  Soil properties, such as wetting collapse behavior and permeability, are
strongly correlated to the soil microstructure. To date, several techniques
including mercury intrusion porosimetry (MIP), can be used to characterize the
microstructure of soil, but all techniques have their own limitations. In this
study, the features of mercury that penetrated and has been entrapped in the
pore network of the specimens through MIP testing were investigated by X-Ray
microtomography (X-$\mu$CT), in order to give an insight into the geometry of
macropores and possible ink-bottle geometry. Two conditions of water content
and density were selected for the compacted Maryland clay. The distribution and
geometry features of mercury entrapped in the microstructure after MIP were
characterized and pore size distributions were also reconstructed. The results
suggest that, for the two conditions studied in this paper, macropores were
evenly distributed within the specimens, and most of them with a non-spherical
shape, and with aspect ratio (ratio between the maximum and minimum thickness
along a given segment) smaller than three. Different dominant entrance pore
size of macropore was obtained from MIP and X-$\mu$CT, due to the specific
experimental protocol used in tests and the effect of ink-bottle geometry. Only
the large pore bodies with high aspect ratio were imaged in X-$\mu$CT, due to
the extrusion of mercury during the process of depressurization and subsequent
sample preparation for X- $\mu$CT. But entire pore space was accessible in MIP.
The difference in dominant entrance pore size was more significant for
specimens with lower void ratio due to a more pronounced aspect ratio.
