A re-evaluation of the filter paper method of measuring soil suction
The filter paper method is a soil suction measurement technique. Soil suction is one of the most important parameters describing the moisture condition of unsaturated soils. The measurement of soil suction is crucial for applying the theory' behind the unsaturated soils. The filter paper method is a laboratory test method, but it is inexpensive and relatively simple. It is also the only known method that covers the full range of suction. With the filter paper method, both total and matric suction can be measured. If the filter paper is allowed to absorb water through vapor flow (non-contact method), then only total suction is measured. However, if the filter paper is allowed to absorb water through fluid flow (contact method), then only matric suction is measured. In this research, the filter paper method was investigated, especially the lower (wet) part ofthe total suction calibration curve. A total suction calibration curve was constructed using sodium chloride, NaCl, salt solutions and S & S No. 589 White Ribbon filter papers. Salt solutions and filter papers were brought to equilibrium through vapor flow (total suction measurement) at isothermal conditions. Equilibrium time and temperature were two weeks and 25°C , respectively. It was found out that total suction is very sensitive below 2.5 pF and the reason for this sudden change in suction arise from double effect ofthe logarithms (i.e., natural logarithm from Kelvin's equation and base ten logarithm from pF scale) and sensitivity of filter papers to water at low suctions. Filter paper is a poorly graded material, so it has more tendency to absorb water at low suctions. Another conclusion from these experiments and literature review was that the calibration curve is highly salt solution-type dependent because every salt solution has a different chemical activity thus different osmotic potential. The filter paper method needs further investigation in regard to equilibrium time and temperature fluctuations.