Methods to assess the health of earthen dams and levees are needed such that maintenance and repair actions can be planned prior to failure and the potential resulting damage. Areas of expertise with applicable technologies include resources in the Applied Geophysics area, the Center for Nondestructive Evaluation (CNDE), and Soil Science. A program was initiated at CNDE in March 2010 with efforts focused on determination of relevant equipment and initiation of purchase orders. Two relevant measurement approaches were selected: ground conductivity meters (GCM) and ground penetrating radar (GPR).
Ground conductivity of the soil (measured in millisiemens/meter), is effected by several factors: moisture of the soil, amount of organic matter, porosity of the soil (related to soil type), clay content of the soil, the amount of electrolytes (salts) in the soil, and soil temperature. The most important of these factors is soil moisture, while other factors such as organic matter have a lesser effect on conductivity. Soil with an increased amount of moisture in it will have a greater conductivity, which in the case of dam inspection could be an indicator of possible potential failure issues which may require further inspection. The factor with the least effect on conductivity appears to be temperature. Due to the low proximity of the meter to the ground, the diurnal temperature variation is simply accounted for by recalibrating the instrument on a routine if not hourly basis throughout the testing day.
The use of a GPR for inspection is similar to GCM in that there are factors within the soil that reduce or inhibit the optimal use of the equipment. An example would be the conductivity of the soil; higher conductive soils tend to reduce GPR’s effectiveness. This may be an issue since plant vegetation tends to increase the moisture content of the inspection area, thereby increasing the conductivity and thereby increasing attenuation of the signal. Since conductivity is a direct relationship to the salt content of the soil, there may be instances where a more humid climate may provide a better environment for the use of the GPR than an arid climate. Drier climates tend to have less leaching of potassium and sodium salts and the less soluble carbonates of calcium and magnesium within a soil which create a higher concentration near the surface area, creating a higher attenuation region.
Like GCM, GPR sensitivity decreases with a decrease in the frequency of the antenna, but depth of penetration increases. This does not render the equipment useless for deeper studies, but does make the resolution of subsurface features more difficult. The suggested maximum depth of penetration for the GPR is most greatly affected by the conductivity of the soil and soil type. There is in general a decrease in attenuation of drier soils over the wetter soils. Frequencies greater than 500 MHz can provide good resolution of shallow features.
Additional uses for both GPR and GCM are continually being sought, such as structural inspection of bridges and buried utilities near new or proposed construction.
Improving levee and dam inspection would have immediate payoffs in terms of saving human lives and reducing the cost of flood damage. Pinpointing defects in levees and dams allows for timely repairs before damage occurs.
The same technology would have many applications beyond levees and dams. It could be used, for instance, in bridge and road construction and maintenance, helping to improve the quality of those areas and lower costs.
In the long term, this research might also reveal other inspection technologies, such as terahertz ray, that have never before been applied to levee and dam inspection.
In 2011, CNDE studied the feasibility of using GPR for levee and dam inspection. The Center purchased state-of-the-art equipment, completed a preliminary literature survey, and assembled a list of potential dams and levees in Story, Boone and Polk counties to include in the study. Using GPR equipment, researchers have gathered baseline data from these sites. When this process is done, the data will then be collected and interpreted.
After the feasibility study is complete, the Center will have more specific recommendations for future. It is anticipated that the research would be extended to other locations. Another recommendation may be to build a test levee with known defects to better understand how various NDE technologies can characterize them. Pending further funding, this in-depth study could be started in short order.
In the long term, CNDE would work with federal, state and local governments to develop a comprehensive, collaborative study of levee and dam inspection and to create guidelines and even regulations. The Center would also work with ISU Extension, other IPRT centers, and InTrans to ensure a broad solution to the problem. It would also investigate how technology and techniques developed for this application could be transferred for use in other areas, such as road and bridge building and maintenance.