Studies on the mechanism of soil consolidation from a view point of micro and macro soil sturctures

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Change in pore size distribution during consolidation processes was investigated using mercury intrusion porosimeter. Two cohesive soils were consolidated under the stress of 39-1,254 kPa in which load iincrement ratio was 1. Consolidation of specimen was interrupted at the end of the primary consolidation, in the midst of the secondary consolidation and 24 hours after the loading. Each specimen was frozen in liquid nitrogen and dried under a vacuum and subjected to MIP test. It was found in volcanic ash soil that the soil volume change during the primary consolidation is mainly due to the compression of the entrapped pores which includes inter-aggregate pores. On the other hand, soil volume change during the secondary consolidation was caused by the compression of free pores which exist in aggregates.Permeability change during secondary consolidation and stress relaxation processes was investigated using a newly developed consolidation-permeation meter in which the stress and strain of specimen can be automatically controlled by computer and applied water head difference can be precisely kept constant. Kaolin and Iwate volcanic ash soil were subjected to the test. It was revealed that reduction rate of permeability with respect t change in void ratio during the secondary consolidation was higher than the rate during the primary consolidation. The reason for such experimental results was considered as follows. There exists aggregate strucuture in these soils and permeating water mainly flows through inter-aggreagate pores. The inter-aggregate pores including wide portions of the pores are compressed in the primary consolidation process which results in relatively lower reduction rate, whereas the narrow portions of the pores are distorted and narrowed in the secondary consolidation process because of compression and deformation of the aggregates which leads to higher reduction rate. Change in permeability was not observed during the stress relaxation process.