Tunnel monitoring with LoRa star technology: your queries answered
Tunneling is a risky and complex affair where it pays to proceed with caution, monitoring progress every step of the way. And in tunnel environments, LoRa star wireless networks can play an important role in delivering timely data easily and cost-effectively. To show how, Worldsensing hosted a free webinar that included presentations from engineering geologist Maria Navarro and Alexandre Cosentino, a project manager with GKM Consultants. This blog answers some of the questions that cropped up during the event.
Questions
Which is the highest sampling rate allowed by the system?
Does the laser node need to be pointing to targets to gather information, or can it point directly to the surface?
How can I know when a laser node needs maintenance?
Why did you choose a thermistor over a thermocouple or resistance temperature detector (RTD) to measure the concrete temperature ?
How much does a typical LoRa network cost?
Do you have a long-term monitoring example?
Is it possible to combine the tunnel boring machine (TBM) data with the data loggers?
How important is continue monitoring electric devices after the construction of a New Austrian Tunneling Method (NATM) and TBM tunnel? Should we continue the monitoring for a short time? Is monitoring needed if the tunnel has no problems during construction?
Do you have any suggestions for measuring tunnel convergence with a method similar to a convergence meter tape, but gathering the data remotely so as not to interrupt tunnel excavation?
What is the best practice for implementing long-term or continuous monitoring on a fragile or old tunnel?
Where should we put the sensors in a tunnel?
Is the technology able to detect existing services in proximity to the tunnel?
Do you recommend installing devices inside an enclosure to protect them in a tunnel environment?
Are you considering fiber optic cables?
How do you transfer data to surface data loggers?
Does the Worldsensing LoRa system automatically manage the 1% duty cycle limitation on the 868 Mhz band?
Is there any limitation for using Worldsensing in long-depth shafts?
What are the scope of technology?
What is the best way to address settlement concerns in a 200-meter rock NATM tunnel?
Which is the highest sampling rate allowed by the system?
Maria Navarro: Worldsensing is suitable for static measurements and the highest sampling rate that can be configured is once every 30 seconds. Bear in mind that if you need to sample every 30 seconds the maximum size of the network will vary depending on the type of Worldsensing architecture. You can find more information in our knowledge base.
Does the laser node need to be pointing to targets to gather information, or can it point directly to the surface?
Maria Navarro: You may need target foils under some installation conditions. Targets may help the laser node measure at the maximum specified accuracy even in favorable conditions. The use of targets for laser distance measurements helps improve the measuring conditions and reach a specified accuracy over long distances or when the lighting is not ideal.
We recommend opaque surfaces rather than reflectant targets and prisms used in surveying. You get signal parameters with each laser node reading that can help you assess the need for target foils.
How can I know when a laser node needs maintenance?
Maria Navarro: The laser distance node measures signal parameters besides distance and temperature. The gain and signal strength parameters can help you schedule cleaning tasks. You do not need to clean the surface for as long as the laser node continues transmitting measurements. You can find more information in our knowledge base.
Why did you choose a thermistor over a thermocouple or resistance temperature detector (RTD) to measure the concrete temperature ?
Alexandre Cosentino: Worldsensing nodes are not compatible with thermocouples. RTDs are typically more accurate than thermistors but are also much more expensive. The accuracy requirements and short-term nature of the measurements did not justify the added cost of using RTDs.
How much does a typical LoRa network cost?
Maria Navarro: The investment needed for a Worldsensing network installation depends on many factors. Prior to deploying it, information such as the following is required:
- The type and number of sensors that need to be automatized and their location.
- The size of the area where the sensors will be placed.
- The sampling rate required during every project phase.
- Project requirements which influence with Worldsensing connectivity management architecture to use (edge or cloud-based).
- Power and connectivity requirements.
Do you have a long-term monitoring example?
Alexandre Cosentino: At GKM, most of our geotechnical instrumentation relates to long-term monitoring projects. You can find project summaries on our website.
Is it possible to combine the tunnel boring machine (TBM) data with the data loggers?
Alexandre Cosentino: We’ve never had the chance to integrate the data into a TBM’s software, but we do not see any reason why it wouldn’t be possible. The Worldsensing gateway allows you to transfer data via Modbus, File Transfer Protocol and more, so it should be compatible for integration with a TBM data management system.
How important is continue monitoring electric devices after the construction of a New Austrian Tunneling Method (NATM) and TBM tunnel? Should we continue the monitoring for a short time? Is monitoring needed if the tunnel has no problems during construction?
Alexandre Cosentino: We can certainly reduce the sample rate of the geotechnical sensor once the construction is complete. But keeping the monitoring active gives you important information. The geotechnical information can be used for other purposes once the tunnel is in operation, such as failure prediction, maintenance planning and monitoring the safety of workers or the public.
Do you have any suggestions for measuring tunnel convergence with a method similar to a convergence meter tape, but gathering the data remotely so as not to interrupt tunnel excavation?
