Tunnel Construction Monitoring: How IoT Wireless Systems Diminish Risks in Construction Projects

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A recent survey conducted by software company TrackVia revealed that 47% of construction managers still use manual methods to collect important project information. The construction sector is a slow adopter of new technologies: although automated data, often enabled through wireless monitoring systems, can make operations more efficient, save costs and reduce risks, the industry remains hesitant to implement novel approaches.

Tunneling projects are one of the most high-risk geotechnical construction sites. Being able to monitor the stability of surrounding structures and underground excavations in real-time is essential to keeping risk-potential low. Operators who rely on manual readings are working with out-of-date information and therefore make partly blind decisions. This poses major risks as potential incidents cannot be detected easily. Despite great advances in instrumentation and monitoring, loss of life of workers and the public due to incidents during tunnel construction projects is still a great threat. The level of risk can be significantly reduced with advanced Internet of Things (IoT) technology.

IoT technology for real-time monitoring

One approach to replace manual readings is to deploy a wireless monitoring system. While “wireless” is not a new concept, working with a system which runs on IoT technology is. IoT monitoring doesn’t rely on 3G or Wi-Fi but on low-power wide area (LPWA) networks such as Sigfox and LoRa, which increases data accuracy and reliability. With IoT wireless monitoring systems, sensors, such as multi-point borehole extensometers, used in a construction project can be connected to wireless data nodes, which transmit the sensor data via gateways to on-site servers. This allows operators to track operations in real time.

Metro projects in North America using IoT wireless monitoring

In North America, some of the biggest metro extension projects such as the Purple Line Rail Link in Washington D.C., the Purple Line Extension in Los Angeles and the Toronto Subway Project, are using IoT wireless monitoring in order to ensure their risk-management systems are reliable and accurate. This kind of monitoring enables operators in these cities to carry out remote, real-time monitoring of in-ground sensors such as piezometers, extensometers, and inclinometers. They are even able to gather data from sensors placed across metallic manholes because the sensors are installed in boreholes drilled into the pavement. Operators can gather data on the stability of the tunnels in a non-intrusive way, through data units that are connected to the sensors inside the manholes, installed at the mouth of the boreholes. These wireless data units are also able to consistently transmit data without requiring traffic cuts to collect readings sporadically. Sporadic readings collected manually are not enough to understand the behavior of a tunnel or excavation station – both in terms of ringing the alarm should something go wrong in the moment, and continuously monitoring the situation to pick up on trends and prepare for any future incidents.

These metro construction projects use Loadsensing to remotely monitor the tunnel excavation, because IoT wireless monitoring systems like this offer a variety of benefits, such as:

Long-range, low-power geotechnical monitoring

Systems running on IoT LPWA networks such as Sigfox and LoRa offer increased data accuracy and reliability. They enable operators to remotely collect and transmit data over long distances (depending on the use case over up to nine miles) without needing much power. The systems are usually battery-powered and can last up to eight years, making them easy to maintain.

Cables, which are still often used in tunnel construction projects, as opposed to wireless approaches, are vulnerable to physical damage, surrounding structures are affected by soil movements induced by tunneling, and design assumptions need to be verified to check that performance is as predicted. Extend cable and cable protection can be used to centralize the data acquisition in traditional data-loggers, but they require much more time for installation, are expensive and are sometimes perceived to be an eyesore. It is also difficult to place cables in hard-to-reach areas, and they are vulnerable to damage over time.

Increased data availability

Within a tunnel, the most interesting data corresponds to the first few days after the excavation. Data gathered manually during this period contains a lot of hidden, or uninterpretable, information, since it cannot follow events exactly as they happened. When systems are based on IoT technology and run on LoRa, this technology enables 157 dB maximum link budget (151 dB in Europe), a metric used to define the quality of the data transmission. Paired with advanced data nodes, which in some cases are highly sensitive (down to -137 dBm) and have a transmission of +20dBm (+14 dBm in Europe), construction projects can ensure efficient communications in conditions where other data reading systems or approaches fail.

This table shows a comparison of some of the wireless options available in the geotechnical monitoring market today:

Wireless Frequency Network type Power Data rate Sensitivity Range**
SmartMesh 2.4 GHz Mesh Medium 250 kbps -95 dBm 40-150 m
XBee

sub 1 GHz

902-928 MHz

868 MHz Europe

Star/

Mesh

Medium 10 kbps -110 dBm 150-400 m
LoRa used by Loadsensing 902-928 MHz

 

868 MHz Europe

Star Low 5469 bps SF7

 

537 bps SF11

-137 dBm

node

 

-141 dBm

gateway

2-15 km

Table 1. Performance of available wireless geotechnical monitoring systems. **Range considered for typical installation on site and with standard antenna.

Due to its long-range, low-power and sensitivity levels, systems like Loadsensing are the most suitable for tunneling projects. The constant aggregation of data from sensors located within and outside the construction area allows for a long-term picture of events to emerge, enabling operators to plan ahead with a more evidence-based approach. Sampling rates are high (with more data), so, even though tunneling is often a slow process, operators can see exactly what is happening over time, while also being warned of any deviations in ground stability, pore water pressure etc. In a station, it is very useful to see the evolution of all the parameters at each phase of excavation. It is then possible to implement remedial actions if something is wrong – for example excessive horizontal displacement, or overload of the ground anchors. This is particularly useful for the control of shotcrete curing, where initial readings can be collected, and pressure cells depressurized accordingly.

