01 | 2021

A flying start

At the end of 2020, HOCHTIEF engineers at the Technical Competence Center performed a technical masterpiece. Working from Essen, they remotely controlled the start of a tunnel boring machine in Singapore, over 10,000 kilometers away.

Stephan Assenmacher and his three-person team were tense as they sat before their monitors in mid-December. Ten cameras were transmitting live images from a 50-meter-deep shaft in Singapore, 10,368 kilometers away. With the help of these images, the team had to carry out a very special mission: remotely control the complex starting sequence of a tunnel boring machine.


They were managing a “flying shield start,” a technology developed by HOCHTIEF for carrying out the first few meters of tunnel construction faster and more cost-effectively. “This was the second time we started the approach remotely,” says Stephan Assenmacher, a mining engineer who is one of the developers of this procedure. Because of the coronavirus lockdown, Assenmacher and his team were once again unable to manage the start in person. “This was a real challenge, because we had to rely on the people working on site to do the things we would otherwise have perceived with our own eyes and ears,” he says.

Even though a tunnel boring machine (TBM) has gigantic dimensions, it has to be maneuvered with a precision measured in millimeters. That applies in particular to the flying shield start. In this procedure for starting the tunneling process, which has been patented by HOCHTIEF, a steel ring is installed behind the shield of the TBM. The ring transmits the force arising from the reaction forces of the startup process onward. A spacer ring that projects from the shield tail connects it with the thrust cylinders so that the connection can transmit forces. This pressure ring, together with the TBM, is pushed forward by means of hydraulic hollow-piston cylinders and threaded rods that are fastened to the pressure ring and to steel girders on the portal wall, which will later become the entrance to the tunnel. By means of this process, the TBM is forced straight into the portal wall. As soon as the shield tail has disappeared into the tunnel it has bored, the TBM can secure and support itself. The pressure ring and the steel girders are then dismantled.

The advantage of the flying start is that it saves time and costs for the construction projectand simplifies the construction and dismantling processes. In conventional processes, a much more complex concrete and steel structure has to support the TBM as it bores the first few meters. Besides, the flying shield start requires much less space than a conventional start with blind rings. That’s why it’s especially suitable for projects like the one in Singapore. There, the 50-meter-deep shaft has a very small diameter that accommodates only the construction of the shield of the TBM without any attachments.


In Singapore, the CIMIC subsidiary Leighton Asia is currently building the Deep Tunnel Sewerage System (DTSS), a 40-kilometer-long underground tunnel with a diameter of 7.65 meters. There’s a reason why this city-state plans to direct its wastewater through such a wide tunnel in the future: There is a shortage of freshwater in Singapore. Consequently, the plans call for as much wastewater as possible to be recycled. And that’s why the pipe system must be able to transport large volumes of water, even those resulting from torrential rainfall, to existing water treatment plants.

For Stephan Assenmacher, this was the thirdand lastTBM start within the project. The fact that it had to be controlled from a desk in Essen was a special challenge that could not have been foreseen. When the tunnel boring machine was built and transported, all of the national boundaries were still open. Then came Covid-19, and Assenmacher had to come up with a whole new approach. Now the experts used cameras to supervise the shield’s slow progress into the earth. “We even had a colleague from Leighton Asia wearing a helmet camera,” Assenmacher says.

However, the TBM could only be started up on site. “From our desks in Essen we sent detailed instructions, as well as the final go-ahead, of course,” he says. Although the cameras transmitted high-resolution images in real time that enabled the team to “identify each crumb of soil in the shaft,” the experts in Essen felt the lack of crucial sensory impressions. As an experienced tunnel builder, Assenmacher normally supervises the TBM’s start with all of his senses. “Sometimes you can hear glitches before you see them,” he explains. Troubleshooting could also have become a problembut fortunately, the start of the TBM was a roaring success. As planned, the flying shield start needed only two and a half shifts for the TBM to reach the position for cementing the annulus so that it could then drive itself forward using its own equipment. It was a brilliant achievement! Stephan Assenmacher cannot yet predict whether flying shield starts will continue to be remote-controlled in the future. “In any case, there’s a trend toward automation,” he says.

Text: Marius Leweke