Search

A permeability test in a monitoring well (MW) is used to assess the hydraulic conductivity, k, of clay. To interpret the test data, the piezometric level (PL) during the test must be known. A previous paper has concluded that due to a long time lag, the water level in the pipe is never a PL and any extrapolation of water level versus time gives an unreliable prediction of the PL. As a result, an incorrect PL yields an incorrect k value. This paper shows that a variable-head permeability test can provide a fair estimate of both the PL and the k value when it is interpreted using the central portion of the velocity graph. The first portion of the graph, when the head difference and the gradients are high, is curved by the expansion of the injection zone (undrained and drained wall displacement with eventually some creep), and thus cannot be used. The last portion of the graph primarily reflects, with some time lag, the natural variations of hydraulic heads in adjacent aquifers, and thus must not be used. [ABSTRACT FROM AUTHOR]

Trapped gas and cavity expansion may affect the data of a variable-head test in a monitoring well. In a low-permeability material, some gas may be trapped in the top of the filter pack. Presence of a gas pocket may be alleged when the initial change in water level in the pipe is less than the theoretical value due to the slug volume. A variable-head test with trapped gas is described by three equations to be solved simultaneously. The gas pocket contraction leads to an underestimation of the hydraulic conductivity, k, especially for small pipes used to test natural or compacted clay. It must be considered, however, that the difference between the theoretical and measured changes in initial water level is due to the sum of trapped gas contraction plus cavity expansion (falling-head) or trapped gas expansion plus cavity contraction (rising head). This difference cannot be used to assess the volume of trapped gas unless an independent mechanical test has established the material deformability. For example, pressuremeter tests can be done in cavities that are prepared with the same care as those used for the installation of piezometers. A simple test that consists of adding or removing known volumes of water in the pipe of the piezometer is described. Field data are provided along with the method to handle them and extract the correct k value. To avoid gas entrapment in the filter pack, a small diameter plastic tube can be installed and used as a gas vent. Du gaz piégé et une expansion de cavité peuvent affecter les données d’un essai à niveau variable dans un piézomètre. Dans un matériau peu perméable, une poche de gaz peut être piégée dans le haut de la lanterne. Sa présence peut être alléguée quand le changement initial de niveau d’eau dans le tuyau est inférieur à la valeur théorique due au volume déplacé. Un essai à niveau variable avec gaz piégé est décrit par trois équations à résoudre simultanément. La contraction de la poche de gaz conduit à sous-estimer la conductivité hydraulique, k, surtout pour les petits tuyaux servant à tester de l’argile naturelle ou compactée. Il faut considérer cependant que la différence entre les variations du niveau d’eau initial, théorique et mesurée, est due à la somme de la contraction du gaz piégé et l’expansion de cavité (niveau descendant), ou à l’expansion du gaz piégé et la contraction de cavité (niveau ascendant). Cette différence ne peut pas servir à évaluer le volume de gaz piégé, sauf si un essai mécanique indépendant a établi la déformabilité du matériau. Par exemple, on peut faire des essais pressiométriques dans des cavités préparées avec le même soin que celles utilisées pour installer les piézomètres. Un essai simple d’addition ou d’extraction de volumes d’eau connus dans le tuyau du piézomètre est décrit. Des données d’essais sont fournies avec la façon de les traiter pour extraire la valeur correcte de k. Pour éviter de piéger du gaz dans la lanterne, on peut installer un petit tuyau de plastique qui servira d’évent. [ABSTRACT FROM AUTHOR]

The drainage of sports fields, highways, farm lands, and covers for wastes corresponds to the drainage of a shallow unconfined aquifer resting on a horizontal or sloping impervious substratum. The seepage, partly saturated and partly unsaturated, is thus described by nonlinear equations that are not easy to solve. A few analytical solutions exist; they were obtained after several simplifying assumptions. Are they realistic? In this paper, comparisons are made between predictions from analytical solutions and those from numerical resolutions (for saturated and unsaturated seepage) under steady and transient states. The analytical solutions predict a water table and flow rates that differ significantly from those of the numerical resolutions, and are sometimes unrealistic. Corrections to the analytical solutions have already been proposed to account for the vadose zone. Despite such corrections, the published solutions to drainage problems may be inaccurate. In engineering projects where the duration of drainage may be critical for the construction schedule, it is recommended to avoid the analytical equations and to use numerical codes that solve the complete differential equations by taking into account the complete soil characteristic curves for hydraulic conductivity and capillary retention, which can be obtained using permeability tests and capillary-retention tests. [ABSTRACT FROM AUTHOR]