Reply to discussion by Haghighi et al. on “Measurements of suction versus water content for bentonite–sand mixtures”Appears in the Canadian Geotechnical Journal 48(2): 336–337 (2011)

The discussers presented filter paper calibration data measured using the vapour equilibrium technique using a salt solution for suctions greater than 300 kPa and through a pressure plate apparatus, in which the filter papers sandwiched between two soil samples were equilibrated to the induced suction values. The data presented agree well with those obtained by the authors for the matric suction (or the in-contact) filter paper calibration up to about 800 kPa and for the total suction greater than 8000 kPa. It should be noted that the accuracy of the total suction measurement using the noncontact technique reduces as total suction decreases. In contrast, the error of the matric suction measurement using the in-contact filter paper increases with increasing matric suction. The total suction (or the noncontact filter paper) calibration data presented by the authors were obtained for total suctions greater than 4000 kPa. It is thought that the errors associated with the measurement of total suction lower than this value are high and are not only caused by the possible temperature gradient during calibration, but also by the error in preparation of the salt solution involving a small amount of salt. No attempt was made by the authors to induce total suction less than 4000 kPa due to the above-mentioned reasons. However, the data presented in Agus (2005) suggest that an extrapolation to total suction values between 2000 and 4000 kPa is possible, but with a maximum possible error of 860 kPa. The use of this technique for measuring total suction of less than 2000 kPa is not recommended. The discussers have shown that in the suction range from 4000 to 8000 kPa the measured filter paper water contents are lower than those presented by the authors for the same total suction values. Filter papers equilibrated to total suctions within this range will normally undergo a wetting process due to the initially relatively dry filter paper used. The amount of water absorbed by the filter paper during the equilibration period is a function of the equilibration time allowed. Equilibrium is thought to be attained after a relatively long equilibration time and is affected by the temperature fluctuation in the calibration system, or more importantly by the temperature gradient between the salt solution, vapor space, and filter paper. The discussers used a 2 week equilibration time as opposed to a 5 week equilibration time adopted by the authors. It is thought that the discussers’ data would be closer to those obtained by the authors if the same equilibration time had been used. The resulting total suction calibration curve will consequently be different from that obtained for the matric suction measurement. Similarly, the equibration time at high matric suctions (i.e., close to 1500 kPa) is thought to be long as water may transfer in vapor form without good contact between the ceramic disk and the filter paper during the matric suction measurement. At high matric suctions, filter paper tends to absorb a smaller amount of absorbed water than it should had a sufficiently long equilibration time not been allowed. Hence, although theoretically the technique can be used to measure matric suction as high as 1500 kPa, the calibration data for matric suctions ranging from 1000 to 1500 kPa are difficult to obtain with sufficient accuracy. In conclusion, reasonable measurement accuracy will be obtained when the filter paper technique is used to measure matric suctions of less than 1000 kPa, or total suctions of greater than 4000 kPa.