700 000 t blasted-out rock at an altitude of 1700 m in the middle of the Swiss Alps –“Linthal 2015” is a mammoth project and easily the largest construction project associated with the Linth-Limmern power plants (Fig. 1). Goal is the construction of a pumped storage hydro-power station for the generation of electricity. To realize this goal, for two years now, two rock caverns (Fig. 2) together with a widely branching network of access tunnels and pressure shafts have been excavated 600 m deep into the interior of the mountain (Fig. 3). 5.5 km belt conveyors are currently used to transport the excavated material away. To minimize dust emissions, the responsible company uses special belt scrapers, which thanks to their patented twist-swing function removes almost 100 % of the material from the belt and minimizes the dust nuisance.
Ten years ago, the PM-10 standard was defined as a directive that specifies limits for inhaled dust, and since then this must be taken into account in addition to the regulations in effect to date. For conveying of huge volumes of excavated material like at the “Linthal 2015” project, the suppression of dust emissions plays an important role – especially as the workers are working up to 600 m deep inside the mountain. The belt conveyors are therefore cleaned during ongoing operation, around the clock, with the help of scrapers. This also prevents material getting stuck to the belt and gradually crumbling off. In this case the lost material would have to be removed manually with shovels.
Moreover insufficiently cleaned belts lead to soiling of the return idlers, which, as a result, can suffer damage in the long term and may have to be replaced. “If the scraping efficiency is poor, we have to clean the belts with water, which makes the material conveyed muddy and difficult to transport,” explains Ernst Kuster, project manager at Marti Tunnelbau AG, which supplies the conveyor belts. In addition, in accordance with the regulations, self-extinguishing belt conveyors have to be used, the rubber material of which, however, is less rugged. To maximize their lifetime nevertheless, efficient cleaning is essential.
One challenge at the “Linthal 2015” project is the restricted space and a slope of 25 degrees, which made installation of the entire conveying system particularly difficult. “To install the belt scrapers in the confined space, we developed a smaller version on site,” explains André Hanke, Sales Manager for Conveying Systems at the conveying systems specialist Schulte Strathaus.
The segmented scrapers are used both as primary and secondary scrapers. To remove the material, the scraper segments comprise polyurethane feet with special rotatable mounted scrapers made of stainless steel with a carbide insert which are fitted into the segment shaft. The individual segments overlap so that none of the conveyed material can slip through. This ensures that the belt is optimally cleaned (Fig. 4).
The special shape and geometry of the polyurethane feet allow a high level of flexibility. This is a prerequisite for the patented twist-swing function. The individual segments adapt excellently to the conveyor belt under the freely configurable contact pressure and swing with the movements of the belt. This not only achieves excellent cleaning results, it also reduces the wear on the segments and the belt (Fig. 5). “Strip scrapers or inflexible segment scrapers that are usually used cannot cope with the unevenness of the belt and therefore always allow a certain amount of the conveyed material to fall through,” explains Hanke.
On account of the limited space at the “Linthal 2015” project, reversible conveyor belts are used, the conveying direction of which can be changed as needed. For these conveyors, special reversing belt scrapers were necessary, which automatically disengage when the direction of the belt changes. Auxiliary blades designed for the opposite direction take over their function. This allows fast changes of direction without breaking the contact between the conveyor belt and the scraper. The system engages and disengages automatically as a result of the friction between the belt and the scraper segments, i.e. without any pneumatic, hydraulic or electrical support.
As particularly in tunnel applications, optimum scraper performance, long service lifetimes and easy maintenance are essential, the scraper system under the name “Starclean” has now become established as the standard system for most major tunnel projects worldwide. For instance, the biggest construction sites in London, Hong Kong, Abu Dhabi, Ecuador, Panama, Madrid and Barcelona as well as the Ceneri and Gotthard Tunnel projects are equipped with systems from Schulte Strathaus.
Since the Gotthardt project at the latest, the preservation of natural resources has played an important role in Switzerland. In advance of the “Linthal 2015” project therefore, preservation and usage plans were drawn up and ecological measures defined together with representatives of the authorities and nature conservation organizations. These also include the recycling of the blasted-out raw material. As around 90 % of the conveyed medium is removed from the belt by the scrapers, the large part of the excavated material can be reused. “80 percent of the blasted rock is further processed to concrete,” says Kuster. First the material is transported to a gravel works and processed there before it is sent to a nearby concrete plant.
3 Pump storage hydro-power plant guarantees reliable power supply even at peak load
Seven years have been estimated for the entire project; at the end of 2014 the first group of machines is to be connected to the power grid. The goal is to optimize the existing plants and to increase the current power output from 480 MW to 1480 MW with the new pump storage power plant. For this purpose the water is flushed from the higher altitude Lake Mutt into a pressure system and drives the turbine, which in turn runs the motor generator. The power generated is fed into the national grid. From the turbine the water reaches the lower lying Lake Limmern, from which it is pumped back up. In this way, excess power can be stored during periods of low consumption and used during periods of peak consumption so that a reliable power supply in line with demand can be guaranteed.