ABSTRACT
The FPM calculates the soil suction indirectly from previous calibration. Basically, the filter paper comes to equilibrium with the soil either through vapor (total suction measurement) or liquid (matric suction measurement) flow. At equilibrium, the filter paper and the soil will have the same suction value. After equilibrium is established between the filter paper and the soil in a relatively constant temperature environment, the gravimetric water content of the filter paper disc is measured, and converted to suction using a calibration curve for the type of paper used. This is the basic approach suggested by the American Society for Testing and Materials (ASTM) standard D5298 for the measurement of either matric suction using the contact filter paper technique or total suction using the non-contact filter paper technique. The ASTM D5298 employs a single calibration curve that has been used to infer both total and matric suction measurements and recommends the filter papers to be initially ovendried (16 h or overnight) and then allowed to cool to room temperature in a desiccator. The ASTM D5298 calibration curve is a combination of both wetting and drying curves. However, because of the marked hysteresis on wetting and drying of the filter paper, the calibration curve for initially dry
1 INTRODUCTION
The filter paper method (FPM) is probably the simplest of the methods available for estimating suction of soils for essentially the full range of interest for fluid and vapor transport and other geotechnical applications (Houston et al.,1994).The method calculates soil suction indirectly by measuring the filter paper water content at equilibrium that is related to soil suction through a predetermined suction calibration curve. The calibration function defines the water retention curve (or characteristic curve) of the filter paper.
