The 500 kV Kavkasioni transmission line, which connects the power systems of Georgia and the Russian Federation, represents a unique engineering structure. Its route passes through extremely complex terrain and crosses numerous avalanche- and landslide-prone areas located on the northern and southern slopes of the Main Caucasus Range.
Based on many years of observations and analysis of accumulated data, it was determined that the least reliable sections of the 500 kV Kavkasioni transmission line are the segments located on these slopes, particularly in the valleys of the Makhari and Klichi Rivers. Along these approximately 20 km sections of the line, numerous serious damages have occurred since commissioning due to the impact of natural factors (avalanches, landslides, ice accretion, and other phenomena), including tower collapses, conductor failures, and other incidents.
Following the proposal of Mr. Sh. Maisuradze, First Deputy General Director of JSC Sakrusenergo, a decision was made to implement measures aimed at preventing such damage and improving the operational reliability of the 500 kV Kavkasioni transmission line. Specifically, in certain sections of the Klichi River valley, located on the southern slopes of the Main Caucasus Range, it was decided to replace conventional steel lattice towers with a cable suspension system.
This solution was applied at Tower No. 224, where all three tower foundations/supports, and at Tower No. 225, where one support, had been destroyed by an avalanche in March 2005. Instead of reconstructing these sections using conventional steel towers, a cable-supported suspension method was selected.
The development of the wire rope suspension option envisaged the construction of a special “cable-supported bridge” at a relatively high elevation across the Klichi River valley, with the overhead line conductors suspended from this structure. This solution was adopted instead of the destroyed Tower No. 224, which had been completely swept away by snow avalanches, as well as other steel towers located in avalanche-prone areas, namely Towers No. 215, No. 220, No. 222, No. 223, and No. 225.
The decision was driven by the necessity to eliminate the mechanical impact of avalanches on the existing towers and conductors within the considered section of the overhead line and, consequently, to improve the operational reliability of the transmission line.
The wire rope suspension system consists of a set of parallel 21 mm diameter steel wire ropes stretched between the two banks of the Klichi River. The ropes are secured by end sockets to the anchor plates, which are fixed to 2.6–3.0 m long ground anchor bolts installed and grouted in 93 mm diameter anchor boreholes. Perpendicular to the wire rope system, three 6.5 m long cross-arms (traverses) are installed along the axis of the overhead line for suspending the V-shaped insulator strings of the individual phase conductors.
The wire rope suspension system consists of 1,000 m long wire ropes, interconnected by line clamps containing B83 grade babbitt metal. The cross-arms are manufactured from No. 20 steel angle sections. The welded anchor plates are fabricated from rolled steel sections, including No. 30 and No. 16 channels, as well as steel plates with thicknesses ranging from 10 to 20 mm.
The steel anchor bolts have a diameter of 56 mm and a length of 2.7–3.0 m. Steel wedges are manufactured from 80 mm thick-walled steel pipes, while the wedge sleeves are made from 88–90 mm thick-walled steel pipes.
For the first time in Georgia, the design and construction of a structure associated with the wire rope suspension of 500 kV overhead transmission line conductors were carried out simultaneously. This was related to exceptional technical complexity and responsibility, primarily due to the severe climatic conditions (with only 4–5 months available during summer and partly spring periods), the extremely limited timeframe required for restoration of the damaged transmission line section, and other challenging circumstances.
The restoration construction and repair work at the damaged section of the 500 kV overhead line near Tower No. 224 were largely completed in 2005. The transmission line has been in operation since then, and no concerns or issues have been identified regarding the reliability of this section to date.
Starting from 2006, design and preparatory works continued for implementing reconstruction measures in the remaining avalanche-prone areas. These activities continued until 2007 and were carried out according to a different engineering concept at Tower No. 220.
The right and middle legs of Tower No. 220, which is an angle tension tower, as well as the conductors of phases A and B attached to them, are located within an avalanche-prone zone. The probability of their damage was considered high, which was confirmed by the fact that two damage incidents had occurred in the past due to avalanche impacts.
To ensure the reliability of this section, a number of protective measures were planned, the most important of which was the installation of conductors without the use of conventional towers. Instead, the conductors were to be secured by means of special tension insulator strings directly anchored into the rock mass.
This project is unique in itself, similar to the so-called “wire rope suspension system” successfully implemented in 2005.
Based on topographical, geodetic, and geological data, it was determined that the attachment points of each phase conductor to the rock face should be located at heights of approximately 85 m and 95 m above the road surface passing near Tower No. 220.
Using geological data on the physical and mechanical properties of the rock mass, together with the electrical parameters of the overhead line, the detailed working technical documentation for the construction part of the project was developed.
Each phase conductor is anchored to the rock using four anchors. The anchors, with a diameter of 56 mm, are installed perpendicular to the prepared rock surface into 93 mm diameter drilled holes. They are initially fixed by applying mechanical force (pre-tensioning) and are then permanently secured by grouting with cement mortar.
The works at Tower No. 220 were completed in September 2007. As a result, the probability of transmission line damage caused by avalanche impacts, as well as emergency outages resulting from the air pressure wave generated by avalanches, was practically reduced to a minimum in this section.
Various measures have been implemented worldwide to ensure the reliable operation of overhead transmission lines located in avalanche-prone areas. However, the implementation of such measures requires significant capital investment and involves considerable technical challenges. It should be noted that their effectiveness is often limited and they do not fully ensure the safe and reliable operation of transmission lines.
The wire rope suspension solutions implemented on the 500kV overhead transmission line in the Klichi River valley were applied for the first time and represent unique engineering structures. Experience gained over the subsequent operational period has demonstrated that this solution has no practical alternative under such conditions.
Therefore, the wider implementation and application of this technology can provide significant technical and economic benefits.