Metro and High-Speed Train Tunnel Construction: Underground and Surface Wireless Monitoring

As more tunneling projects are taking off across different continents, finding a solution to cover both underground and surface monitoring has become imperative. Considering the different engineering challenges, safety hazards and insurance regulations in tunneling projects, a wireless monitoring system based on Internet of Things (IoT) technology is the answer. 

Worldsensing organized a webinar for engineers, consultants, project managers and sensor manufacturers involved in tunneling projects who want to learn more on how wireless technology can address the specific needs of tunneling projects. 

The main topics covered were: advantages and limitations of current methods used to monitor tunneling construction projects, benefits of automation, how monitoring is performed using different wireless methods, a detailed visual presentation of how IoT technology is used in tunnel construction, and the key considerations when choosing a monitoring solution. Case studies about IoT wireless monitoring in tunnel construction in Europe and the United States were also discussed. 

A variety of wireless technologies with different performance results

Most wireless technologies depend on local area networks (LAN) which include well-known technologies such as Bluetooth or WiFI. These are mostly used for short distances (between 50 and 100 meters) but are not sufficient to cover the monitoring needs of tunnel construction or, in general, the geotechnical market. Cellular technologies such as 3G and 4G can reach longer distances and allow for frequent data collection but their drawback is that mobile devices present high power consumption rates. 

Both LAN and cellular technologies are dependent on network signal and line of sight (LOS), which can be difficult to achieve in tunneling projects. In tunneling projects working with this kind of technology, and in order to reduce consumption, data is usually sent once a day or weekly, so it is not suitable for real-time or near-real-time data acquisition.

In contrast, LoRa (Long Range communication) is based on IoT (Internet of Things) technology so it can address the limitations on the range and power consumption of LAN and cellular technologies. With LoRa, data can be transmitted over long distances (up to 15km with line of sight and 4km in a tunnel) while only needing very little power. Batteries can last very long as devices only ‘wake up’ when they have to read and transmit data and go back into sleeping mode after.

“LoRa (Long-Range) has become the de facto technology for Internet of Things (IoT) networks worldwide because of its long-range, low-power capabilities” Maria Navarro, Senior Engineering Geologist. 

LoRa also offers the possibility of deploying private networks, enabling the user to have control and not being dependent on the large operators.

Wireless monitoring systems: architecture overview

Maria presents Worldsensing as an example of a wireless monitoring system based on LoRa to illustrate how these systems work. Worldsensing incorporates a data logger (wireless node) and a gateway to the sensors deployed on the site. Sensors are installed at their specific locations and wired to dataloggers which collect data from them. This data is broadcasted via radio signal to the gateway. The radio server and the Worldsensing data server are hosted in the gateway and it is possible to access to data readings, basic visualization, and network management via web. Additional software layers can be added by the client to manage alarms and to build further calculations by sending data from the gateway automatically to their platform via FTP or Modbus protocol or API Calls.

Application for tunnel construction monitoring 

Juan describes how a low-power wide-area network wireless system like Worldsensing is applied in tunnel construction monitoring. Using an example of a cut-and-cover tunneling project, he details the corresponding wireless solution for each key monitoring need. The example illustrates the total length for a metro station or a tunnel segment for a high-speed train and focuses on single point sensors and in-ground sensors.

“During tunneling, ground movements are expected and nearby buildings will respond to these movements. Wireless solutions can automate all the instrumentation frequently used to monitor this motion” Juan Pérez, Senior Geotechnical Engineer.

Wireless tiltmeters can be used to monitor the tilt of the buildings and the angular distortion related to damage criteria. This kind of 3D movement can be monitored with total stations. 

Movement across existing surface cracks or new cracks can be monitored with a potentiometer connected to a 1-channel analog node, such as the Worldsensing Piconode, to gather readings. A vibrating wire crackmeter connected to a vibrating wire node can also be used. 

In the surveillance of buildings’ response to tunnelling, it’s essential to measure settlement (and heave in some cases). Besides geodetic techniques like total stations, settlement can be monitored with liquid level settlement systems. Each settlement cell is a pressure sensor and can be read with an analog node. These systems maintain the accuracy of the readings even when installed in building basements and areas with no line of sight. 

Monitoring ground movements behind diaphragm walls and above tunnels is done with multipoint borehole extensometers (MPBX) measuring vertical deformation at various depths. Displacement sensors installed in the MPBX may be vibrating wires or potentiometers that can be read with analog or vibrating wire nodes.

In cut-and-cover tunneling projects it is crucial to monitor groundwater levels since the dewatering works conducted to allow the excavation can produce changes in the pore water pressure that can, in turn, generate settlements on the surface and affect the surrounding buildings. Water level is checked at different monitoring points using piezometers, usually vibrating wire, and the corresponding wireless node. The distance between monitoring points is typically more than 100 m so long-range radio is required to collect near real-time data.

The performance of some structural elements such as the force in ground anchors should also be monitored using load cells with a 1-channel analog node or a vibrating wire used to read the cell. To analyze settlement, vibrating wire strain gauges connected to vibrating wire nodes will monitor strain and stresses in structural members.

Battery-powered wireless nodes can automatically read chains of in-place inclinometers to measure lateral deformation in diaphragm walls or behind them. 

Juan also explains how wireless technologies contribute to monitor the key parameters inside the tunnel considering the different tunneling methods utilized (TBM, NATM, etc.). 

Future trends demand automation

The panelists discussed the current situation and future trends expected for the tunneling sector. According to Maria, tunnel tenders have begun to prescribe automation either by including a percentage of instrumentation to be automated or by indirectly prescribing higher sampling rates, which leads to an automated instrumentation system.

Automation gives valuable information about the evolution of soil and rock behavior and the global behavior of tunnel sections through all the stages. In the near future, monitoring will lean towards automated readings. In high-density operation environments, wireless monitoring is the best solution to acquire reliable data, because it provides high sampling rates that enable project teams to detect any defects using (near) real-time data. 

More data acquisition will lead to the development of more accurate predictive models and more knowledge about soil and structural behavior. Matching this data with other relevant information such as excavation parameters will provide an integral vision of how the excavation is performing. This could help reduce the response time to incidents and minimize risks for workers, urban tunnel environments and citizens.

In Maria’s view, although IoT has penetrated into the geotechnical monitoring market in the last decade, there is still a long way to go: “We could say that now all systems (manual, wiring and wireless) coexist in tunnel projects,” she says. “For complex scenarios like tunnel excavations, with several contractors and systems involved, a wireless automated monitoring solution is the most effective and least risky solution to ensure risk-management systems are reliable and accurate”. 

Data intelligence will be an important area of focus as it will give additional value to monitoring projects. Current models are fed with monitoring data but, according to Maria, adding data intelligence to the models is still about 5 years away.