Comparison of Water Retention Curves for Clayey Soils Using Different Measurement Techniques

Abstract

Soil water retention curves were determined using four different techniques for suction measurement. Transistor psychrometer and non-contact filter paper techniques were used for total suction determinations; pressure plates and in-contact filter paper techniques were used for matric suction measurements. The soil water retention curves (WRC) were determined for bentonite, speswhite kaolin and a mixture of kaolin and sand (70 percent:30 percent by mass). Specimens were prepared at their liquid limits and the WRC were determined for drying from this initial state. At least four measurements were made for each point on the WRC to ensure reliable results. The transistor psychrometer was found to be suitable for testing suctions between 200 kPa and 18,000 kPa. However, results were found to be very sensitive to temperature change. The pressure plate gave reliable results but was limited to 1500 kPa and tests took long time periods. Osmotic suction was also determined for kaolin using a saturation extract technique followed by measurement of electrical conductivity. The results for total suction determinations showed good agreement with the sum of the matric and osmotic suction measurements. The filter paper was used successfully for both total and matric suction determinations. The results suggest that there is not a need for separate calibration curves for matric and total suction, but that a single curve can be applied to both, if the equilibration time for total suction measurements is sufficient. It was found that reasonable agreement between techniques was obtained using published calibration curves for filter paper measurements.