Evaluation of Filter Paper Calibration

The filter paper technique has been used in the geotechnical community for the determination of soil suction. Depending on the sort of contact existing between both, the filter paper can measure either matric or total suction, absorbing water until equilibrium is reached. By means of the calibration curve, one may infer suction values in the soil from the water content of the filter paper. In order to obtain this calibration, several equipments can be used to impose a known suction on the filter paper. This article presents calibration data of five batches of Whatman No. 42 filter paper, obtained through the use of pressure plate, suction plate and NaCl solutions. It has been noticed that one of the batch presents a calibration curve which differs from calibrations proposed in the literature. In order to better visualize the effect of the difference between calibration curves, information on the retention curve of a residual soil of gneiss is presented, using the calibration obtained in this work. Results suggest that when the filter paper method is used as suction quantifier, it is important to get at least one point of calibration, comparing it to the calibrations proposed in literature.