A coupled analysis of cavity and pore volume changes for pulse tests conducted in soft clay deposits

Abstract

SUMMARYThis paper presents a new interpretation method for pulse tests, a field permeability test that allows for rapid measurements of hydraulic conductivity k in aquitards. This new method applies to soft clay deposits. To initiate a pulse test, a known volume of water is injected into the sand filter of a monitoring well isolated by a packer. The resulting pressure increase yields an outward movement of the sand filter cavity wall. After presenting the usual interpretation methods and their limits, this paper proposes a new interpretation method based on a coupled analysis of the pressure and displacement fields in the soil using the Biot–Darcy formulation. A series of analytical and numerical non‐dimensional velocity graphs, normalized plots of the mean hydraulic head difference versus its rate of change, are given. For a linear elastic material, these type curves show relatively small variations with the sand filter aspect ratio and the Poisson ratio of the tested clay. The type curves are also found to be independent of the clay compressibility (mv) and k, an important result. A series of pulse tests conducted in a soft marine clay deposit near Montreal, Canada, are interpreted with the proposed method. The hydraulic conductivity values calculated from these tests are closely correlated with independent estimates obtained using long‐duration variable‐head tests. Compared with previous interpretation methods, the proposed method allows soil volume changes to be reconciled with cavity expansion phenomena and the range of type curves available for the interpretation of test data to be constrained. Copyright © 2013 John Wiley & Sons, Ltd.

Stress and strain fields for overdamped slug tests in aquifer materials, and resulting conservation equation

“…Therefore, the stress‐strain conditions for clay predict a curved velocity graph at the beginning of a slug test (irreversible strains), and subsequently a straight line when the volumetric strain becomes elastic. This curvature is regularly observed with slug tests in clay where it lasts 1 to 2 days for a 1 to 2 month‐long test, and also with pulse tests in aquitards where it lasts during almost all test of duration 1 to 2 hours, long before being proven here using geomechanics.…”

Influence of seasonal hydraulic head changes on slug tests conducted in shallow low-permeability soils

“…On all sites but one, two MWs were installed at the upper and lower thirds of the clay layer. A detailed description of their installation was presented by Duhaime (2012) and Duhaime and Chapuis (2014). A schematic representation is shown in Fig.…”

“…interpretation method, the K value calculated from the numerical test data would be identical to the value used in the model. Part of the difference between the calculated K values and those used in the model comes from the impact of clay volume changes on the middle third of the velocity graph (e.g., Duhaime and Chapuis, 2014). Seasonal head changes can also have an influence on the slope and intercept of the straight line.…”

“…Based on Eq. 1 and a procedure similar to that used by Duhaime and Chapuis (2014) for pulse tests, Duhaime…”

“…For example, in Lachenaie, pulse tests conducted by installing a packer into the 52.5-mm riser pipe last less than two hours. On the other hand, these shorter tests tend to magnify the importance of the transient flow, which feeds pore volume change in the clay, with respect to the steady state flow component used to calculate K (Bredehoeft and Papadopulos, 1980;Duhaime and Chapuis, 2014).…”