4.2: Line peg marking for sespensition tower
Directional pegs are essential for correct alignment of tower center line along longitudinal
and transverse directions for angle tower. On angle towers these are rotated by an angle
equal to half the angle of line deviation.
Figure 4.3: Directional Peg marking at Angle point.
4.1.4 Survey Methodology & Precision:
All elevations shall be referenced to benchmarks established by the survey of India. Survey
operations shall begin and end at benchmarks approved by the Employer.
During the levelling of the profile, check surveys will be affected at intervals not exceeding
50 km with benchmarks of known elevations. The difference in elevations as surveyed by
the contractor and as declared by Survey of India for these benchmarks shall not exceed the
precision required for 3rd order surveys e ≤ 24k, where k is the distance between
benchmarks in km and e is the difference between elevations in mm.
In the absence of suitable benchmarks, the leveling shall be done by two independent
leveling parties working in opposite directions along the same line. The difference in
elevations between the two surveys shall not exceed the precision required for 3rd order
surveys as stated above.
All-important objects and features along the transmission line center line (railways,
highways, roads, canals, rivers, transmission lines, distribution lines, telephone lines etc.)
shall be surveyed and located with a positional accuracy of 1:2000 between points of known
horizontal position.
4.3 Pit Marking
Pit marking is a step in the construction of transmission line foundations that involves
marking the locations of poles and digging pits for them. It's based on surveyed data and
determines the alignment of the transmission line.
Figure 4.4 Survey: A check survey is conducted to locate the tower positions on the ground
Why it's important:
o Pit marking is crucial for determining the exact locations of the foundation pits
before foundations can be installed at ground level.
o It helps to ensure that the transmission line is aligned correctly.
The pit marking shall be carried out according to the pit marking drawing which is prepared
from the excavation plan details in foundation drawing of the tower. The size of the pit, in
case of open cut foundation, is worked out by adding 150 mm to the sides of the base pad on
all the four sides for allowing working space. No margin is necessary for undercut
foundations. The excavation of pit marking drawing indicates the distance of centers, sides
and corners of the pit with reference to the Centre point of the tower. These distances are
measured, and each pit boundary is marked in the field by means of spade or pick axe along
the sides of the pit. A pit marking chart is then made showing all relevant dimensions of the
pits for each type of tower foundation type for reference for pit marking. Typical foundation
drawing including excavation plan and Methodology for marking four corners of one of the
foundation pits is given below:
Figure4.5: Typical Foundation drawing including excavation plan
Methodology for marking four corners of one of the foundation pits:
From the dimensions shown in the drawing, the triangle ACF is first marked with the help of
a measuring tape. The distance BE equal to p (width of the pit) is marked on the ground. The
triangle ACF is then marked by shifting the point B and without changing the point C and F.
The distance CB’ equal F, is then marked. The side BC and B’E both equal to F,and then
marked. The procedure is repeated for marking other pits. The dimension M shown in the
drawing is the Centre to Centre distance between stubs of the tower at their lowest point. The
dimension N is the diagonal distance between the ends of the stub of the tower. The
excavation pit marking drawing is prepared on the basis of these dimensions.
Figure 4.6: Excavation Plan for Tower Foundation
Figure 4.7: Typical PIT MARKING Chart
4.4 Contouring of Tower locations in hilly/Undulated terrain
o Contouring in undulating terrain" refers to the process of mapping the elevation of a
land surface with significant ups and downs (like rolling hills) by drawing contour
lines that connect points of equal elevation, where the lines will be closer together in
steeper areas and further apart on flatter sections, providing a detailed representation
of the terrain's undulations on a map.
o The levels up or down of each pit centre with respect to centre of tower
location shall be recorded at intervals of 2m using total stations/ GPS/ digital
theodolite and digitized contour plans shall be made. Based on the digitized
elevation plans, the quantities of benching & protection work vis-à-vis
possible unequal leg extensions shall be optimized using suitable computer-
aided techniques/ software. Required tower and foundation details, cost data
for comparative valuation for benching & protection work vis-à-vis unequal
leg extensions are to be considered for the same. The leg extension and
benching details are to be incorporated in the final tower schedule.
o Accuracy: Careful field measurements are important to ensure the accuracy
of the contour map, especially in complex terrain.
