TIME DOMAIN REFLECTOMETRY
FOR INNOVATIVE SOIL CHARACTERIZATION

Principle

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 hole development.

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.

Freeze-thaw Monitoring

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

LRFD Calibration

LRFD has been used increasingly and has become mandatory for all bridge projects funded by the Federal Highway Administration (FHWA). Compared with the allowable-stress design (ASD) method, LRFD can achieve a compatible reliability between the bridge superstructure and substructure. The uncertainty of load and resistance are quantified separately and are reasonably incorporated into the design process.

An example histogram of collected bias of drilled shaft resistance and fitted statistical distributions

Thermo-TDR Sensor

Knowledge of the thermal properties of soil and soil thermal conductivity are fundamental to understanding the heat transfer process in soil, which is critical for many applications such as active geothermal structures. Thermal properties of soils include thermal conductivity, diffusivity, and heat capacity properties. Thermal conductivity is the most frequently used and is affected by several factors, such as moisture content, degree of saturation, dry density, and mineral components.

A thermo-TDR probe can function both as a regular Time Domain Reflectometry (TDR) probe, for measuring moisture and density, and a dual-heat probe, for measuring thermal conductivity, thermal diffusivity, and volumetric heat capacity. It is, therefore, of great importance to the study of soil geothermal behavior, which is greatly affected by the soil thermal properties, moisture content, and dry density.

Photo of thermo-TDR probe

Active Research Grants

· Calibration of LRFD Geotechnical Axial Resistance Factors for California, PI, $222,606, California Department of Transportation, Aug. 22, 2014-Jun. 29, 2016.

· Use of Geothermal Energy for De-icing Approach Pavement Slabs and Bridge Decks, PI, $193,063, Texas Department of Transportation, April 2015-August 2016.

· CyperSEES: Type 2: Integrative Sesing and Prediction of Urban Water for Sustainable Cities (iSPUW)), Co-PI, $1,196,295, National Science Foundation, Oct. 1, 2014 - Sept. 30, 2018.

· Pilot Implementation Using Geofoam for Repair of Bridge Approach Slabs and Adjoining Roadway, Co-PI, $152,702, Texas Department of Transportation, April 2015-August 2017.

Completed Research Grants

· Geotechnical Services - Data Collection, Co-PI, $23,661, Texas Department of Transportation, August 2012-August 2013, Completed.

· Laboratory Study of Thermal Properties of Soil for Sustainable Energy Applications, PI, $10,000, UTA REP Fund, June 2013-May 2014

· Pilot Implementation Project Using Geofoam, Co-PI, $59,821, Texas Department of Transportation

· Structural Capabilities Of No-Dig Manhole Rehabilitation, Co-PI, $200,000, Water Environment Research Foundation

· Structure Health Monitoring of I-10 Twin Span Bridge, Co-Principal Investigator, funded by Federal Highway Administration, Nov. 11, 2007-Dec. 31, 2010, 500,000$.

· Field Instrumentation and Testing to Study Setup Phenomenon of Piles Driven into Louisiana Clayey Soils, Co-Principal Investigator, Fund from Louisiana Department of Transportation and Development, Sept., 2010-Aug., 2014, 388,953$.

· Calibration of Resistance Factors for Drilled Shafts for the New FHWA Design Method, Co-Principal Investigator, Fund from Louisiana Department of Transportation and Development, Dec., 2010-Dec. 2011, 75,000$.