TIME DOMAIN REFLECTOMETRY
Time Domain Reflectometry (TDR) sensors utilize the propagation of electromagnetic wave in soils to measure its dielectric and electrical properties. It is has been widely used in soil science, hydrology, and geotechnical engineering to measure moisture content (ASTM D6565) based on the fact that water has much larger dielectric constant (81) than that of the air (1) or soil solids (around 3 to 5). It works by generating a small-magnitude electromagnetic field excitation and measures the material dielectric response. As the advanced TDR electronics are now capable of generating fast rising pulse of a few picoseconds, TDR signals contain broad frequency band information on material dielectric responses from a few megahertz to gigahertz. The information commonly used from a TDR signal, however, is the reflection points, which are related to the speed of electromagnetic wave in the soil and are subsequently used for determining the apparent dielectric constant Ka; and the long term signal level, which is related to the energy attenuation and subsequently used to determine the electrical conductivity ECb. This is due to the fact that both quantities can be easily obtained from a TDR waveform. The dielectric constant and electrical conductivity are strongly related to the physical and mechanical properties of materials.
An example TDR waveform and interpretation methods
Bridge Scour Monitoring
Scour is a major threat to bridge safety. Bridge failures cost millions of dollars
each year as the result of not only the direct costs of replacing and restoring these
bridges but also the indirect costs related to the disruption of transportation network.
Bridge scour is the removal of sediment such as sand and rocks from around bridge abutments
or piers due to the erosion forces created by swiftly moving water. The scouring process is
a dynamic process. It typically reaches its maximum depth at the peak of a flood; the scour
hole is backfilled as sediments deposit during the flood recession. Therefore, post flood
survey, such as by sonar or radar, does not provide the most critical information on scour
evolution. Real-time monitoring of bridge scour is essential to provide timely surveillance
of scour condition. The real-time data can also be used to study the mechanisms of the scour
In this research, a scour-TDR sensor was developed and tested both in the laboratory and field.
Simulated bridge scour test in a sand-water column. The scour-TDR sensor consists of a coated flat-strip supported on a u-shape fiber glass channel.
Installation of the scour-TDR sensors on the bridge deck above the Great Miami River in Ohio
A geotechnical boring rig was used to install the scour-TDR sensor on the bridge deck at great Miami Bridge in Ohio. The field TDR bridge scour monitoring system was installed in September, 2009. Since then, the system has continuously served with encouraging performance. In two months after the installation, a maximum scour depth of 0.5 m was recorded among five monitored locations.
Specially designed tube-TDR and strip-TDR sensors can be used to measure the free-thaw process in specimens prepared in the lab and frost-depth in the field.
A PVC tube TDR sensor for monitoring freeze-thaw process in soil specimen prepared in miniature compactor. Two copper metal strips were attached to the tube inner side to act as the TDR waveguide.
Dielectric constant and electrical conductivity monitored during freezing (red) and thawing (black) process in cylinder soil specimens
A flexible strip-TDR sensor that can measure moisture profile