IDENTIFICATION OF PREEXISTING CRACKS ON SOIL FRACTURE SURFACES USING DYE

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. The first set of samples examined was formed by compacting aggregate beds to selected macroporosities to obtain samples with a statistically homogeneous macrostructure. Natural soil aggregates, collected from zero and conventional traffic treatments, were also tested. Results obtained illustrate that compaction reduces both structural porosity and fragmentation. An increase in the impact energy resulted in a decrease in the proportion of the fracture surface that was stained, showing that more 'new' surface area was being produced.