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Approximate Analysis of the Borehole Permeameter in Unsaturated Soil
Author(s) -
Philip J. R.
Publication year - 1985
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/wr021i007p01025
Subject(s) - permeameter , borehole , hydraulic conductivity , capillary action , vadose zone , geotechnical engineering , flow (mathematics) , geology , mechanics , mathematics , soil water , soil science , geometry , thermodynamics , physics
An approximate analysis of the steady constant‐head uncased borehole permeameter in homogeneous unsaturated soil is presented. A bulb‐shaped region of saturated soil, the “saturated bulb,” adjoins the water‐filled length of the hole. The problem is solved by matching approximate models of the “inner” saturated flow within the bulb and of the “outer” flow in the surrounding unsaturated soil. The quasilinear analysis, with sorptive number α characterizing the capillary properties of the soil, is applied to the outer, unsaturated flow. Certain approximations made are geometrical, and others simplify the physics by treating gravity and capillarity as separable. The results agree well with the limited body of relevant detailed numerical solutions, and the model is consistent also with saturated flow results and formulae. In general, the capillary properties of the soil cannot be ignored: for a borehole of radius 0.05 m, the error committed in ignoring capillarity increases from 2.8 to 280% as α decreases from 10 to 0.1 m −1 . The concepts and methods (the saturated bulb, use of the quasi‐linear analysis, matching inner and outer flows) apply to a range of steady mixed saturated‐unsaturated flow systems with water applied under positive hydrostatic pressure to an initially unsaturated soil mass. The study leads to some doubt about the practicality of using the borehole permeameter to measure saturated hydraulic conductivity in the absence of an independent determination of α.