Microscopic Changes in Soil Structure During Compression

Compressed specimens of soil were examined microscopically to determine the nature of the changes occurring in moist soil during the application of pressure. A small compression chamber was filled loosely with moist soil aggregates, and the pressure was applied gradually in a controlled manner. The compressed material was removed at various stages and dried. Polished sections were then prepared. The results have been preserved in a series of photomicrographs and show that the volume changes are attributable in large part to plastic deformation of the aggregates. Deformation occurred readily at the lower plastic limit, causing a progressive closing of the interaggregate spaces as the pressure was increased. At water contents below this limit deformation appeared to be localized in the areas of contact between aggregates and consisted mainly of flattening of the aggregates against one another. The incomplete closing of the interaggregate spaces at low water contents was attributed to the increased shearing strength of the material. The theory has been advanced that during compression localized stresses in excess of the shearing strength occur in the contact areas between aggregates and that the resulting flattening causes a dimunition of stress on account of the distribution of the load over a greater area; deformation ceases when the shearing stress falls below the shearing strength. According to this theory, the flattening phenomen acts as a check against unlimited deformation at any given pressure.