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| Titel |
Evaluation of clay content in soils for pavement engineering applications using GPR |
| VerfasserIn |
Fabio Tosti, Claudio Patriarca, Andrea Benedetto, Evert C. Slob, Sébastien Lambot |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250083209
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| Zusammenfassung |
| Clay content significantly influences the mechanical behavior of soils, thereby playing an
important role in many fields of applications such as civil engineering, geology and
agriculture. In the area of pavement engineering, clay content in structural bearing courses of
pavement frequently causes damages and defects, such as transversal and longitudinal cracks,
or other faults. The main consequence is a lowering of both the road safety and operability,
with the number of expected accidents increasing.
In this study, ground-penetrating radar (GPR) laboratory tests were carried out to predict
the clay amount in pavement structural layers under different clay and moisture conditions.
GPR data processing is performed using two different methods. The first method is based
on the Fresnel theory and focuses on the Rayleigh scattering of the radar waves.
The approach is based on a different scattering of the various components of the
frequency spectrum, mostly depending on both the soil texture and variation in soil
moisture content. For the application of this method, we used a pulse radar with
ground-coupled, 500 MHz centre-frequency antennas in a common offset, bistatic
configuration. The transmitter and receiver were linked by optic fiber electronic
modules.
The second method is based on full-waveform inversion of the ultra wideband radar data.
In particular, a specific radar-antenna electromagnetic model is used to filter out antenna
effects and antenna-medium interactions from the raw radar data and retrieve the response of
the soil only, expressed in terms of a layered medium Green’s function. To estimate
the medium geometrical and electrical values, an optimization inverse problem is
formulated.
For the application of that second method, we used a vector network analyzer (VNA) as
continuous-wave stepped-frequency radar system to acquire data in the 500-3000 MHz
frequency range. A doubled-ridged broadband horn antenna operating in far-field conditions
was used as transmitter and receiver, and was connected to the radar using a high-quality
coaxial cable.
Typical road materials for subgrade and sub-base courses were used. In particular, three
types of soils classified, respectively, as A1,A2,A3 by AASHTO were used and adequately
compacted in electrically and hydraulically isolated boxes. A copper sheet was laid at the
bottom of the experimental boxes to control the bottom boundary conditions in the
electromagnetic model. Basically, two significant cases were considered for each soil type,
taking into account the 0% and the 25% by weight of bentonite clay, respectively. Water was
gradually added and GPR measurements were carried out for all moisture steps until the
maximum saturation level was reached.
Concerning the Rayleigh scattering method, analyses show a high consistency of the
results with respect to our expectations. A negative correlation between the shift of the
frequency spectrum peaks and the clay amount was demonstrated, by virtue of its strong
hygroscopic properties. Similarly, the full-waveform inversion technique allowed to measure
reliable electric parameters. Generally, different responses (e.g. electric conductivity and
permittivity) of the 0% clay-member cases compared to those of the analogous clayey soil
samples highlight the large potentiality of both methods for the detection of clay. |
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