Abstract
Hydraulic conductivity measurement through a fixed wall permeameter is a common practice to obtain the fluid transmissibility characteristics of soil matrix; however, sidewall leakage due to rigid wall effect may significantly influence the observed values for coarse-grained soils. In this study, the boundary flow error was identified through characterizing the geometrical properties of voids adjacent to the sidewall, and a parameter known as the boundary void ratio (eb) was proposed to account for this effect. The findings suggest that a fixed wall cell containing coarse soils would unavoidably generate extra voids at the interface between soil grains and inner rigid wall, contributing to a larger eb at the wall than void ratio within the soil bed; the measured hydraulic conductivity is increased primarily due to the apparatus-induced error. A two-dimensional geometric model was then established to estimate the eb value for uniformly sized coarse soils confined by a rigid permeameter wall, based on which a method was obtained for eliminating the boundary flow error from a fixed wall cell. The mathematical method was finally validated against experimental data from existing literature. It can be concluded that the boundary condition at sidewall featuring unwanted gaps lead to overestimation of the coefficient of permeability; however, the proposed correction method could adequately eliminate the boundary flow error for uniformly sized coarse-grained soils tested within a rigid wall cell.
Highlights
Soil mass made up of solid particles with necessary voids can allow water to pass through [1, 2]
Hydraulic conductivity measurement through a fixed wall permeameter is a common practice to obtain the fluid transmissibility characteristics of soil matrix; sidewall leakage due to rigid wall effect may significantly influence the observed values for coarse-grained soils
There are two typical types of permeameter used for measuring the hydraulic conductivity in the laboratory, namely, fixed wall permeameter [3] and flexible-wall permeameter [4]
Summary
Soil mass made up of solid particles with necessary voids can allow water to pass through [1, 2]. A flexible-wall device comprises a triaxial cell, in which the bed is enclosed by a flexible membrane and subjected to controlled confining pressure It can effectively eliminate boundary flow issue, and the measured data are close to the real hydraulic conductivity of unconfined bed. Kango et al [20] carried out a series of permeability tests for natural soils with different permeameters to particle size ratios (i.e., Dp/dg, where Dp is the permeameter diameter and dg is the particle diameter) and found that the influence of increasing porosity is more pronounced than surface are at the wall for Dp/dg < 40, and the hydraulic conductivity of confined bed (k) would be greater than unconfined conditions (k∗). Further investigation is required to extend its application to soils having different grain dimensions
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