Using the filter paper method to determine soil suction data for sands and low plasticity clays

Knowledge of the soil water retention (SWR) data is important for study unsaturated soils (UNSAT). This article evaluates a simple and economical laboratory measurement method for determining matric suction in distinct UNSAT and suction levels: filter paper method (FPM). The FPM estimates the soil suction, s, indirectly by measuring the gravimetric water content of filter paper, w, at hydraulic equilibrium with the soil using predetermined filter paper SWR data (calibration function). Several filter paper calibrations for the Whatman No. 42 (W42) filter paper have been published in the literature. The article assesses the applicability of some previous published W42 calibrations using matric suctions determined by the contact FPM test considering distinct W42 calibrations and measured by other techniques for non-plastic sands and low-plasticity soils and suction range. The results with under and over soil suction estimates by FPM method depend on the used calibration. To improve suctions FPM determinations, an exponential function for the W42 filter paper is suggested with potential to be used for determining SWR data for both very low (< 10 kPa) and low and intermediary (10-2000 kPa) suction values.