Alexandre Cosentino: The laser distance node from Worldsensing could measure the distance between two points remotely, without interrupting the construction or the operation of the tunnel.
What is the best practice for implementing long-term or continuous monitoring on a fragile or old tunnel?
Maria Navarro: Before installing a monitoring system in an old tunnel, you should carry out a study to determine the status of the structure and check for potential pathologies and their magnitude. The study should throw up information regarding the parameters that need to be monitored, such as crack extension and water infiltration.
The monitoring system should be able to provide information on the current status of the structure and its evolution, and also if you need to undertake structural actions such as repairing structural elements or reinforcing and monitoring them.
Where should we put the sensors in a tunnel?
Maria Navarro: You should have a monitoring plan that specifies the type and number of sensors to be installed, the accuracy and other specs of the readings, the threshold that will apply to each type of control, and a contingency plan. This plan will usually be created during the design phase previous to excavation. It should take into account the excavation method, type of terrain and, in urban areas, the surrounding structures and their expected movements.
Once the excavation is done, depending on the evolution of readings, you may decide to reinforce sections and install more sensors to ensure critical areas are well covered.
Is the technology able to detect existing services in proximity to the tunnel?
Maria Navarro: The detection is usually done in the design phase, but it can be done during construction if there is uncertainty of the depth of certain services, for example. The most extensive technique is through ground-penetrating radar, a terrestrial radar used to detect objects below the surface based on the transmission of electromagnetic ultra-wide waves.
Do you recommend installing devices inside an enclosure to protect them in a tunnel environment?
Maria Navarro: Worldsensing is a robust solution, rated IP67 and IP8, and its design is suitable for harsh environments, so it is not necessary to protect the equipment in special enclosures. If a project requires the equipment to be protected, it is important not to use metallic enclosures as they interfere with radio communication, decreasing the signal.
Are you considering fiber optic cables?
Maria Navarro: Using fiber optics sensors for monitoring construction is an alternative, but this is a completely different technology to the LoRa radio systems used by Worldsensing. There are advantages in using this monitored, distributed and continuous technique, but it implies covering the structure with long distances of cable.
Worldsensing is a wireless monitoring solution where the objective is to install loggers near sensors without long cables. The nodes communicate with the gateway through radio and centralize information on a data server.
How do you transfer data to surface data loggers?
Maria Navarro: There are many sensors that are typically used for tunnel construction monitoring in city environments. These can be divided into surface sensors and in-ground sensors. For surface sensors, such as crack meters, load cells, strain gauges, tiltmeters and others, data loggers are installed near the sensors to avoid long cable distances.
With in-ground sensors, such as piezometers, extensometers and in-place inclinometers, once the sensor has been installed inside a borehole the cable is wired to a data logger on the surface. Usually these sensors are installed in manholes that house the logger and protect the system. All the loggers that are installed on the surface communicate with gateways also on the surface.
Gateways can be connected to the internet, through a mobile connection or ethernet cable, or to a local area network. The system is the same for sensors inside a tunnel. Loggers are installed near the sensors and communicate with gateways in the tunnel. Usually, due to a lack of mobile network coverage, the connection is through ethernet.
Does the Worldsensing LoRa system automatically manage the 1% duty cycle limitation on the 868 Mhz band?
Maria Navarro: This is done automatically when configuring the node with the mobile app. The standard configuration ensures Worldsensing complies with radio regulations.
Is there any limitation for using Worldsensing in long depth shafts?
Maria Navarro: Usually Worldsensing is extensively used for surface monitoring and tunnel construction. Some installation good practices can be found in our knowledgebase, but as a rule of thumb it is important to always locate the gateway above nodes. As our edge devices and gateways use omnidirectional antennas, we always recommend installing them in vertical position.
For a long depth shaft, though, due to its vertical alignment we recommend installing node and gateway antenna perpendicular to the alignment of the shaft, as this could improve the radio coverage.
What are the scope of technology?
Maria Navarro: LoRa is a wireless technology that uses low radio frequencies that operate in an unlicensed radio spectrum. One of the advantages of this technology is that it’s long range and low power. LoRa is used in several markets, including smart cities and industrial automation. Worldsensing is based on LoRa star technology and is focused on the wireless automation of geotechnical and structural sensors.
We offer data loggers that are compatible with most of the sensors on the market and we automate readings over long distances consuming little power. These readings are received by gateways and can be visualized through web-based software and integrated into third-party systems.
What is the best way to address settlement concerns in a 200-meter rock NATM tunnel?
Maria Navarro: When monitoring a rock NATM tunnel during construction it is important to have a comprehensive view of the parameters involved in the excavation. Surveying methods are used in ground surface and structural monitoring, but some areas can be difficult to cover as these systems require line of sight.
In those cases, using settlement gauges or tiltmeters attached to beams could be an option to obtain the inclination and thus calculate settlement. Also, installing extensometers will provide important information regarding vertical movements in depth. Combining these readings with the information provided by incremental extensometers and the convergence control of the excavation section will tell you about vertical movements induced by the excavation.
