Tunnel construction monitoring for the U5 metro line extension in Germany

Challenge

The U5 Stadtbahn Europaviertel is a metro line extension project that aims to provide public transport for a new city district with commercial areas, a park, 3,800 households and 30,000 people in Frankfurt. The underground section of the U5 metro line involves two tunnels with a total length of 1,720 m (860 m for each tunnel). Because of fragile infrastructure surrounding the site, and the general risks of tunnel excavation, PORR Group, the building contractor selected by project owner SBEV, needed to install monitoring equipment without impacting the construction work or disrupting city traffic.

Photo courtesy of SBEV GmbH/Klaus Helbig

Solution

Project partner Angermeier Ingenieure installed around 100 geotechnical sensors and 400 geodetic monitoring points to carry out precise monitoring across the tunnel project. Multi-point extensometers were located at several points along the tunnel under public areas and roadways. And to automate data acquisition without installing long and expensive cables, the extensometers were connected to Worldsensing vibrating wire 5-channel data loggers. A wireless biaxial tiltmeter, which serves as a sensor and node in one, was installed to monitor the tilt of surrounding structures.

The battery-powered data loggers, installed in a covered manhole to prevent vandalism, send data wirelessly to a gateway installed on a temporary office on site. From there, the network transmits the data to an evaluation and visualization system.

For connectivity, Worldsensing uses LoRa, a long-range, low-power wireless technology used by IoT networks worldwide. Features of the system include:

• Radio signals up to 32 times stronger than those of other wireless monitoring systems–strong enough to penetrate through metallic manholes.
• A star network topology that can cover a range of up to 2.5 miles/4 kilometers in a tunnel without any repeaters.
• Low-power edge devices that can last for up 10 years or until project completion, around 2025.

Benefits

The monitoring solution eliminates the need for expensive cabling that could disrupt traffic and construction work. Setting up the system is simple and data can be acquired even from sensors inside manholes.

Ultimately, the monitoring system contributes to the safety not only of workers but also of people who live and work in the city center.