A significant strength-controlling property of aggregated soils is structural porosity such as cracks. A technique is presented to identify structural porosity on fracture surfaces using methylene blue dye. Soil aggregates were immersed in the stain, returned to their original water content, and equilibrated. They were then fractured under a dynamic load using an impact rig. The fragments were collected and sieved to selected size ranges. The proportion of the fragment surface that was stained from each size range was measured using a colour image analyser adapted for this experiment. Results obtained illustrate that compaction reduces both structural porosity and fragmentation.