Pressure-Pulse Test for Field Hydraulic Conductivity of Soils: Is the Common Interpretation Method Adequate?

Abstract

Pressure-pulse tests are performed in boreholes in either impervious rock or clayey formations. According to the usual theory, they give the hydraulic conductivity, k, and the storativity, S, of the tested material. However, comparative testing programs have shown that the usual interpretation method may give k values that are quite different from those given by other methods. It gives also unrepresentative (much too low) values of S for the tested formations. Several reasons for the discrepancies are examined. First, the theory came from a heat conduction problem but the mass and energy transfers in a pulse test are quite different from those in the heat conduction problem. Thus the usual interpretation method is derived from equations that do not represent correctly the physical phenomena involved in a pulse test. A correct theory should use equations that consider medium deformation to be partly instantaneous (undrained) and partly delayed (drained) during and after the pulse. This is not the case in the commonly used theory. It should also consider that both the testing system and the formation wall are deformed during the pulse, which is ignored by the usual theory. Besides this theoretical inadequacy, the usual interpretation method is frequently affected by an inaccurate knowledge of the local piezometric level for the test. This level is frequently guessed prior to testing, and this estimate may be incorrect. However, a velocity graph technique can be used to analyze the test data and establish the real piezometric level for the test. Considering the inadequacy of the usual equations and interpretation method, it is proposed to interpret the test data as those of a slug test after having determined the local piezometric level using the velocity graph method. Hvorslev's method is used after introducing an imaginary open tube with an inside diameter that is defined by the volume of injected water associated with the initial pulse. A detailed example is treated where this interpretation method yielded a k-value close to that obtained from a constant-head test in the same injection cavity, both values being 50 times lower than the k-value derived from the usual superposition method.

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.…”

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

“…The soil skeleton deformation is also negligible for most variable‐head permeability tests conducted in Champlain clays and liners , and for some pulse tests in highly overconsolidated clay deposits. Chapuis and Cazaux gave an example of a pulse test conducted in Vaivre, France, which was successfully interpreted with the velocity graph technique, a method based on a differential form of the Hvorslev model.…”

“…Both the interpretation methods of Bredehoeft and Papadopulos , as modified by Neuzil [], and of Chapuis and Cazaux [] are based on the calculation of a virtual riser pipe section from the ratio of the volume of water injected in the cavity to the hydraulic head increase. This virtual section is then used to obtain k in conjunction with the interpretation methods applicable for variable‐head tests.…”

“…In the theoretical developments leading to Equation , because of the constant total stress hypothesis, the hydraulic head variation is directly related to changes in pore pressure and vertical effective stress (d h = d p / γ w = d σ′ z / γ w , where σ′ z is the vertical effective stress). Therefore, one may question the ability of analytical solutions using Equation to correctly represent the cavity expansion.…”

“…In this paper, a linear elastic u–p model is used to study the influence of deformations during pulse tests and to develop a new interpretation method for pulse tests. Before introducing the u–p model equations, the interpretation methods of Chapuis and Cazaux , and Bredehoeft and Papadopulos , as well as some details regarding the execution of pulse tests, are described. The u–p model is then introduced using the simple case of a hypothetical field permeability test involving purely radial flow and plane strain deformations.…”