A drive point application of the Guelph Permeameter method for coarse-textured soils

The Guelph Permeameter (GP) is a widely used well/auger-hole method for in-situ determination of field-saturated hydraulic conductivity (K-fs) and sorptive number (alpha*) in the vadose or unsaturated zone. Its application can be difficult, however, in gravelly materials which are problematic to auger, and in un-cohesive sandy materials which collapse during the augering process or upon wetting of the unlined well. We circumvented these issues, by replacing the GP well with a conically tipped well screen or drive point which is pushed, driven, rotated or vibrated to the desired depth. We also developed a steady flow analysis to account for the fact that water discharge through the drive point screen is solely radial, rather than both radial and vertical as in the original GP method. The HYDRUS-2D numerical simulation model was used to determine appropriate shape values (C-DP) for the drive point analysis; and an empirical regression equation was developed to convert the discrete Cm, values into continuous shape functions for porous materials with negligible-moderate, strong and very strong capillarity. In a well-sorted medium-coarse sand, the drive point method produced K-fs, and alpha* values which were plausible and consistent with other data from the field site. The original GP method, on the other hand, under-estimated the drive point results by factors of 2.1-3.1, which may have been caused by progressive collapse of the unlined GP well, and/or gradual sinking of the GP outflow tip into the unprotected well base. It was concluded that the drive point GP method can provide convenient and accurate estimates of K-fs and alpha* in materials where hydraulic properties are not substantially altered by the disturbance of drive point installation. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.