Cohesive red soil of high water content was compacted in a mold and change in pore water pressure was observed. The pore water pressure at the bottom of the specimen was at first very high just after compaction and then it decreased very rapidly. The pressure continued to decrease and finally approached a negative constant value. Several experiments were conducted to investigate the reason for such a phenomenon. Increase in bulk volume of specimen and height of soil skeleton were observed after

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... compaction. Water exudation which followed the expansion of soil volume was also observed. Soil was stored at high/low temperature before compaction and the change in pore water pressure after compaction was compared at room temperature. Soil was compacted in a chamber filled with different gases such as carbon dioxide and surfer hex fluoride whose solubility to water is different from air. It was revealed that expansion of entrapped air and soil skeleton coincides with the rapid reduction of pore water pressure and residual high pressure may be the cause of water exudation. The cause for long lasting reduction in pore water pressure was found to be solution of entrapped air to pore water which takes a long time.