General

River Crossing involved in any Transmission line, is most important link of the line. It has to be safe & sound from design considerations to take care of field conditions, vagaries of nature such as cyclones/storms, whirl winds, flood & change of course of rivers etc.

Site Selection

The River Crossing site should be selected judiciously and carefully. It is desirable to cross rivers where :

• River is tamed by a bridge, preferably on down stream side to avoid fury of floods.
• River banks at the crossing point are at a level higher than the HFL recorded to ensure safety of Rx towers during floods.
• Special well or pile foundations should be avoided as far as possible to minimize the cost.
• The soil is firm & dry.
• The river crossing span should be within the maximum designed span of Rx Towers.

Type of River Crossing Towers

The major River crossings should be done on Suspension Type Towers, suitably designed for all wind conditions i.e. for transverse load incorporating wind load depending upon the height of the tower and upon the height of clamping point of Conductor and Earth wire, Vertical load, and longitudinal load including the effect of diagonal wind (at 45°) load conditions. The 'Anchor Towers' designed for Dead-end conditions should be provided on both the sides of crossing as shown below—

Fig. 10-I

Design Criteria

10.4.1 The main points considered for design of River Crossing and Anchor towers are described below :

• The normal Transmission Line Towers are designed with the assumption that wind would be acting on conductor, Insulator & tower structure in longitudinal transverse direction. But, for very high towers/structures of river crossing, the load on towers is evaluated after taking into account height factor and basic wind pressure etc. The towers for river crossing should be designed for diagonal wind load conditions also, which is an additional feature to take care of high velocity winds during severe storms/cyclones etc. Behavior of wind will, however, depend on the river crossing locations and the profile of crossing.
• The towers are so designed that the minimum factor of safety of individual member as well as over all structure is not less than 2.5.
• Shielding angle is; less than 30° for 132 KV and 220 KV towers and 20° for 400 KV towers.
• Over load factor for vertical loads is taken as 1.29
• The wind on, towers is assumed to be acting at an angle to the line which may produce the maximum combined effect of conductor and tower loadings in the tower members.
• Longitudinal load under broken wire condition due to power conductor and ground wire for Rx towers is taken as 100% of the tension of conductor and ground wire at 32°C with no wind. This is multiplied by Impact Load Factor of 2.65.
• Broken wire conditions should be assumed as follows :
  • Suspension Towers (Rx Towers):
    Ground wire or any one conductor broken whichever is more stringent for a particular' member in the tower
  • Anchor Towers
    Dead-end type towers designed for any combination of ground wire and two conductors on the same side or three conductors on the same side and on the same span whichever is more stringent for a particular member in the tower.

Parameters

The main desirable features/parameters are being described briefly hereinafter :

1. River Crossing Towers (Rx Towers)

1. The main design parameters for River Crossing (Rx) Towers which should be clearly specified and whih are presently being used in UPSEB are as follows:

2. The Salient features of Rx Towers presently being used in UPSEB are as follows:

If the Rx Towers are located in the meandering zone of, the river, Special Well-Type or Pile-type foundations are provided. However, with normal or wet soil with unit weight of 940 Kg per Cu. Meter and bearing capacity of 13675 Kg per Sq. Meter assuming angle of repose as. 15° the open cast fully submerged RCC Foundations (Step type) should be provided.

2. Anchor Tower

1. Main features of Anchor Towers which are presently being used in UPSEB are as follows :

2. Foundations for Anchor Towers
   Anchor towers are desirable to be placed outside meandering zone of the rivers. In such cases the foundation normally provided is open cast fully submerged RCC (Step type).

Deviation on River Crossing Towers

10.6.1 No deviation is allowed on River Crossing Towers. All the towers from Anchor Tower to Anchor Tower on River crossing portion should be in straight line. The Anchor Towers should be placed square to River crossing portion. If any deviation is required to be given, it should be given on the normal line side only.

Calculation for Important Parameters

10.7.1 Position and method for calculations of following parameters are also enclosed for reference & guidance :

Guideline for Preparation of Proposal for River Crossing

The river crossing proposal should be submitted from the field covering following details :—

1. Profile:

1. Profile of river crossing and Anchor towers along with position of adjacent towers of the line of either side of the river crossing.

2. The profile should clearly indicate reduced level of HFL mark, Rx and Anchor tower locations, nearby roads, bunds/embankment etc, if any.

3. The year of HFL mark and source of information.

4. Sag curve of 75°C and no wind for each span from Anchor to Anchor towers.

5. Clearance of lowest power conductor from HFL mark for maximum sag calculated at 75° and no wind. The minimum clearance required is 6.1 meters for 132 KV, 7.00 meters for 220 KV and 8.84 meters for 400 KV lines for non-navigable rivers.

2. Plan:

1. Plan showing Rx Towers, Anchor towers, adjacent tower with span, roads, bridges, bunds and other important structures etc.

2. Type of soil and its bearing capacity.

3. Meandering zone of the river.

4. Whether Rx and/or Anchor towers will remain normally submerged in water, should also be indicated clearly.

3. Calculations for Uplift :

On Anchor towers shall also be submitted on the profile & plan sheet itself.

4. Narrative:

Narrative about the geographical conditions of the area and whether there would be any problem for right of way etc. should also be submitted along with the proposal.

