THE EFFECTS OF ENCAPSULATED AIR ON FALLING HEAD PERMEAMETERS

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Abstract
The theory developed in Part 1, for the effects of expansion and contraction of encapsulated air in soils as water pressure changes, is applied to falling head perme-ameters. On commencement of flow from a static ponded column that contains air, water pressure changes at distances from the exit surface are delayed, reflecting the dampening effect of the air. Pressure and flow are described by a diffusion coefficient equal to K/C, where K is the hydraulic conductivity and C is the compressibility of the encapsulated air. Experimental results confirm the model, and times for pseudoequilibrium (as contrasted to the steady state of a constant head permeameter) are reached only after a time interval that is a function of K/C and the length of the column. At pseudo-equilibrium the diameter of the inlet manometer affects the conditions in the column, and when the inlet manometer diameter is small incorrect values of K for the column are derived from the conventional equation. A proposed method of use for falling head permeameters is suggested: the proposal allows for a-delay before readings occur, the length of delay being a function of the properties of the medium, for example about 13.5 min for a hypothetical clay core, where K is 1 mm/h, C is 0.005/m, and length is 0.3 m. © Williams & Wilkins 1987. All Rights Reserved.

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