FACS: How Webuild innovates tunnel building

More than two years of studies, tests, and field trials conducted between Italy and Australia. Two years of research and development to patent a new way to install precast concrete segments, which provide form the walls of tunnels through which trains, cars, and even water travel. To address the immense water pressures within some tunnels crucial to hydroelectric plants, the Webuild Group first studied and then developed the Force-Activated Coupling System (FACS). The project was carried out by engineers and technicians with a specific goal: to provide a safer and more secure way to build tunnels for Snowy 2.0 the largest hydroelectric project under development in Australia. The project, owned by Snowy Hydro Limited, is being constructed by Future Generation, the joint venture partnership of Webuild and subsidiaries Clough and Lane in the Snowy Mountains, in the state of New South Wales.

Snowy 2.0 is designed ensure the production of clean energy capable of meeting the needs of 500,000 households simultaneously for approximately a week, involves the construction of a network of nearly 30 kilometers of tunnels connecting two existing reservoirs, Tantangara and Talbingo, accompanied by the construction of an underground power station with pumping capabilities. When there is demand, the system transfers water from the higher dam to the lower one to produce electricity. The FACS was designed to ensure that the tunnel through which the water surges can resist the pressure. It is an innovative system that is bound to set a new standard in the industry.

The FACS was developed in response to the specific conditions envisaged by the Snowy 2.0 project, especially those of the Inclined Pressure Shaft (IPS), a hydraulic tunnel that will be 1.65 kilometres long with a diameter of 10 metres and a slope of 25 degrees. As it surges through the IPS, water will cover a vertical drop of 750 metres, exerting very high pressure on the walls of the tunnel.

When the hydropower plant will be operational, it is calculated that the water pressure can reach values of about 30 bar, equivalent to the weight of a column of water about 300 metres high acting on a single point. This led to the need to devise an alternative solution that enable the tunnel lining to withstand these powerful internal forces.

“The individual (pre-cast concrete) segments must collaborate as a whole to resist the actions generated by the water pressure in the tunnel,” says Nicola Valiante, Director of Design Services who oversaw the development of the FACS. “The challenge, therefore, was to ensure that the segments constituting the tunnel have a greater pre-compression capacity, making the entire lining more solid. Hence the birth of the FACS solution.”

This new system consists in having each segment equipped with two pin-socket steel couplers with disc springs. A set (one male coupler with pins and one female coupler with sockets) is installed on the longitudinal sides of each segment. When segments are interlocked with one another, the system creates a compressed force with the coupling mechanism. When the passing of the water applies pressure on the walls, the segments are more flexible and can expand accordingly.

“The work was very long,” says Valiante. “(It was) divided into a first research phase and a second development phase that included both design and prototyping activities, as well as many experiments conducted on a reduced scale to fine-tune every single component of the system.”

At the end of the design and laboratory testing phase, Webuild engineers and technicians moved on to a real-scale application and the construction of the first tunnel segment prototypes with the FACS system. In Sondrio, Italy, a first experimental phase was conducted with the production of real-scale tunnel segments.

“This phase allowed us to fine-tune both individual components and verify the functioning of each segment,” he says.

In the second phase, the process shifted to Australia, where experimental production of tunnel segments was initiated, leading to the construction of an entire tunnel ring, assembled with all the elements of the system.

Innovation is never something achieved by a single individual. It the result of a collaborative process born from a winning idea. Such was the case for the FACS. After the initial study phase, it saw the collaboration of dozens of engineers and technicians, both young and old.

“In every innovative project, we involve young individuals because we believe it is an important formative activity,” says Valiante. “It is also an occasion to handover knowledge and experience for future projects.”