VARIATION OF TEXTURAL POROSITY OF A CLAY-LOAM SOIL DURING COMPACTION

The effect of compaction on the porosity of aggregates was studied in the laboratory using mercury porosimetry and backscattered electron scanning images (BESI) of polished thin sections. Porosity was divided into structural and textural porosity, and then textural porosity into lacunar and clayey porosity. For aggregates equilibrated at a water potential of -1 kPa and pressures of 50 and 200 kPa, compaction of structural porosity resulted in an increase of textural porosity. For aggregates equilibrated at a water potential of -1 kPa and an applied pressure of 600 kPa, the structural porosity strongly decreased but did not result in a variation of textural porosity. For aggregates equilibrated at water potentials of -63 and -10(3) kPa and the three values of pressure studied, textural porosity was unaffected by compaction whatever the evolution of structural porosity. The BESI indicated that the increase in textural porosity which was recorded by mercury porosimetry for aggregates equilibrated at a water potential of -1 kPa and applied pressures of 50 and 200 kPa was due to the formation of relict structural pores during compaction. The relict structural pores resulted from partial distortion of the structural pores within the original aggregates. These relict structural pores still had the morphology of structural pores on BESI, but they were accessible to mercury through the necks of textural pores identified as lacunar pores. Results also indicate that massive structure frequently seen in the field for these soils and interpreted as resulting either from structural collapse during rewetting or from compaction actually resulted mainly from wheel compaction.