Transportation Defined: Time Domain Reflectometry

Transportation Defined: Time Domain Reflectometry

April 9, 2013

The TDR cable can be seen above alongside the inclinometer casing.

You might remember that we blogged about inclinometers back in February – they’re the instruments used to measure slope inclination or movement at specific locations.

Well, today we have another technology that we want to tell you about called Time Domain Reflectometry.

Like inclinometers, TDR can help our geotechnical experts and geologists measure movement of a slope or a landslide. However, TDR technology looks and operates a little differently from inclinometers.

How does it work and what is it used for?

TDR technology consists of a coaxial cable (similar to the one that's coming out of your television set – but more heavy duty) that’s embedded or grouted within a vertically drilled hole (it might help if you can recall our milkshake straw analogy from the inclinometer post).

Alternately, a TDR cable can be attached to a grouted inclinometer casing (see photo above). That coaxial cable that's in the ground is then connected to a cable tester that sits outside the drilled hole.

You should know that the cable is installed in a precise way so that the zones of ground movement (shear planes) in the landslide mass are intersected.

Once the cable is in the ground, it will be able to detect ground movement that’s happening within the vicinity of the cable. If there is movement in the landslide mass, the cable becomes kinked, stretched or even sheared.

This damage creates a change in the impedance of the cable, which can be detected by the cable tester, says ADOT Engineering Geologist Nick Priznar.

“The cable tester sends an electronic voltage pulse that is reflected like radar from a damaged location in the coaxial cable,” he said. “The returned signal will show where changes have occurred along the length of the cable and can be compared to the signal obtained originally when the cable was undamaged. This makes it possible to determine the depth, progression and general magnitude of movement of the landslide zone that was intersected.”

TDR technology is often used in conjunction with inclinometers. That’s because the lifespan of the inclinometer casing is limited – slide plane movement can shear or distort the casing, eliminating its stable base or reducing its diameter to a point where the inclinometer probe cannot pass through the casing.

When, or if, that happens, the TDR system can be used to continue to monitor the movement/displacement. That’s because the TDR cable is usually more resilient to displacement than the inclinometer casing.

However the two systems are not equivalent, according to Priznar. While the inclinometer data provides actual measures of deflections the TDR cable tester can only provide depth and relative magnitude of displacement.

How are TDRs being used on US 89?

At this time ADOT engineers are in the process of developing the best strategy for implementing TDR technology for US 89.

In anticipation of utilizing this technology, TDR cables are being be secured along the outside diameter of select slope inclinometer guide casings (again, see photo above).

Transportation Defined is a series of explanatory blog posts designed to define the things you see on your everyday commute. Let us know if there's something you'd like to see explained ... leave a comment here on the blog or over on our Facebook page!