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Measuring Horizontal and Vertical Hydraulic Conductivity of Soil With the Double-Tube Method¹

ABSTRACT

Horizontal and vertical hydraulic conductivity, symbols K_h and K_v, respectively, of anisotropic soils in which the principal conductivity components are in the horizontal and vertical directions, can be measured with the double-tube method. The principle is based on manipulation of flow direction in the artificially saturated soil region below the auger hole. As with the original double-tube method, the presence of a water table is not required.

If the water levels in the inner and the outer tube differ, the streamlines in the resulting flow system below the auger hole have both horizontal and vertical portions. Thus, the hydraulic conductivity measured with the conventional double-tube method from this flow system on the basis of assumed isotropy, symbol K_dt, is some resultant of K_h and K_v. If the water levels in the inner and the outer tube are the same, the flow in the soil below the inner tube is, initially, vertically downward. Piezometers are used to measure the vertical hydraulic gradient and knowing the intake rate, K_v can be computed. By transformation to equivalent isotropic systems, the relationship between K_dt/K_v and K_h/K_v can be determined and expressed in the form of a graph. Thus, knowing K_dt and K_v, the conductivity K_h can be evaluated from that graph.

Laboratory studies on artificial anisotropic soil consisting of alternating 0.5-cm. layers of coarse and fine soil were performed to verify and exemplify the method.

¹ Contribution from the Soil and Water Conservation Research Division, ARS, USDA.

² Research Hydraulic Engineer, U. S. Water Conservation Laboratory, Tempe, Ariz.

Received for publication April 15, 1963. Accepted for publication July 22, 1963.