Minimal maintenance

Having to regularly replace or maintain a monitoring system requires putting workers at greater risk, and decreases reliability and overall safety as there is a constant possibility that the system might go down suddenly and with it all risk-management operations too. This threatens lives.

Systems that employ low-power, wireless hardware have high durability and adaptability and are thus a good option because they require much less upkeep and are adjusted to specific environmental conditions, making their fallibility less significant. This applies to the sensors as much as the network and the software. Long-range, low-power wireless technologies, such as LoRaWan, used by IoT networks worldwide, are the most reliable option. Adaptability in the wireless data units, with operating frequency bands that are adjustable to each territory requirement is also key. It is additionally useful for wireless monitoring solutions to be compatible with multiple types of sensors. This means that no hardware needs to be replaced gratuitously, again putting lives at risk and stalling operations, even if it is produced by a different company to the one providing the wireless monitoring network and software.

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Typically, in tunneling projects, a mesh network is installed, which tends to be more vulnerable due to the existence of critical paths where the density of nodes is not enough to balance the network. In other words, if some of the nodes are damaged or unavailable, other parts of the network are majorly affected. Star topology networks are a good alternative here because the loss of one of the nodes does not have an impact on the rest of the network, making the whole system more resilient and reliable.

Monitoring of surrounding tunnel areas

Monitoring the stability of the surrounding ground structures and buildings during a tunnel excavation with tiltmeters, settlement systems or crack meters is a crucial risk-management area in geotechnical construction projects. One of the major concerns for contractors and designers in a tunneling project is the potential nuisance caused to local residents by noise or ground movements. Wireless nodes can be a very useful measuring tool for this when placed on buildings or boreholes surrounding the excavation area. By using these nodes, it is possible to collect readings from tiltmeters, liquid settlement gauges and other types of measurement tools installed in the basements of the buildings, or via nodes placed at the mouth of the borehole in a manhole. These readings can then be transmitted to a gateway installed on top of the building, with no need for constant upkeep or manual readings. Importantly, the evaluation of the data collected and transmitted by geotechnical sensors through these wireless nodes not only increases the safety level of the project, but also reduces the nuisance often caused to locals by soil movements induced by tunneling.

With wireless monitoring systems, operators can monitor not only the direct excavation area but also the surroundings, giving them access to real-time data that can alert them when something goes wrong, allowing them to sound the alarm immediately and halt operations in order to prevent, for instance, a nearby building from becoming unstable and therefore unsafe.

Flexibility and durability

To withstand the harsh working conditions of tunneling construction sites, devices used to measure the situation onsite need to be highly durable. Wireless data nodes are specifically designed to be installed in tough environments. In comparison, cables tend to be a lot more vulnerable to damage. Durability is key to ensuring that minimal maintenance is required: operators need to make sure the units they want to deploy are IP67 certified and tested from -40 to +176 F, making them highly robust and protected in any scenario. Hardware durability is essential for effective risk-management because data needs to be constantly gathered to ensure that operators know exactly what is happening in the tunnel at any given moment. Any interruption to this data-aggregation through equipment failure therefore increases risks greatly.

Flexibility is also important. Sensors can give operators flexibility in their monitoring because once installed, they facilitate the easy deployment of data-loggers. Wireless nodes are very moveable, meaning that they can be easily removed if another kind of construction activity needs to be carried out in the same location, for example waterproofing or installing the final tunnel lining. Flexibility and durability of wireless monitoring systems are thus key to ensuring that data aggregation, crucial to risk-management, can be carried out constantly, while also cutting operational costs, as equipment rarely needs to be maintained or replaced.

The future of tunneling?

In the past, the loss of human life was not an uncommon part of tunnel construction projects; today, this is no longer acceptable, and this is largely down to technological advances based on the Internet of Things that have made risk-management easier and much more accurate. The next frontier of risk-management in instrumentation and monitoring for tunneling projects is probably real-time data assimilation into a computational model of the project. This will enhance the quality of the information available and allow operators to make educated, evidence-based decisions while tunneling.

In the future, we will see operations becoming digitized through operational intelligence (OI). When a wireless monitoring system is connected to advanced IoT software, such as an OI solution, operators can integrate sensor data, other data sources, assets, teams as well as existing systems into one overarching system, in order to make centralized, data-informed decisions. Through constantly receiving updated project insights, operators can access information and KPIs allowing them to predict events and “try out” solutions in the software, before implementing them in real-life. As such, digitizing construction projects with smart sensors not only enables operators to track how operations are going in real time, but it also allows companies to predict what is needed next by displaying real, planned and projected production needs. Wireless monitoring technologies will thus continue to give tunneling construction operators the power to protect not only their workforce and local residents, but also their critical assets too.

About the author

Juan Pérez is a Geotechnical Engineer and Product Owner of the globally recognized wireless monitoring system, Loadsensing. Juan has a background in geotechnical and structural instrumentation and has been leading the product development of wireless monitoring at IoT pioneer Worldsensing since 2013.

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