Vertical pore structure profile of a compacted clayey soil

Abstract

An investigation was conducted to determine the vertical profile of the pore structure of a compacted clayey soil. Specimens were prepared at the dry and wet sides of optimum water content using standard compaction effort. Thin sections were prepared for microscopic analysis along vertical profiles throughout the depth of the specimens. The relative quantities of pores in the compacted specimens were determined using data obtained from image analysis of the thin sections. The parameter Pore Area Ratio (PAR) was introduced as the quotient of the area of pores to the total area for a given image to quantify pore structure. The results of the analysis indicated that PAR varied significantly from the top to the bottom of the specimens. This trend was strongest in the dry of optimum and near optimum water content specimens and less pronounced in the wet of optimum specimens, in particular under very wet conditions. The common standardized impact compaction testing procedure (i.e., standard compaction test) resulted in significant variation in the vertical pore profile of the specimens due to the cumulatively higher energy applied to the specimens near the bottom and lower energy applied to the specimens near the top. Larger and more connected pores were present in the top layers of the specimens, whereas the pores were smaller and less connected in the bottom layers of the specimens. In general, the PAR decreased with the depth of the specimens with the highest PAR observed at the top and the lowest PAR observed at the bottom of the specimens. The differences in PAR were commonly twofold to threefold between the top and bottom layers of the soils.