4.5 Benching and Revetment
When the line passes through hilly/undulated terrain, levelling the ground may be required
for casting of tower footings. All such activities are termed benching and shall include
cutting of excess earth and removing the same to a suitable point of disposal as required by
Employer. Benching shall be resorted to only after approval from Employer. Volume of the
earth to be cut shall be measured before cutting and approved by Employer for payment
purposes. Further, to minimise benching, unequal leg extensions/raised chimney shall be
considered and provided if found economical. The proposal is submitted by the Contractor
with detailed justification to the Employer.
In the context of a transmission line construction, "benching" and "revetment" refer
to earthwork techniques used to stabilize slopes, particularly in hilly or uneven terrain,
where the transmission towers are being erected, by creating flat platforms (benches) on the
hillside and adding protective wall structures (revetments) to prevent soil erosion and ensure
the stability of the tower foundations; essentially preparing the ground for proper tower
placement in challenging topography.
Figure 4.8: Benching
Key points about benching and revetment in transmission lines:
Purpose: To provide a stable foundation for transmission towers in areas with steep
slopes or loose soil, preventing potential landslides or tower instability
Benching: Involves cutting flat platforms (benches) into the hillside at specific
intervals to create a level area for the tower base. Allows for proper placement and
anchoring of the tower foundation. As per the contour drawing of location,
Raising/Lowering of centre peg may also be done at site for optimization of
Benching (Cutting/Filling) as per the available Leg extension and Raised Chimney
foundation.
Revetment: A protective wall constructed on the slope face, often using stone,
concrete, or gabions, to prevent soil erosion and further stabilize the benched area. It
is also used along riverbanks where erosion could affect tower foundations.
Figure 4.9: Protection of Tower Footing
Figure 4.10: Revetment Wall
Figure 4.11: Section of Revetment
Important considerations:
o Site survey: Detailed geological surveys are crucial to determine the appropriate
design and extent of benching and revetment needed.
o Engineering calculations: Proper engineering calculations are necessary to ensure
the stability of the designed benching and revetment structures.
o Environmental impact: Construction techniques should be environmentally
sensitive to minimize disruption to the natural landscape.
The levels up or down of each pit centre with respect to centre of tower location shall be
recorded at intervals of 2m using total stations/ DGPS/ digital theodolite and digitized
contour plans shall be made. Based on the digitized elevation plans, the quantities of
benching & protection work vis-à-vis and possible unequal leg extensions / Raised Chimney
shall be optimized using suitable computer-aided techniques/ software or manual method
and submitted to the Employer. Required tower and foundation details, cost data for
comparative valuation for benching & protection work vis-à-vis unequal leg extensions shall
be provided by the Employer to the Contractor during execution stage.
Protection of Tower and Tower Footing:
The work shall include all necessary stone revetments, concreting and earth filling above
ground level, the clearing from site of all surplus excavated soil, special measures for
protection of foundation close to or in nalas, river bank /bed, undulated terrain, protection of
up hill / down hill slopes required for protection of tower etc., including suitable revetment
or galvanised wire netting and meshing packed with boulders. The top cover of stone
revetment shall be sealed with 1:2:4 nominal mix concrete. Contractor shall recommend
protection at such locations wherever required. Details of protection of tower/tower footing
are given in drawing enclosed with these specifications for reference purpose only.
For some of the locations in nalas, riverbed or undulated terrain etc., boulders of minimum.
150mm size bounded and packed in galvanised wire net/mesh of 8 SWG wire and 152
square (maxm.) mesh are to be provided. These stones are provided in crates size of
2.0mx2.0m or as deemed suitable for a particular location. Measurement is taken in cubic
meters and 15% deduction made for void from cage/stack measurements.
Figure 4.12: Gabion Wall
4.6 Leg Extensions and Chimney Extensions
In a transmission line, a "leg extension" refers to an additional length added to a tower leg to
accommodate uneven ground levels, essentially extending the leg downwards to reach a
stable footing, while a "chimney extension" is a vertical extension added to the top of a
transmission tower foundation, typically used to increase conductor clearance above
obstacles like buildings or other structures.
Figure 4.13: Unequal Leg extension Multi Circuit tower in undulated terrain.
Key points about leg extensions:
o Purpose: When a transmission line crosses terrain with significant elevation
changes, some tower legs need to be longer than others to ensure proper stability on
the uneven ground.
o Design consideration: Engineers calculate the required length of leg extensions to
maintain the necessary tower stability and conductor clearance.
Below is a photograph showing chimney extension above the leg extension of tower for
meeting the stable ground in highly undulated terrain.
Figure 4.14: Chimney extension Multi Circuit tower
Figure 4.15: Typical Raised Chimney Foundation
Figure 4.16: Single line Diagram of Unequal Leg Extension tower
