Volume 31, Issue 6 (November 2008)
A Revised Contact Filter Paper Method
The filter paper method, ASTM Standard D5298-03 is generally accepted to be an inexpensive, technically simple, and reasonably accurate method that could be used to measure a wide range of soil suction. The method, however, is dependent upon the accuracy of the calibration curve that relates filter paper water content to soil suction. Additionally, applying a contact stress to the filter papers significantly influences this calibration curve. The effect of five separate contact stresses, namely, 0, 0.4, 1, 2, and 4 kPa on the calibration curve of Whatman 42 filter paper were examined. The study demonstrates that a contact stress of 1 kPa will ensure direct contact between the filter papers and the pore-water of the test specimens without significantly altering the ASTM calibration curve. The contact filter paper method was used to measure the matric suction of compacted Indian Head till specimens over a range of 20 to 300 kPa. This range was chosen to understand the performance and sensitivity of the filter paper method in the low suction range. A comparison of published calibration equations was then undertaken to show the drastically different matric suction estimates that can be obtained by the improper selection of published calibration equations.