APPROXIMATE ANALYSIS OF THE BOREHOLE PERMEAMETER IN UNSATURED SOIL

An approximate analysis of the steady constanthead uncased borehole permeameter in homogeneous unsaturated soil is presented. A bulb-shaped region of saturated soil, the "saturated bulb", adjoins theehole. The problem is solved by matching approximate models of the "inner" saturated flow within the bulb and the "outer" flow in the surrounding unsaturated soil. The quasilinear analysis, with sorptive number a characterizing the capillary properties of the soil, is applied to the outer, unsaturated, flow. The results agree well with relevant detailed numerical solutions, and the model is consistent also with saturated flow formulae. In general, the capillary properties of the soil cannot be ignored: for a borehole off radius 0.05 m, the error committed in ignoring capilarity increases from 2.8% to 280% as a decreases through the practical range 10 to 0.1 m-1. The concepts and methods apply to a range of steady mixed saturated- unsaturated flow systems.

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