Interaction of Water with Filter Paper Cellulose/fiber Structure in Soil Suction Measurements

Abstract

The filter paper technique of soil suction measurement is evaluated in this research. The main objective of this study is to evaluate the interaction between water and filter paper at molecular level through an extensive literature review and laboratory testing. Wetting and drying calibration data were obtained using tests based on a no-contact moisture transfer method between filter papers and salt solutions of different concentrations and de-ionized water. The Whatman No.42 and Schleicher and Schuell No. 5892 filter paper types were studied. Sodium Chloride (NaCl) solutions of different concentrations were used to generate the suction needed for calibration. A series of experiments were conducted with differing equilibration periods. Wetting calibration curve equations were proposed based on the equations of Fredlund and Xing (1994) for both filter papers. An abrupt change in suction is observed in the wetting calibration curves, which is a likely indication of change in moisture sorption mechanism with the break point lying close to the Fiber Saturation Point of cotton linters measured using DSC and NMR in previous studies. The drying calibration data did not show a clear trend. Thus, no calibration curves have been fitted for the drying calibration data of both papers. Observation of the trend of the values for the drying calibration over the different equilibration periods clearly showed the data shifted inward showing less water content with longer equilibration period. This implies that the drying filter papers required longer equilibration periods and reaffirms that hysteresis decreases with increase in equilibration period. Critical values obtained in the calibration data have been associated with the type of water in the filter paper. Literature review and the data obtained in this experiment indicate that the filter paper method is dictated by the water-filter paper interaction at the molecular level in addition to the thermodynamics that dictates the moisture transfer and equilibrium conditions. Further study on this area might prove useful in the clear understanding of the filter paper method.