Stone columns provide valuable ground improvement in soft soils in numerous projects around the world. They increase the stiffness of the subsoil and reduce time needed for consolidation. This last aspect is due to the fact that the main drainage direction switches from being vertical to being radial, significantly reducing the length of the drainage path. However, driving the installation mandrel into the ground causes changes in the structure of the subsoil, usually referred to as smear zones, which decrease the drainage performance of granular inclusions. The implementation of a needle to measure the impedance (electrical resistivity) in-flight in the geotechnical centrifuge at ETH Zürich enables an insight to be made into the pore size distribution with depth close to a stone column constructed in-flight. © 2010 Taylor & Francis Group, London.