Abstract:
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X-ray computed tomography (CT) provides nondestructive three-dimensional visualization and
characterization, creating images that map the variation of X-ray attenuation within objects,
which relates closely to bulk density.
In this work it was considered interesting to use this technique as a tool for geotechnical
inspection sample. CT scans of soil samples of Castelló d'Empúries (close to Figueres, Spain)
were quantitatively evaluated to determine wet bulk density.
The CT scans performed over Castello soil samples were obtained as part of a larger
experimental program related to the Soft Soil Project (Arroyo et al., 2012). This project had as
a main objective the evaluation of sample disturbance produced by various sampling methods.
A state-of-art of X-ray computed tomography applied to geosciences was perfomed. It was
found that X-ray attenuation depends on the bulk density of the sample, its porosity, water
content and chemical composition in a complex way. Therefore, experimental calibration was
necessary to unambiguosly determine the wet bulk density of the Castello soil from the CAT
values. A complementary experimental campaign using several reconstituted soils was
performed for that purpose.
Once the calibration was established it was applied to the CT scans of the Castello soil samples
to obtain detailed profiles of wet bulk density for the tube samples and specific values for the
lab test specimens. For the late wet bulk density values had been previously obtained from lab
procedures. It was the possible to check the quality of the results obtained from CT scan
analysis. Very good agreement was attained between both measurements. This means that
the calibration and the post-process of the CT scan data were successful.
Specimen quality classification results of the Soft Soil Project were reviewed, since the initial
results were unreliable. The reviewing process included the sample disturbance criterion
applied (Lunne et al. 1997) and all the laboratory tests results. The newly obtained CAT density
was useful to identify some mistakes in the original test interpretation. After correcting then,
the results were re-analyzed. A new quality classification for the specimens was obtained and it was checked for consistency against the results of the laboratory tests. This study confirms
the applicability of the Lunne criteria of sample quality well away from the geotechnical setting
for which they were originally proposed.
The CT scan images and the sample quality assessment results indicate that, for the Castello
soil, there is no advantage in using Osterberg piston-type samplers instead of simpler pushed
Shelby tubes. |