A heavy soil consisting of hydrated halloysite is widely distributed along the coast of the Okhotsk Sea in the northern part of Hokkaido.
The soil is very compact, impervious and unfit for plant growth.
For the improvement of the soil tile draining has been practiced.
According to the common theory of tile draining, it is unnatural that the effect on draining is actually observed in such a soil.
The authors clarified the mechanism of draining of the heavy clay soil at Komukai, Monbetsu City in September 1961 and June 1962.
The results are as follows :
1) The sub-soil is very compact and a column-like structure is developed. Cracks are observed inthe upper 70cm.
2) Excepting the water in cracks, the pF of soil moisture is evaluated as more than 2.5. Therefore, the soil moisture cannot be drained by tile drains.
3) Melting snow and rain water are drained through cracks in sub-soil which join each other. The permeability of sub-soil including cracks is evaluated at the order of 10-3 cm/sec.
4) Except the case of heavy rainfall, there is no movement of drainage water to tile drains through mold along the hard pan via the disturbed and refilled earth of the drain trench.
5) Sub-soil breaking can release the compact structure of the heavy clay soil and increase its water holding capacity and permeability.
6) Therefore, the design of drainage in the heavy clay soil should be based upon these facts. The drainage system involving tile drains and sub-soil breaking is considered desirable view from land improvement.

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Author has been interested in moisture movement in unsaturated soils, and investigated this problem.
Through these investigations, author used many tensiometers for measuring soil moisture tension, and felt it very necessary to equip the most proper apparatus and, espesially, porous cup for each purpose.
So, auther made such apparatuses and cups, and in the process, could get some experiences and knowledge concerning the following problems.
1) Easy production of porous cups in laboratory.
2) Methods of testing porous cups.
3) Some methods of measuring tension by tensiometer, especially on advantages and disadvantages of null method and normal method.
4) Analysis on the problem of, time lag acompanying normal method measurement.
(Theoretical equation for adjusting time lag proposed by author was examined by experiment and proved to be correct.)

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