Composting is one of the oldest bioconversion process used by human beings; however some knowledge of the engineering understanding of the process is still lacking. One reason is that until now, little attention has been paid to the phenomenon of compaction and its effects on initial physical properties before composting. The aims of this study are to experimentally acquire evolution profiles between depth, bulk density, Free Air Space (FAS), air permeability and thermal conductivity in composting materials, and to evaluate the impact of moisture content and particle size on these four physical properties. The depth-bulk density relationship was obtained using a Schaub-Szabo device and the relationships between depth and the other parameters were then obtained using a modified air pycnometer. The study was carried out on a mixture of urban sludge and recycled wooden palettes used as bulking agents. Two moisture contents (50% and 65%) and two meshes of bulking agents (< and > 20 mm) were tested, and their influence on physical characteristics were brought to light. We also found that bulk density and thermal conductivity both increased with depth while FAS and air permeability both decreased with it. In all cases, strongest in-depth evolutions for the four parameters cited above were observed in the mixture made of thin particles (< 20 mm) and of high moisture content (65%). Moreover, we got the first data on the relationship between thermal conductivity and depth in a pile of organic waste.