The slope stability of a sea dike with a gentle grade was studied in this research. The seepage water through the dike researches at the surface of the downstream slope and exudes on the surface.
Firstly, it was supposed that the stability of soil grains on the slope is dependent on the gradient of the slope, hydraulic gradient in the dike and others.
Then, as a part of the study of slope stability, the state of a thin sheet flow on the surface of downstream slope was theoretically discussed, being suggested by solving the energy eguation concerning thin sheet flow. The equation of non-uniform flow is as follows : dh/dx=sini-2αq (x) ∫^x₀q (x) dx/gh²-3ν∫^x₀q (x) dx/gh³/cosi-α {∫^x₀q (x) dx} ²/gh³=F₁ (h, x) /F₂ (h, x)
Then, it was explained that this equation may be solved by the method of iso-inclination or numerical calculation.
Secondly, assuming that the seepage water through soil grains exudes on the surface of downstream slope and flows together with the ohter exude water, it was supposed that the tractive power of thin sheet flow had influence on the stability of soil grains.
Thereby, the equilibrium expression on the stability was obtained as follows : a₁·tanψ+a₂·tan (θ-ψ) +a₃·I_f·h/D {cosθ+sinθ·tanψ} =a₁·tanψ_s
Some coefficients were determined by sand model experiments so as to put this theoretical expression to practical use.

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The authors examined the availability of the auger hole method for measuring the field permeability for tile drainage projects. The results were compared with the values of permeability calculated from tile drainage tests in the same test field and also with those obtained from the data of laboratory test.
From the point of view of the permeability, the soil texture of tested field was separated into two strata at the depth of 65cm. So two series of auger holes were dug, 50 cm and 80 cm deep respectively, and the layered analysis was applied.
It was found that Hooghoudt's fomula gave smaller values than other fomulas by two orders. It may be owing to the unreasonableness of Hooghoudt's formula which was based on horizontal flow into the hole.
In the case of the monolayer anlysis of upper layer, Ernst's and Kirkham's formulas nearly coincided with each other, and for lower layer, a little smaller values were obtained in comparison with the values of permeability obtained by laboratory tests. The values inversely calculated from the tile drainage formulas nearly corespond to those for the lower layer obtained by the auger hole method. This is pretty guessed from the fact that the water percolating into tiles passes through the upper and lower layers, and it was understood that the auger hole method is practically adoptable as the permeability test for tile drainage projects.

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