Modified Tube Diameters for the Double‐Tube Apparatus

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Abstract
Experiences with the double‐tube method indicated that the diameter of the outer tube should be larger than the theoretical minimum because of soil disturbance in the annular space between the inner and the outer tube. Field studies were conducted in Adelanto loam and in river sand using outer tubes of various diameters in combination with an inner tube with a constant diameter. The results indicated that, for an inner‐tube diameter of 12.7 cm, an outer‐tube diameter of 25.4 cm was sufficient to eliminate most of the effects of soil disturbance in the measured value of the hydraulic conductivity. Values of 10.2 cm and 20.3 cm for the diameters of the inner and outer tubes, respectively, also appeared to be satisfactory.

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