The construction of new tunnels in urban areas require constant monitoring both from the ground and of the tunnel itself during construction and commissioning. Typical instruments include multi-point borehole extensometers, piezometers, pressure cells and in-place inclinometers. They are used to track overpressures, total pressures, convergence, settling and more. Historically, the instruments were installed and only monitored with manual readouts. This method is simple but time consuming and, more importantly, does not provide real-time data.
A more recent approach was to connect instruments to standalone loggers. The data loggers would collect data at regular intervals and workers go once per day to collect all the day’s data. This method provides more data than manual readings but is still a far cry from real-time. Adding internet or network connectivity to the loggers has proven to be a surprisingly difficult problem to solve.
The most obvious solution is to wire every instrument back to the entrance or the shaft where power and cellular or Wi-Fi networks are usually available. This often proves to be cost-prohibitive because as a tunnel progresses, longer cables are required. A way to alleviate this is to use digital instruments remotely controlled multiplexers so that only one bus cable is deployed rather than one cable per instrument.
More efficient solutions have begun emerging in recent years, often arising from requirements in the mining industry. Battery-powered radio-enabled loggers are becoming commonplace. This new crop of products allows engineers to have direct access to their data and make appropriate decisions as needed. The nodes collect data and retransmit it wirelessly to a strategically located gateway that has internet connectivity. There are a few considerations when selecting or designing a data acquisition system for this purpose such as radio frequency bands, radio range, network type and power requirements.
The exact frequency bands available will be decided mostly by local or national regulations. Most products will operate either in the 850-950 MHz band, such as Worldsensing’s Loadsensing products or Ackcio’s products. Other products, such as Senceive’s flatmesh line of products, operate in the 2.4 Ghz band. Typically, 900 MHz will propagate more easily in underground applications but both frequency bands work well. Additionally, overlap with other radios should be avoided to prevent interference. The 900 MHz band being a license-free band in North America, the exact frequency band should be carefully selected due to the fact that radios, control equipment or machinery could already be using it. Wi-Fi uses the 2.4 GHz band so, again, care should be taken to avoid interference between the data logging system and any Wi-Fi system that might operating in the tunnel.
Typical range for either frequency band will be between 100 and 500 m with a straight line of sight inside of the tunnel and less if line of sight cannot be guaranteed. In order to increase the range, some products, such as Worldsensing’s Loadsensing products use high-power LoRa radios that have a reasonably long range for most tunneling applications but only work by transmitting data from the node directly to the gateway (point to point). Other products, such as Senceive’s, Newtrax’s or Ackcio’s use a mesh network where data can be relayed by consecutive nodes along a tunnel. The main advantages are that it increases the effective range of the product by making the data hop from one to the next and that it allows for data transmission around bends and corners. The only major risk that is incurred by this method is that if one or several nodes are damaged, data will stop being relayed to the gateway. This can be mitigated by increasing the density of nodes so that each node has at least 2 neighbors it can transmit its data to. This gives a level of redundancy sufficient for most projects but can increase costs.
Power requirements are also a key consideration. On any of the aforementioned nodes, battery life is typically of several years but this varies dramatically with the type of instrument and sampling rate. Vibrating wire instruments require very little power whereas more modern digital instruments such as Measurand’s SAA drain much more power, leading to battery lives of 10 years and 2 months respectively.
Overall, new data acquisition and communications technologies have rendered the access to geotechnical data easier and more cost-effective than ever. Keeping in mind these few considerations will help engineers properly select and deploy the right technology for their project.
Vincent Le Borgne is R&D Manager with GKM Consultants in Sainte-Julie, Canada. He can be reached at firstname.lastname@example.org.