Application of X-ray Computed Tomography to Cone Penetration Tests in Sands

X-ray computed tomography (x-ray CT) was applied to visualise the local deformation patterns caused by miniature cone penetration tests (CPTs) in a calibration chamber filled in with a fine grained quartzic sand and pressurized with vacuum. A reference test was conducted during which the calibration chamber was scanned at several stages of cone penetration. The measured X-ray attenuations were translated into local sand density maps with a resolution of 0.25 x 0.25 x 1mm by using the calibration established by scanning layered sand samples of different densities. Test results showed that a dilating zone having the shape of a sphere was formed ahead of the cone tip before being penetrated by the cone. Friction along the cone shaft did not generate visible density changes. The final dilating zone consisted of several coaxial cylinders of different density, all having a rounded tip. The absence of contractance at proximity to the cone was attributed to the behaviour of the Baskarp sand. When sheared in its dense state under high confining pressure, this fine sand dilates. Also, its particle shape does not promote crushing and re-arrangement of crushed grains in a denser packing. 3 other scanner tests were performed. Decreasing the confining pressure resulted in, despite a decrease in the tip resistance, no large differences in the size of the area affected by density changes and magnitude of density changes. By lowering the sand density, the size of the dilating area was found to be significantly smaller and the loosening much less. The use of rigid boundaries only resulted in a small decrease in the dilating area despite causing a large increase in cone resistance. Using X-ray CT, it was possible to quantify the local loosening generated by CPT in sands under axisymmetrical stress conditions. Deformation patterns different from those observed in plane strain experiments were observed.