FACTORS IN PERMEABILITY CHANGES OF SOILS AND INERT GRANULAR MATERIAL

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Abstract
Permeability changes were studied with a glass cylinder permeameter operating under constant head in a constant temp. room (20[degree]C), using dist. water which had been recently boiled to remove air. Soils always exhibited an initial period of decreasing permeability, followed by a period of increasing permeability, and that followed by another period of decreasing permeability. Soil from which air was first evacuated did not exhibit a period of increasing permeability. The chemical constituents of the leachates decreased continuously, but at a decreasing rate. Inert sands exhibited an initial period of increasing permeability, followed thereafter by a constant permeability. Conclusions:[long dash]1) When water starts percolating through a previously unsatd. soil, air is trapped which cannot be displaced by that water. 2) The max. effect of trapped air appears to be in pores of intermediate size. 3) This trapped air is removed only by soln. in the water percolating through the soil. The ease with which the air is dissolved depends on the capacity of the water to absorb air and on the time of contact of that water with the air, and, more important, with the amt. of percolating water passing through per unit amount of trapped air. 4) Percolating waters pass through or around such trapped air, but the coeff. of permeability is greatly depressed thereby, increasing as air is dissolved. 5) In the field, it appears that such air would not be dissolved appreciably by normal rainfall or irrigation, thus allowing air-free infiltration. Exceptions would be water-spreading basins where water completely covers the surface for considerable periods and below the surface of ground-water table. 6) Initial decreases in permeability are associated with the instability of soil under the action of the percolating waters. With Placentia loam topsoil, prior wetness without percolation did not decrease initial permeability but did cause permeability to drop more rapidly.

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