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Contractors Use RFID Sensors to Measure Strain in Seattle Rail Tunnels

The sensors, provided by Phase IV, are being embedded in prefabricated concrete sections of the tunnels' lining, and will be used to verify the structure's integrity.
By Claire Swedberg

First, the tags are being read before they are incorporated into a section of liner, to capture a base strain measurement in open air. The tag's ID and initial strain measurement are stored in the Phase IV software. Two sensors are then welded to pieces of steel that are added to the liner's existing rebar cage, which is cast into the concrete. The sensors are interrogated again after the concrete is poured and the tunnel segment is removed from the forms.

Construction workers take another reading once the tunnel segments are delivered onsite and the concrete has been fully cured. Six curved liner sections are assembled to a ring encircling the tunnel, with thousands of such rings making up each of the two tunnels. The sensor, positioned about 1 inch beneath the surface of the concrete, can be read again after that section of liner is installed in the tunnel, and finally, after the passages are cut, in order to determine whether there are significant changes in the strain measurements. If such changes are discovered, Braun says, additional engineering will be required to ensure that there is sufficient support for the lining around the passages.

To read the tags, Phase IV designed a kit that includes a backpack with a ThingMagic RFID reader and an antenna with a yellow telescoping handle that ensures users can reach the tags embedded in difficult-to-access liner segments.
To read the tags, Phase IV designed a backpack with a ThingMagic RFID reader built in and a telescoping wand antenna that ensures users can reach the tags, which may be in a difficult-to-access location, such as at the top of the tunnel ceiling. After the lining panels are cured, Braun explains, a mark is painted at the tag's location to alert those equipped with RFID readers to where they need to point their reader antennas.

Phase IV provided software for capturing, interpreting and storing the sensor measurements. That software resides as a standalone solution on a PC in a backpack that is kept at the construction site. The company also provided firmware that is loaded onto the reader so that the strain results can be displayed on the device's screen, as well as forwarded to the back-end software via a USB connection.

Once construction is complete and Sound Transit commences rail operation, Dalgleish says, the transit company's own staff will be able to use the tags to monitor the tunnel's structural soundness, based on the amount of strain to individual liner sections.

RFID sensors are not built into every section of concrete liner, Braun notes, but rather in 38 segment rings where the passages are being cut, with two sensors embedded in three of the six sections within each of those rings (totaling about 200 sensors). The six sensors in each ring provide some redundancy, he explains, in case any of the sensors are damaged during the installation process, and also serve as a means to check the sensors' accuracy.

The liner sections are still in the process of being molded and then installed, Braun reports, and no passages have yet been cut. The tags can be read well through the concrete, he adds, as long as the reader is located about a foot from the surface of the panel where the tag was embedded. Workers are painting a mark above each embedded tag.

Phase IV Engineering will display the SensTag strain sensor at this year's RFID Journal LIVE! conference and exhibition, being held next week in San Diego, Calif. The company is also an RFID Journal Awards finalist, in the category of Best New Product (see Finalists Unveiled for Ninth Annual RFID Journal Awards).

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