Precautions During Construction Work of River Crossing :

• The following points may be borne in mind and implemented religiously before taking up construction activities for river crossing
  • The tower parts should be checked with approved drawings and bill of material (B.O.M.) about correctness of fabrication and supply.
  • The survey and profile should be thoroughly checked at site. It should be ensured that the river crossing portion from Anchor-to-Anchor are in straight line. No deviation is allowed on River Crossing Towers and anchor towers on the river crossing side, i.e. the anchor towers should be placed square to the river crossing portion.

During construction right from survey to final sagging and commissioning. The following precautions may be taken to ensure execution & completion of River Crossing properly from all considerations :—

During Foundation:

1. The pits should be as per approved drawing.

2. The template should be placed and installed properly as per drawing for proper setting of stubs and the following dimensions should be checked before concreting: -

1. Base width

2. Base Width (back-to-back)

3. Diagonals

3. Proper Form Boxes or moulds should be used. If necessary shoring and shuttering should be used to avoid falling of surrounding earth and its mixing with concrete.

4. Depending on the condition, proper arrangement should be done for dewatering of pits, preferably by power driven pumps.

5. The concrete in 1:2:4 ratio should be mixed in mechanical mixer, preferably power driven. Hand mixing should not be allowed.

6. Proper laying and filling of concrete should be ensured. It should be done continuously, uniformly and properly with power drive vibrators.

7. The concreting of a particular leg should be done continuously and uninterruptedly to ensure proper bonding.

8. It is also preferable that concreting of 2 diagonally opposite legs is done simultaneously.

9. The levels of all the 4 legs should be checked before concreting during concreting and after completion of concreting job also.

10. The curing should be done properly and continuously for a period of at least 10 days.

11. Proper back filling by the same soil should be ensured.

12. The stub setting template should be opened only after the back filling.

During Erection of Towers:

1. The erection work should be done panel wise as per drawing.

2. It should be ensured that all the bolts & nuts of correct sizes are provided on all the members as per drawings and no hole is left blank.

3. All the cleats, gusset plate and pack washers should be provided as per drawing.

4. After erection of each panel and sections, the levels and verticality should be checked and corrective measures be taken, if necessary.

5. If the erection of tower is partial then it should be anchored properly and suitably before resuming work on next day.

6. After completion of erection each & every bolt should be tightened properly and checked thoroughly.

7. Punching & hammering of left over threads should be done.

8. Insulator strings should be assembled on the ground and hoisted properly before stringing. Chipped or cracked insulators should not be used.

During Stringing:

1. Before stringing it should be ensured that all the bolts & nuts have been tightened and punching and hammering of the left over threads has also been done.

2. The placement of Rx & Anchor Towers has been done as per approved survey and profile and that all the towers of River Crossing portion from Anchor to Anchor are placed in straight line i.e. square to the river crossing portion. It may be noted that no deviation is permitted.

3. River Crossing span is as per approved profile and within the maximum designed spans.

4. Levels & verticality of all the towers is Okay.

5. The stringing should be done as per standard norms & practices by using ground rollers, aerial rollers (with high quality Ball rollers for minimum friction), Snatch Pully Blocks, Hydraulic pressures, proper T & P and equipment, preferably by Tension Stringing Equipment.

6. It should be ensured that E/W is strung and securely clamped to the tower before the conductors are drawn up in the order of top conductor first then the middle and bottom conductors at the end.

7. It is also desirable that paying & tensioning of both the conductors on a particular cross arm is done is simultaneously, so as to maintain identical tension time history.

8. The traveler surface in contact with aluminium surface of conductor should be of such a material that the conductor is not damaged due to contact with material such as neoprene rubber or Stainless steel.

9. The conductor should not be rubbed with any rough ground surface and it should not also damage the standing crops.

10. During pulling out operation the tension in each conductor and E/W shall not exceed designed working tension of the conductor at the actual prevailing temperature.

11. After being pulled the conductor and E/W shall not be allowed to hang in the stringing blocks for more than 96 hours before being pulled to the specified sag. During the time the conductors and E/W are on the stringing block before sagging in, it should be ensured that the conductors and E/W are not damaged due to clashing, vibration or other causes.

Final Sagging:

1. The conductors and E/W shall be sagged in accordance with the approved stringing charts before final clamping. The conductors shall be pulled-up to the desired sag and left in travelers at least for one hour after which the sag shall be checked and adjusted if necessary before transferring the conductors from the travelers to the suspension clamps.
2. The conductors shall be clamped within 36 hours of sagging in.
3. Clamping as per initial stringing chart.
4. Dynamo-meters should be used in checking the tension in the conductors and EAV.
5. The sage shall also be checked when the conductors have been drawn up and transferred from aerial blocks to the insulator clamps.
6. Sagging operations shall not be carried out under wind, extremely low temperature or other adverse conditions which prevent satisfactory sagging.

Final Checking, Testing & Commissioning:
The following points should be kept in view before commissioning

• Sufficient backfilled earth is lying over each foundation pit and it is adequately compacted.
• Concrete chimneys and their copings are in good and finally shaped conditions.
• All the tower members are correctly used strictly according to finally approved drawings and are free from any defect or damage, whatsoever.
• All the bolts are fully tightened and they are properly punched.
• The stringing of the conductors and earth wire has been done as per the approved sag tensions charts and desired clearances are available.
• No damage, minor or major to the conductors, earth wire accessories and insulator strings, still unattended are noticed.