Abstract
There are some reclaimed lands from the sea in Kojima Bay from which we selected, as experimental fields, one paddy field with combined mole-pipe drainage and two paddy fields with pipe drainage. We investigated the relation among cracks, percentage of shrinkage due to drying and permeability at the three experimental fields from the results of our study on such problems as circumstances of the fields, irrigation, laying out of drainage and meteorological conditions since reclamation.
Consequently, the coefficient of permeability was found to be K=10-3-10-2 cm/sec, 10-3 cm/sec and 10-4 cm/sec in the field with combined mole-pipe drainage system and the two fields with pipe drainage system, respectively.
The amount of cracks was found to be largest in the combined mole-pipe drainage system, and there was a difference in the amount of cracks between the two fields with pipe drainage system.
There was little difference in the maximum length of cracks (about 80 cm) among the three experimental fields. The cracks are presumed to have been caused by a drought in 1951.
Therefore, the magnitude of cracks and the coefficient of permeability depend on conditions in past-day circumstances, i. e. laying out of drainage system.
It has been explained that the combined mole-pipe drainage system has a great effect on the acceleration of cracks because water moves downward through the subsoil rapidly.
