Porous stones in permeability measurement: drawbacks and solution

Abstract

Porous stones are commonly used in geotechnical laboratory testing to provide solid support at the two ends of the tested specimen. Although porous stones can have several advantages, they can induce a hydraulic impedance that might alter the flowrate results. Numerous suggestions and recommendations were issued to avoid some of the problems encountered when using porous stones, such as clogging and stones' low permeability coefficient ( Kp). However, it is proven in this technical note that the existence of a soil–porous stones interaction prevents the elimination of porous stones' influence, leading to unreliable permeability results. A practical solution based on manufacturing unique porous stones (MS) is suggested in an attempt to eliminate such an influence. The porous stones' influence is highlighted by running excess pore pressure dissipation tests under triaxial conditions with calibrated beads. The results show a soil–porous stones interaction and a flowrate increase up to 700% due to MS. This interaction is addressed through permeability measurements of three soil types in a flexible-wall permeameter using standard porous stones (SS) and the newly manufactured stones (MS). At least one order of magnitude of difference in the soil permeability measurement and a decrease in the dissipation test time are observed with MS. This technical note also validates the limitation of the superposition approach for correcting the soil's permeability ( Ksoil). It is shown that porous stones can induce a signature directly related to the soil type and applied pressure.