An enormous Soviet-era hydro dam project in Tajikistan has been revitalised thanks to modern engineering ingenuity.
This article was originally published in the August 2024 issue of create with the headline ‘Flying high’.
The right tool for the job. In the right place. At the right time. It’s an engineer’s definition of happiness.
But what if that place is 1300 m above sea level in the Tajik mountains? And what if that job is to haul 3000 t an hour of alluvial material between cliffs 650 m apart while building a 335 m-tall embankment dam?
It’s a job that tests the boundaries of possibility. And that’s the fun part, according to the engineers behind the Flyingbelt conveyor.
“All the tools and materials were already there to be found,” Project Director Fabio Guerino told create. “They just needed to be processed and matched in ways that would work best.”
The Rogun Hydropower Project is enormous. The Central Asian mountain range is remote, and the high-altitude terrain is as rugged as it gets. But the multibillion-dollar project promises to transform lives across the entire nation.
Two of the six planned 600 MW hydroelectric turbines are already operational. Once completed, the 3600 MW project promises to double Tajikistan’s overall energy production.
It’s an outcome worthy of the effort and inspiration needed to get the megaproject off the ground, Guerino said. But getting things off the ground proved one of many challenges for the principal contractor.
How do you move all that material and infill into place in a fast, reliable, efficient and safe manner at such a challenging site? The answer comes down to how strong you can make a piece of string.
The 650 m-long single-span conveyor belt, suspended by four track ropes, is designed to withstand an overall tension of about 8000 kN.
The challenge
The Rogun Dam sits in the Vakhsh River valley, some 110 km from Tajikistan’s capital, Dushanbe.
When completed, it will be the tallest embankment rockfill dam in the world – overtaking the current holder of that title, the Nurek Dam, also in Tajikistan, and also taking the overall title for world’s tallest dam from China’s Jinping-I dam by 30 m.
Work on the megaproject began in 1976 under the former Soviet Union. Progress was curtailed in 1993 – as the Soviet Union dissolved – before restarting in 2016 under the Tajik government. Italian civil infrastructure group Webuild was brought in to complete the project.
With the plant slated to achieve full operational capacity in the 2030s, solving the materials transportation challenges to fill around 80 million m3 of dam structure plays a critical role in meeting that deadline.
It’s about linking supply to demand. Different grades of infill material must be transferred from one side of the valley to the construction site on the other.
“The complexity of the geomorphology and geological conditions in the project area makes the usual method of building roads and hauling with heavy trucks a very slow, costly and unsustainable prospect,” Guerino said.
A suitable road would have to be hacked out of the steep, rugged landscape over several kilometres. And the fuel consumption of the haulage trucks is daunting.
A long line of conveyor belts is already in use to bypass the boulders, cliffs and switchbacks that dominate the landscape. It carries embankment material between stockpiles and to the dam location.
But extending this to the final 650 m span across the neighbouring Obishur valley needed extra thought.
The Flyingbelt
Webuild foresaw the use of a conveyor system for the advanced phases of construction from the start.
Production rates were set and the necessary haulage capacities were calculated. These calculations pointed to a possible solution: a 650 m-long single-span suspended conveyor belt.
“The Flyingbelt itself is an extension of the conveyor system currently operating at the site,” Deputy Technical Manager Francesco Celeste said. “But its use will be of paramount importance to raise the dam up to its final configurations.”
Conveyor belts have a well-established role on construction sites; these traditionally run along supports placed on the ground.
Suspended conveyor technology is nothing new, but is usually found in mining operations and cement production facilities – and none of these inhabit locations as extreme as the Vakhsh Valley.
“Only one other example of a suspended conveyor exists, on a construction site in Italy,” Celeste said. “But this is a much lower scale of system in capacity and length.”
It first had to be determined if a suitably large suspended conveyor was even possible. A site survey confirmed the original alignment of the system and the final location of the loading and unloading stations.
The physical nature of the site had to be understood. A geotechnical and geophysical investigation was carried out to assess the existing geomechanical conditions of the abutments beneath the foundations. Then came an exploration of the physical possibilities of such a single-span conveyor system itself.
Under pressure
Designed by Italian company Agudio, the Flyingbelt is a conveyor kept suspended by four track ropes designed to withstand an overall tension of about 8000 kN.
The success of the idea depends on the strength and resilience of its lock-coil steel wire ropes.
“The concept of the system is that the ropes are built to last; no replacement is planned as it implies a substantial stop of production,” Celeste said.
“Those types chosen have three layers of Z-shaped wire [with a] height of 5.2 mm, for an external diameter of about 65 mm. That kind of wire rope is typically used for the track ropes of aerial tramways”.
The setup has been designed to avoid the need for regular re-tensioning to minimise maintenance. But doing so is feasible if necessary.
“The wire ropes are able to ensure a very low elongation, which allowed us to consider a reduced range of tensions at the design stage of the system.”
The ropes are anchored around drums at both the loading and unloading stations’ large reinforced concrete structures.
Once strung up and tensioned, three months was allowed for the anticipated permanent stretching to take place. A close eye was kept out for any difference in elongation between the four lines; only then was the conveyor system attached.
Tense situations
The necessary 8000 kN tension isn’t just about accommodating the weight of the steel rope, conveyor and rubble. Thermal effects on the steel and weave of the cable itself had to be considered due to the location’s extreme temperature variations; Tajikistan’s mountains can be more than 30°C in summer and below -20°C in winter. Then there’s the intermittent additional load of snowfall and ice.
Above all, there is a need to cope with dynamic load scenarios associated with heavy structures in seismically active areas.
“It may sound surprising, but the rope suspension system itself does not require any sensor,” Celeste said. “The monitoring of the rope inclination is performed manually using an inclinometer, as commonly done for aerial tramways.”
But the suspension bridge is vertically and laterally flexible.
“Its location naturally leads to a high exposure to wind that may trigger significant effects.”
The kind of movements the 650 m-long system would be subject to had to be carefully modelled. To prevent rotations of the suspension along its axis, the line has been fitted with a series of hung counterweights.
Hauling alluvial material
With the suspension bridge established, its conveyor works like any other, Celeste said. A sensor network closely monitors its performance, but this is no different to any “classic” ground setup. Its performance is determined by the needs of the dam.
It can transport embankment materials up to 400 mm in size at a speed of four metres per second. This delivers about 3000 t per hour from a transfer chute on loading to a discharge pulley.
Celeste said the Flyingbelt can sustain this full-load performance at wind speeds of about 70 km/h. And it can avoid damage from gusts of about 160 km/h if emptied and deactivated.
“Considering both the scenarios mentioned above, the span tends to sag horizontally by about three metres in operation and by more than 20 m out of operation.
“Although the latter values are important displacements, the system is designed to cope with them and the conveyor belt smoothly follows the suspended structure.”
The Flyingbelt will remain operational for the remainder of the dam’s construction, carrying materials in different directions according to need.