This contribution shows how the unsaturated properties of a uniform sandy material are used to create stable vertical surfaces when building dyke models for testing in a geotechnical drum centrifuge. The model is constructed inside a 1.0 m (length) × 0.54 m (depth) × 0.36 m (width) box with a semi-circular base, which must then be tilted through 90° to place the model into the channel of the centrifuge drum. A construction procedure has been developed to create unsaturated conditions in the sand so that the suctions are large enough to ensure that the 0.5 m high vertical walls remain stable for a short period of time until the centrifuge test can begin. This has been backed up by 3D numerical finite element calculations of the performance of the model in an unsaturated state. © 2013 Taylor & Francis Group, London.